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Copyright © 2005 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.
This document is a companion to the WSDL 2.0 specification (Web Services Description Language (WSDL) Version 2.0 Part 1: Core Language [WSDL 2.0 Core], Web Services Description Language (WSDL) Version 2.0 Part 2: Adjuncts [WSDL 2.0 Adjuncts]). It is intended for readers who wish to have an easier, less technical introduction to the main features of the language.
This primer is only intended to be a starting point toward use of WSDL 2.0, and hence does not describe every feature of the language. Users are expected to consult the WSDL 2.0 specification if they wish to make use of more sophisticated features or techniques.
Finally, this primer is non-normative. Any specific questions of what WSDL 2.0 requires or forbids should be referred to the WSDL 2.0 specification.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
This is a W3C Last Call Working Draft of Web Services Description Language (WSDL) Version 2.0 Part 0: Primer. It has been produced by the Web Services Description Working Group, which is part of the W3C Web Services Activity. If the feedback is positive, the Working Group plans to submit this specification for consideration as a W3C Candidate Recommendation along with the rest of the WSDL 2.0 documents.
Comments on this document are to be sent to the public public-ws-desc-comments@w3.org mailing list (public archive) until 19 September 2005.
A diff-marked version against the previous version of this document is available. Issues about this document are documented in the Last Call issues list maintained by the Working Group. A list of formal objections against the set of WSDL 2.0 Working Drafts is also available.
Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This document has been produced under the 24 January 2002 Current Patent Practice as amended by the W3C Patent Policy Transition Procedure. Patent disclosures relevant to this specification may be found on the Working Group's patent disclosure page. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) with respect to this specification should disclose the information in accordance with section 6 of the W3C Patent Policy.
1. Introduction
2. WSDL 2.0 Basics
3. Advanced Topics I: Importing
Mechanisms
4. Advanced Topics II: Extensibility
and Predefined Extensions
5. Advanced Topics III:
Miscellaneous
6. References
A. Acknowledgements
(Non-Normative)
1. Introduction
1.1 Prerequisites
1.2 Structure of
this Primer
1.3 Notational
Conventions
2. WSDL 2.0 Basics
2.1 Getting
Started: The GreatH Hotel Example
2.1.1 Example Scenario: The GreatH Hotel
Reservation Service
2.1.2 Defining a WSDL 2.0 Target
Namespace
2.1.2.1
Explanation of
Example
2.1.3 Defining Message Types
2.1.3.1
Explanation of
Example
2.1.4 Defining an Interface
2.1.4.1
Explanation of
Example
2.1.5 Defining a Binding
2.1.5.1
Explanation of
Example
2.1.6 Defining a Service
2.1.6.1
Explanation of
Example
2.1.7 Documenting the Service
2.1.7.1
Explanation of
Example
2.2 WSDL
2.0 Infoset, Schema and Component Model
2.2.1 WSDL 2.0 Infoset
2.2.2 WSDL 2.0 Schema
2.2.2.1
WSDL 2.0 Element Ordering
2.2.3 WSDL 2.0 Component Model
2.2.3.1
WSDL 2.0 Import and Include
2.3 More on Message
Types
2.3.1 Inlining XML Schema
2.3.2 Importing XML Schema
2.3.3 Summary of Import and Include
Mechanisms
2.4 More on
Interfaces
2.4.1 Interface Syntax
2.4.2 Interface Inheritance
2.4.3 Interface Faults
2.4.4 Interface Operations
2.4.4.1
Operation Attributes
2.4.4.2
Operation Message References
2.4.4.2.1
The messageLabel Attribute
2.4.4.2.2
The element Attribute
2.4.4.2.3
Multiple infault or outfault
Elements
2.4.4.3
Understanding Message Exchange
Patterns (MEPs)
2.5 More on
Bindings
2.5.1 Syntax Summary for Bindings
2.5.2 Reusable Bindings
2.5.3 Binding Faults
2.5.4 Binding Operations
2.5.5 The SOAP Binding Extension
2.5.5.1
Explanation of
Example
2.5.6 The HTTP Binding Extension
2.5.6.1
Explanation of Example
2.5.7 HTTP GET Versus POST: Which to Use?
3. Advanced Topics I: Importing
Mechanisms
3.1 Importing WSDL
3.2 Importing Schemas
3.2.1 Schemas in Imported Documents
3.2.2 Multiple Inline Schemas in One Document
3.2.3 The schemaLocation Attribute
3.2.3.1
Using the id Attribute to Identify Inline
Schemas
4. Advanced Topics II: Extensibility
and Predefined Extensions
4.1 Extensibility
4.1.1 Optional Versus Required
Extensions
4.2 Features and
Properties
4.2.1 SOAP Modules
4.2.2 Abstract Features
4.2.3 Properties
4.3 Defining New
MEPs
4.3.1 Confirmed Challenge
4.4 RPC
Style
4.5 MTOM and
Attachments Support
5. Advanced Topics III:
Miscellaneous
5.1 Enabling Easy Message Dispatch
5.2 Web Service
Versioning
5.2.1 Compatible
Evolution
5.2.2 Big Bang
5.2.3 Evolving a Service
5.2.4 Combined Approaches
5.2.5 Examples of Versioning and Extending a
Service
5.2.5.1
Additional Optional Elements Added in
Content
5.2.5.2
Additional Optional Elements Added to a
Header
5.2.5.3
Additional Mandatory Elements in
Content
5.2.5.4
Additional Optional Operation Added to
Interface
5.2.5.5
Additional Mandatory Operation Added to
Interface
5.2.5.6
Indicating Incompatibility by Changing the
Endpoint URI
5.2.5.7
Indicating Incompatibility by Changing the
SOAP Action
5.2.5.8
Indicating Incompatibility by Changing the
Element Content
5.3 Describing Web Service Messages That
Refer to Other Web Services
5.3.1 The Reservation Details Web Service
5.3.2 The Reservation List Web Service
5.3.3 Reservation Details Web Service Using
HTTP Transfer
5.3.4 Reservation List Web Service Using HTTP
GET
5.4 Multiple Interfaces
for the Same Service
5.5 Mapping to
RDF and Semantic Web
5.5.1 RDF Representation of WSDL 2.0
5.6 Notes on
URIs
5.6.1 XML Namespaces and Schema
Locations
5.6.2 Relative URIs
5.6.3 Generating Temporary URIs
6. References
6.1 Normative References
6.2 Informative References
A. Acknowledgements
(Non-Normative)
This primer assumes that the reader has the following prerequisite knowledge:
familiarity with XML (Extensible Markup Language (XML) 1.0 (Second Edition) [XML 1.0], XML Information Set [XML Information Set]) and XML Namespaces (Namespaces in XML [XML Namespaces]);
some familiarity with XML Schema (XML Schema Part 1: Structures [XML Schema: Structures] XML Schema Part 2: Datatypes [XML Schema: Datatypes]);
familiarity with basic Web services concepts such as Web service, client, and the purpose and function of a Web service description. (For an explanation of basic Web services concepts, see Web Services Architecture [WS Architecture] Section 1.4 and Web Services Glossary [WS Glossary] glossary. However, note the Web Services Architecture document uses the slightly more precise terms "requester agent" and "provider agent" instead of the terms "client" and "Web service" used in this primer.)
Section 2 starts with a hypothetical use case involving a hotel reservation service. It proceeds step-by-step through the development of a simple example WSDL 2.0 document that describes this service:
The types
element describes the kinds of messages
that the service will send and receive.
The interface
element describes what
abstract functionality the Web service provides.
The binding
element describes how to
access the service.
The service
element describes where to
access the service.
After presenting the example, it moves on to introduce the WSDL 2.0 infoset, schema, and component model. Then it provides more detailed coverage on defining message types, interfaces, bindings, and services.
Section 3 explains the WSDL 2.0 importing mechanisms in great details.
Section 4 talks about WSDL 2.0 extensibility and various predefined extensions.
Section 5 covers various topics that may fall outside the scope of WSDL 2.0, but shall provide useful background and best practice guidances that may be useful when authoring a WSDL 2.0 document or implementing the WSDL 2.0 specification.
This document uses several XML namespaces, some of which are defined by standards, and some are application-specific. Namespace names of the general form "http://greath.example.com/..." represent application or context-dependent URIs [IETF RFC 3986].Note also that the choice of any namespace prefix is arbitrary and not semantically significant (see [XML Information Set]).
Following the convention for XML syntax sumary in [WSDL 2.0 Core], this primer uses an informal syntax to describe the XML grammar of a WSDL 2.0 document:
The syntax appears as an XML instance, but the values indicate the data types instead of values.
Characters are appended to elements and attributes as follows: "?" (0 or 1), "*" (0 or more), "+" (1 or more).
Elements names ending in "…" indicate that elements/attributes irrelevant to the context are being omitted.
This section introduces the basic concepts used in WSDL 2.0 through the description of a hypothetical hotel reservation service. We start with a simple scenario, and later add more requirements to illustrate how more advanced WSDL 2.0 features may be used.
Hotel GreatH (a fictional hotel)) is located in a remote island. It has been relying on fax and phone to provide room reservations. Even though the facilities and prices at GreatH are better than what its competitor offers, GreatH notices that its competitor is getting more customers than GreatH. After research, GreatH realizes that this is because the competitor offers a Web service that permits travel agent reservation systems to reserve rooms directly over the Internet. GreatH then hires us to build a reservation Web service with the following functionality:
CheckAvailability. To check availability, the client
must specify a check-in date, a check-out date, and room type. The
Web service will return a room rate (a floating point number in
USD$) if such a room is available, or a zero room rate if not. If
any input data is invalid, the service should return an error.
Thus, the service will accept a checkAvailability
message and return a checkAvailabilityResponse
or
invalidDataFault
message.
MakeReservation. To make a reservation, a client must
provide a name, address, and credit card information, and the
service will return a confirmation number if the reservation is
successful. The service will return an error message if the credit
card number or any other data field is invalid. Thus, the service
will accept a makeReservation
message and return a
makeReservationResponse
or
invalidCreditCardFault
message.
The next several sections proceed step-by-step through the process of developing a WSDL 2.0 document that describes the desired Web service. However, for those who can't wait to see a complete example, here is the WSDL 2.0 document that we'll be creating.
Example 2-1. WSDL 2.0 Document for the GreatH Web Service (Initial Example)
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace= "http://greath.example.com/2004/wsdl/resSvc" xmlns:tns= "http://greath.example.com/2004/wsdl/resSvc" xmlns:ghns = "http://greath.example.com/2004/schemas/resSvc" xmlns:wsoap= "http://www.w3.org/2005/08/wsdl/soap" xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:wsdlx= "http://www.w3.org/2005/08/wsdl-extensions"> <documentation> This document describes the GreatH Web service. Additional application-level requirements for use of this service -- beyond what WSDL 2.0 is able to describe -- are available at http://greath.example.com/2004/reservation-documentation.html </documentation> <types> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="http://greath.example.com/2004/schemas/resSvc" xmlns="http://greath.example.com/2004/schemas/resSvc"> <xs:element name="checkAvailability" type="tCheckAvailability"/> <xs:complexType name="tCheckAvailability"> <xs:sequence> <xs:element name="checkInDate" type="xs:date"/> <xs:element name="checkOutDate" type="xs:date"/> <xs:element name="roomType" type="xs:string"/> </xs:sequence> </xs:complexType> <xs:element name="checkAvailabilityResponse" type="xs:double"/> <xs:element name="invalidDataError" type="xs:string"/> </xs:schema> </types> <interface name = "reservationInterface" > <fault name = "invalidDataFault" element = "ghns:invalidDataError"/> <operation name="opCheckAvailability" pattern="http://www.w3.org/2005/08/wsdl/in-out" style="http://www.w3.org/2005/08/wsdl/style/iri" wsdlx:safe = "true"> <input messageLabel="In" element="ghns:checkAvailability" /> <output messageLabel="Out" element="ghns:checkAvailabilityResponse" /> <outfault ref="tns:invalidDataFault" messageLabel="Out"/> </operation> </interface> <binding name="reservationSOAPBinding" interface="tns:reservationInterface" type="http://www.w3.org/2005/08/wsdl/soap" wsoap:protocol="http://www.w3.org/2003/05/soap/bindings/HTTP"> <fault ref="tns:invalidDataFault" wsoap:code="soap:Sender"/> <operation ref="tns:opCheckAvailability" wsoap:mep="http://www.w3.org/2003/05/soap/mep/soap-response"/> </binding> <service name="reservationService" interface="tns:reservationInterface"> <endpoint name="reservationEndpoint" binding="tns:reservationSOAPBinding" address ="http://greath.example.com/2004/reservation"/> </service> </description>
Before writing our WSDL 2.0 document, we need to decide on a WSDL 2.0 target namespace URI for it. The WSDL 2.0 target namespace is analogous to an XML Schema target namespace. Interface, binding and service names that we define in our WSDL 2.0 document will be associated with the WSDL 2.0 target namespace, and thus will be distinguishable from similar names in a different WSDL 2.0 target namespace. (This will become important if using WSDL 2.0's import or interface inheritance mechanisms.)
The value of the WSDL 2.0 target namespace must be an absolute URI. Furthermore, it should be dereferenceable to a WSDL 2.0 document that describes the Web service that the WSDL 2.0 target namespace is used to describe. For example, the GreatH owners should make the WSDL 2.0 document available from this URI. (And if a WSDL 2.0 description is split into multiple documents, then the WSDL 2.0 target namespace should resolve to a master document that includes all the WSDL 2.0 documents needed for that service description.) However, there is no absolute requirement for this URI to be dereferenceable, so a WSDL 2.0 processor must not depend on it being dereferenceable.
This recommendation may sound circular, but bear in mind that the client might have obtained the WSDL 2.0 document from anywhere -- not necessarily an authoritative source. But by dereferencing the WSDL 2.0 target namespace URI, a user should be able to obtain an authoritative version. Since GreatH will be the owner of the service, the WSDL 2.0 target namespace URI should refer to a location on the GreatH Web site or otherwise within its control.
Once we have decided on a WSDL 2.0 target namespace URI, we can begin our WSDL 2.0 document as the following empty shell.
Example 2-2. An Initial Empty WSDL 2.0 Document
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace= "http://greath.example.com/2004/wsdl/resSvc" xmlns:tns= "http://greath.example.com/2004/wsdl/resSvc" . . . > . . . </description>
<description
Every WSDL 2.0 document has a description
element
as its top-most element. This merely acts as a container for the
rest of the WSDL 2.0 document, and is used to declare namespaces
that will be used throughout the document.
xmlns="http://www.w3.org/2005/08/wsdl"
This is the XML namespace for WSDL 2.0 itself. We assign it as
the default namespace for this example by not defining a prefix for
it. In other words, any unprefixed elements in this example are
expected to be WSDL 2.0 elements (such as the
description
element).
targetNamespace=
"http://greath.example.com/2004/wsdl/resSvc"
This defines the WSDL 2.0 target namespace that we have chosen for the GreatH reservation service, as described above. Note that this is not an actual XML namespace declaration. Rather, it is a WSDL 2.0 attribute whose purpose is analogous to an XML Schema target namespace.
xmlns:tns=
"http://greath.example.com/2004/wsdl/resSvc"
This is an actual XML namespace declaration for use in our
GreatH service description. Note that this is the same URI that was
specified above as the value of the targetNamespace
attribute. This will allow us later to use the tns:
prefix in QNames, to refer to the WSDL 2.0 target namespace of the
GreatH service. (For more on QNames see [XML
Namespaces] section 3 Qualified
Names.)
Now we can start describing the GreatH service.
We know that the GreatH service will be sending and receiving messages, so a good starting point in describing the service is to define the message types that the service will use. We'll use XML Schema to do so, because WSDL 2.0 processors are likely to support XML Schema at a minimum. However, WSDL 2.0 does not prohibit the use of some other schema definition language.
WSDL 2.0 allows message types to be defined directly within the
WSDL 2.0 document, inside the types
element, which is
a child of the description
element. (Later we'll see
how we can provide the type definitions in a separate document,
using XML Schema's import
mechanism.) The following
schema defines checkAvailability
,
checkAvailabilityResponse
and
invalidDataError
message types that we'll need.
In WSDL 2.0, all normal and fault message types must be defined as single elements at the topmost level (though of course each element may have any amount of substructure inside it). Thus, a message type must not directly consist of a sequence of elements or other complex type.
Example 2-3. GreatH Message Types
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace= "http://greath.example.com/2004/wsdl/resSvc" xmlns:tns= "http://greath.example.com/2004/wsdl/resSvc" xmlns:ghns = "http://greath.example.com/2004/schemas/resSvc" . . . > ... <types> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="http://greath.example.com/2004/schemas/resSvc" xmlns="http://greath.example.com/2004/schemas/resSvc"> <xs:element name="checkAvailability" type="tCheckAvailability"/> <xs:complexType name="tCheckAvailability"> <xs:sequence> <xs:element name="checkInDate" type="xs:date"/> <xs:element name="checkOutDate" type="xs:date"/> <xs:element name="roomType" type="xs:string"/> </xs:sequence> </xs:complexType> <xs:element name="checkAvailabilityResponse" type="xs:double"/> <xs:element name="invalidDataError" type="xs:string"/> </xs:schema> </types> . . . </description>
xmlns:ghns =
"http://greath.example.com/2004/schemas/resSvc"
We've added another namespace declaration. The ghns
namespace prefix will allow us (later, when defining an interface)
to reference the XML Schema target namespace that we define for our
message types. Thus, the URI we specify must be the same as the URI
that we define as the target namespace of our XML Schema types
(below) -- not the target namespace of the WSDL 2.0
document itself.
targetNamespace="http://greath.example.com/2004/schemas/resSvc"
This is the XML Schema target namespace that we've created for
use by the GreatH reservation service. The
checkAvailability
,
checkAvailabilityResponse
and
invalidDataError
element names will be associated with
this XML Schema target namespace.
checkAvailability
,
checkAvailabilityResponse
and
invalidDataError
These are the message types that we'll use. Note that these are defined to be XML elements, as explained above.
Although we have defined several types, we have not yet indicated which ones are to be used as message types for a Web service. We'll do that in the next section.
WSDL 2.0 enables one to separate the description of a Web service's abstract functionality from the concrete details of how and where that functionality is offered. This separation facilitates different levels of reusability and distribution of work in the lifecycle of a Web service and the WSDL 2.0 document that describes it.
A WSDL 2.0 interface
defines the abstract interface
of a Web service as a set of abstract operations, each
operation representing a simple interaction between the client and
the service. Each operation specifies the types of messages that
the service can send or receive as part of that operation. Each
operation also specifies a message exchange pattern that
indicates the sequence in which the associated messages are to be
transmitted between the parties. For example, the in-out
pattern (see WSDL 2.0 Predefined Extensions
[WSDL 2.0 Adjuncts] section
2.2.3 In-Out)
indicates that if the client sends a message in to the
service, the service will either send a reply message back
out to the client (in the normal case) or it will send a
fault message back to the client (in the case of an error). We will
explain more about message exchange patterns in 2.4.4.3 Understanding Message Exchange
Patterns (MEPs)
For the GreatH service, we will (initially) define an interface
containing a single operation, opCheckAvailability
,
using the checkAvailability
and
checkAvailabilityResponse
message types that we
defined in the types
section. We'll use the in-out
pattern for this operation, because this is the most natural way to
represent a simple request-response interaction. We could have
instead (for example) defined two separate operations using the
in-only
and out-only
patterns (see WSDL 2.0 Predefined Extensions
[WSDL 2.0 Adjuncts] section
2.2.1 In-Only
and section 2.2.5 Out-Only),
but that would just complicate matters for the client, because we
would then have to separately indicate to the client developer that
the two operations should be used together as a request-response
pair.
In addition to the normal input and output messages, we also
need to specify the fault message that we wish to use in the event
of an error. WSDL 2.0 permits fault messages to be declared within
the interface
element in order to facilitate reuse of
faults across operations. If a fault occurs, it terminates whatever
message sequence was indicated by the message exchange pattern of
the operation.
Let's add these to our WSDL 2.0 document.
Example 2-4. GreatH Interface Definition
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace= "http://greath.example.com/2004/wsdl/resSvc" xmlns:tns= "http://greath.example.com/2004/wsdl/resSvc" xmlns:ghns = "http://greath.example.com/2004/schemas/resSvc" . . . xmlns:wsdlx="http://www.w3.org/2005/08/wsdl-extensions"> . . . <types> ... </types> <interface name = "reservationInterface" > <fault name = "invalidDataFault" element = "ghns:invalidDataError"/> <operation name="opCheckAvailability" pattern="http://www.w3.org/2005/08/wsdl/in-out" style="http://www.w3.org/2005/08/wsdl/style/iri" wsdlx:safe = "true"> <input messageLabel="In" element="ghns:checkAvailability" /> <output messageLabel="Out" element="ghns:checkAvailabilityResponse" /> <outfault ref="tns:invalidDataFault" messageLabel="Out"/> </operation> </interface> . . . </description>
<interface name = "reservationInterface"
>
Interfaces are declared directly inside the
description
element. In this example, we are declaring
only one interface, but in general a WSDL 2.0 document may declare
more than one interface. Thus, each interface must be given a name
that is unique within the set of interfaces defined in this WSDL
2.0 target namespace. Interface names are tokens that must not
contain a space or colon (":").
<fault name =
"invalidDataFault"
The name
attribute defines a name for this fault.
The name is required so that when an operation is defined, it can
reference the desired fault by name. Fault names must be unique
within an interface.
element =
"ghns:invalidDataError"/>
The element
attribute specifies the schema type of
the fault message, as previously defined in the types
section.
<operation
name="opCheckAvailability"
The name
attribute defines a name for this
operation, so that it can be referenced later when bindings are
defined. Operation names must also be unique within an interface.
(WSDL 2.0 uses separate symbol spaces for operation and fault
names, so operation name "foo" is distinct from fault name
"foo".)
pattern="http://www.w3.org/2005/08/wsdl/in-out"
This line specifies that this operation will use the in-out pattern as described above. WSDL 2.0 uses URIs to identify message exchange patterns in order to ensure that the identifiers are globally unambiguous, while also permitting future new patterns to be defined by anyone. (However, just because someone defines a new pattern and creates a URI to identify it, that does not mean that other WSDL 2.0 processors will automatically recognize or understand that pattern. As with any other extension, it can only be used among processors that do recognize and understand it.)
style="http://www.w3.org/2005/08/wsdl/style/iri"
This line indicates that the XML schema defining the input message of this operation follows a set of rules as specified in IRI Style that ensures the message can be serialized as an IRI.
wsdlx:safe="true" >
This line indicates that this operation will not obligate the client in any way, i.e., the client can safely invoke this operation without fear that it may be incurring an obligation (such as agreeing to buy something). This is further explained in 2.4.4 Interface Operations.
<input messageLabel="In"
The input
element specifies an input message. Even
though we have already specified which message exchange pattern the
operation will use, a message exchange pattern represents a
template for a message sequence, and in theory could consist of
multiple input and/or output messages. Thus we must also indicate
which potential input message in the pattern this particular input
message represents. This is the purpose of the
messageLabel
attribute. Since the in-out
pattern that we've chosen to use only has one input message, it is
trivial in this case: we simply fill in the message label "In" that
was defined in WSDL 2.0 Predefined Extensions
[WSDL 2.0 Adjuncts] section
2.2.3 In-Out
for the in-out
pattern. However, if a new pattern is defined that involve multiple
input messages, then the different input messages in the pattern
could then be distinguished by using different labels.
element="ghns:checkAvailability"
/>
This specifies the message type for this input message, as
defined previously in the types
section.
<output messageLabel="Out" . .
.
This is similar to defining an input message.
<outfault ref="tns:invalidDataFault"
messageLabel="Out"/>
This associates an output fault with this operation. Faults are
declared a little differently than normal messages. The
ref
attribute refers to the name of a previously
defined fault in this interface -- not a message schema type
directly. Since message exchange patterns could in general involve
a sequence of several messages, a fault could potentially occur at
various points within the message sequence. Because one may wish to
associate a different fault with each permitted point in the
sequence, the messageLabel
is used to indicate the
desired point for this particular fault. It does so indirectly by
specifying the message that will either trigger this fault or that
this fault will replace, depending on the pattern. (Some patterns
use a
message-triggers-fault rule; others use a
fault-replaces-message rule. See WSDL 2.0 Predefined
Extensions [WSDL 2.0
Adjuncts] section 2.1.2
Message Triggers Fault and section 2.1.1
Fault Replaces Message.)
Now that we've defined the abstract interface for the GreatH service, we're ready to define a binding for it.
Although we have specified what abstract messages can be exchanged with the GreatH Web service, we have not yet specified how those messages can be exchanged. This is the purpose of a binding. A binding specifies concrete message format and transmission protocol details for an interface, and must supply such details for every operation and fault in the interface.
In the general case, binding details for each operation and
fault are specified using operation
and
fault
elements inside a binding
element,
as shown in the example below. However, in some cases it is
possible to use defaulting rules to supply the information. The
WSDL 2.0 SOAP binding extension, for example, defines some
defaulting rules for operations. (See Web Services Description
Language (WSDL) Version 2.0 Part 2: Adjuncts [WSDL 2.0 Adjuncts],
Default Binding Rules.)
In order to accommodate new kinds of message formats and transmission protocols, bindings are defined using extensions to the WSDL 2.0 language, via WSDL 2.0's open content model. (See 4.1 Extensibility for more on extensibility.) WSDL 2.0 Part 2 [WSDL 2.0 Adjuncts] defines binding extensions for SOAP 1.2 [SOAP 1.2 Part 1: Messaging Framework] and HTTP 1.1 [IETF RFC 2616] as predefined extensions, so that SOAP 1.2 or HTTP 1.1 bindings can be easily defined in WSDL 2.0 documents. However, other specifications could define new binding extensions that could also be used to define bindings. (As with any extension, other WSDL 2.0 processors would have to know about the new constructs in order to make use of them.)
For the GreatH service, we will use SOAP 1.2 as our concrete message format and HTTP as our underlying transmission protocol, as shown below.
Example 2-5. GreatH Binding Definition
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace= "http://greath.example.com/2004/wsdl/resSvc" xmlns:tns= "http://greath.example.com/2004/wsdl/resSvc" xmlns:ghns = "http://greath.example.com/2004/schemas/resSvc" xmlns:wsoap= "http://www.w3.org/2005/08/wsdl/soap" xmlns:soap="http://www.w3.org/2003/05/soap-envelope"> . . . <types> . . . </types> <interface name = "reservationInterface" > ... </interface> <binding name="reservationSOAPBinding" interface="tns:reservationInterface" type="http://www.w3.org/2005/08/wsdl/soap" wsoap:protocol="http://www.w3.org/2003/05/soap/bindings/HTTP"> <operation ref="tns:opCheckAvailability" wsoap:mep="http://www.w3.org/2003/05/soap/mep/soap-response"/> <fault ref="tns:invalidDataFault" wsoap:code="soap:Sender"/> </binding> . . . </description>
xmlns:wsoap=
"http://www.w3.org/2005/08/wsdl/soap"
We've added two more namespace declarations. This one is the
namespace for the SOAP 1.2 binding extension that is defined in
WSDL 2.0 Part 3 [SOAP 1.2 Part 1:
Messaging Framework]. Elements and attributes prefixed
with wsoap:
are constructs defined there.
xmlns:soap="http://www.w3.org/2003/05/soap-envelope"
This namespace is defined by the SOAP 1.2 specification itself.
The SOAP 1.2 specification defines certain terms within this
namespace to unambiguously identify particular concepts. Thus, we
will use the soap:
prefix when we need to refer to one
of those terms.
<binding
name="reservationSOAPBinding"
Bindings are declared directly inside the
description
element. The name
attribute
defines a name for this binding. Each name must be unique among all
bindings in this WSDL 2.0 target namespace, and will be used later
when we define a service endpoint that references this binding.
WSDL 2.0 uses separate symbol spaces for interfaces, bindings and
services, so interface "foo", binding "foo" and service "foo" are
all distinct.
interface="tns:reservationInterface"
This is the name of the interface whose message format and
transmission protocols we are specifying. As discussed in 2.5 More on Bindings, a reusable
binding can be defined by omitting the interface
attribute. Note also the use of the tns:
prefix, which
refers to the previously defined WSDL 2.0 target namespace for this
WSDL 2.0 document. In this case it may seem silly to have to
specify the tns:
prefix, but in 3.1 Importing WSDL we will
see how WSDL 2.0's import mechanism can be used to combine
components that are defined in different WSDL 2.0 target
namespaces.
type="http://www.w3.org/2005/08/wsdl/soap"
This specifies what kind of concrete message format to use, in this case SOAP 1.2.
wsoap:protocol="http://www.w3.org/2003/05/soap/bindings/HTTP"
This attribute is specific to WSDL 2.0's SOAP binding extension
(thus it uses the wsoap:
prefix). It specifies the
underlying transmission protocol that should be used, in this case
HTTP.
<operation
ref="tns:opCheckAvailability"
This is not defining a new operation; rather, it is referencing
the previously defined opCheckAvailability
operation
in order to specify binding details for it. This element can be
omitted if defaulting rules are instead used to supply the
necessary information. (See the SOAP binding extension in WSDL 2.0
Part 2 [WSDL 2.0 Adjuncts]
section 4.3
Default Binding Rules .)
wsoap:mep="http://www.w3.org/2003/05/soap/mep/soap-response">
This attribute is also specific to WSDL 2.0's SOAP binding
extension. It specifies the SOAP message exchange pattern (MEP)
that will be used to implement the abstract WSDL 2.0 message
exchange pattern (in-out)
that was specified when the opCheckAvailability
operation was defined.
When HTTP is used as the underlying transport protocol (as in
this example) the wsoap:mep
attribute also controls
whether GET or POST will be used as the underlying HTTP method. In
this case, the use of
wsoap:mep="http://www.w3.org/2003/05/soap/mep/soap-response"
causes GET to be used by default. See also 2.5.7 HTTP GET Versus POST: Which to
Use?.
<fault
ref="tns:invalidDataFault"
As with a binding operation, this is not declaring a new fault;
rather, it is referencing a fault (invalidDataFault
)
that was previously defined in the opCheckAvailability
interface, in order to specify binding details for it.
wsoap:code="soap:Sender"/>
This attribute is also specific to WSDL 2.0's SOAP binding
extension. This specifies the SOAP 1.2 fault code that will cause
this fault message to be sent. If desired, a list of subcodes can
also be specified using the optional wsoap:subcodes
attribute.
Now that our binding has specified how messages will be
transmitted, we are ready to specify where the service can
be accessed, by use of the service
element.
A WSDL 2.0 service specifies a single interface that the service will support, and a list of endpoint locations where that service can be accessed. Each endpoint must also reference a previously defined binding to indicate what protocols and transmission formats are to be used at that endpoint. A service is only permitted to have one interface. (See 5.4 Multiple Interfaces for the Same Service for further discussion of this limitation.)
Here is a definition for our GreatH service.
Example 2-6. GreatH Service Definition
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace= "http://greath.example.com/2004/wsdl/resSvc" xmlns:tns= "http://greath.example.com/2004/wsdl/resSvc" xmlns:ghns = "http://greath.example.com/2004/schemas/resSvc" xmlns:wsoap= "http://www.w3.org/2005/08/wsdl/soap" xmlns:soap="http://www.w3.org/2003/05/soap-envelope"> . . . <types> . . . </types> <interface name = "reservationInterface" > . . . </interface> <binding name="reservationSOAPBinding" interface="tns:reservationInterface" . . . > . . . </binding> <service name="reservationService" interface="tns:reservationInterface"> <endpoint name="reservationEndpoint" binding="tns:reservationSOAPBinding" address ="http://greath.example.com/2004/reservation"/> </service> </description>
<service
name="reservationService"
This defines a name for this service, which must be unique among service names in the WSDL 2.0 target namespace. The name attribute is required. It allows URIs to be created that identify components in WSDL 2.0 description. (See WSDL 2.0 Core Language [WSDL 2.0 Core] appendix C URI References for WSDL 2.0 constructs.)
interface="tns:reservationInterface">
This specifies the name of the previously defined interface that these service endpoints will support.
<endpoint
name="reservationEndpoint"
This defines an endpoint for the service, and a name for this endpoint, which must be unique within this service.
binding="tns:reservationSOAPBinding"
This specifies the name of the previously defined binding to be used by this endpoint.
address
="http://greath.example.com/2004/reservation"/>
This specifies the physical address at which this service can be
accessed using the binding specified by the binding
attribute.
That's it! Well, almost.
As we have seen, a WSDL 2.0 document is inherently only a partial description of a service. Although it captures the basic mechanics of interacting with the service -- the message types, transmission protocols, service location, etc. -- in general, additional documentation will need to explain other application-level requirements for its use. For example, such documentation should explain the purpose and use of the service, the meanings of all messages, constraints on their use, and the sequence in which operations should be invoked.
The documentation
element allows the WSDL 2.0
author to include some human-readable documentation inside a WSDL
2.0 document. It is also a convenient place to reference any
additional external documentation that a client developer may need
in order to use the service. It can appear in a number of places in
a WSDL 2.0 document (see 2.2.1
WSDL 2.0 Infoset), though in this example we have only
demonstrated its use at the beginning.
Example 2-7. Documenting the GreatH Service
<?xml version="1.0" encoding="utf-8" ?> <description . . . > <documentation> This document describes the GreatH Web service. Additional application-level requirements for use of this service -- beyond what WSDL 2.0 is able to describe -- are available at http://greath.example.com/2004/reservation-documentation.html </documentation> . . . </description>
<documentation>
This element is optional, but a good idea to include. It can contain arbitrary mixed content.
at
http://greath.example.com/2004/reservation-documentation.html
The most important thing to include is a pointer to any additional documentation that a client developer would need in order to use the service.
This completes our presentation of the GreatH example. In the following sections, we will move on to look into more details of various aspects of WSDL 2.0 specification.
In computer science theory, a language consists of a (possibly infinite) set of sentences, and each sentence is a finite string of literal symbols or characters. A language specification must therefore define the set sentences in that language, and, to be useful, it should also indicate the meaning of each sentence. Indeed, this is the purpose of the WSDL 2.0 specification.
However, instead of defining WSDL 2.0 in terms of literal
symbols or characters, to avoid dependency on any particular
character encoding, WSDL 2.0 is defined in terms of the XML
Infoset [XML Information
Set]. Specifically, a WSDL 2.0 document
consists of a description
element information item (in
the XML Infoset) that conforms to the WSDL 2.0 specification. In
other words, a sentence in the WSDL 2.0 language is a
description
element information item that obeys the
additional constraints spelled out in the WSDL 2.0
specification.
Since an XML Infoset can be created from more than one physical
document, a WSDL 2.0 document does not necessarily correspond to a
single physical document: the word "document" is used
figuratively, for convenience. Furthermore, since WSDL 2.0 provides
import
and include
mechanisms, a WSDL 2.0
document may reference other WSDL 2.0 documents to facilitate
convenient organization or reuse. In such cases, the meaning of the
including or importing document as a whole will depend (in part) on
the meaning of the included or imported document.
The XML Infoset uses terms like "element information item" and "attribute information item". Unfortunately, those terms are rather lengthy to repeat often. Thus, for convenience, this primer often uses the terms "element" and "attribute" instead, as a shorthand. It should be understood, however, that since WSDL 2.0 is based on the XML Infoset, we really mean "element information item" and "attribute information item", respectively.
The following diagram gives an overview of the XML Infoset for a WSDL 2.0 document.
Figure 2-1. WSDL 2.0 Infoset Diagram
The WSDL 2.0 specification supplies a normative WSDL 2.0 schema, defined in [XML Schema: Structures], which can be used as an aid in validating WSDL 2.0 documents. We say "as an aid" here because WSDL 2.0 specification [WSDL 2.0 Core] often provides further constraints to the WSDL 2.0 schema. In addition to being valid with the normative schema, a WSDL 2.0 document must also follow all the constraints defined by the WSDL 2.0 specification.
This section gives an example of how WSDL 2.0 specification constrains the WSDL 2.0 schema about the ordering of top WSDL 2.0 elements.
Although the WSDL 2.0 schema does not indicate the required
ordering of elements, the WSDL 2.0 specification (WSDL 2.0 Part 1
[WSDL 2.0 Core] section
"XML
Representation of Description Component") clearly states a set
of constraints about how the children elements of the
description
element should be ordered. Thus, the order
of the WSDL 2.0 elements matters, in spite of what the WSDL 2.0
schema says.
The following is a pseudo-content model of
description
.
<description> <documentation />? [ <import /> | <include /> ]* <types />? [ <interface /> | <binding /> | <service /> ]* </description>
In other words, the children elements of the
description
element should be ordered as follows:
An optional documentation
comes first, if
present.
then comes zero or more elements from among the following, in any order:
include
import
extensions
An optional types
follows
Zero or more elements from among the following, in any order:
interface
binding
service
extensions.
Note the term "extension" is used above as a convenient way to refer to namespace-qualified extension elements. The namespace name of such extension elements must not be"http://www.w3.org/2005/08/wsdl".
The WSDL 2.0 Infoset model above illustrates the required structure of a WSDL 2.0 document, using the XML Infoset. However, the WSDL 2.0 language also imposes many semantic constraints over and above structural conformance to this XML Infoset. In order to precisely describe these constraints, and as an aid in precisely defining the meaning of each WSDL 2.0 document, the WSDL 2.0 specification defines a component model as an additional layer of abstraction above the XML Infoset. Constraints and meaning are defined in terms of this component model, and the definition of each component includes a mapping that specifies how values in the component model are derived from corresponding items in the XML Infoset. The following diagram gives an overview of the WSDL 2.0 components and their containment hierarchy.
Figure 2-2. WSDL 2.0 Components Containment hierarchy
In general, the WSDL 2.0 component model parallels the structure
of the required XML Infoset illustrated above. For example, the
Description, Interface, Binding,
Service and Endpoint components
correspond to the description
, interface
,
binding
, service
, and
endpoint
element information items, respectively.
Since WSDL 2.0 relies heavily on the component model to convey the
meaning of the constructs in the WSDL 2.0 language, you can think
of the Description component as representing the meaning of the
description
element information item, and hence, it
represents the meaning of the WSDL 2.0 document as a whole.
Furthermore, each of these components has properties
whose values are (usually) derived from the element and attribute
information item children of those element information items. For
example, the Service component corresponds to the
service
element information item, so the Service
component has an {endpoints} property whose value is a set of
Endpoint components corresponding to the endpoint
element information item children of that service
element information item. (Whew!).
The WSDL 2.0 component model is particularly helpful in defining
the meaning of import
and include
elements. The include
element allows you to assemble
the contents of a given WSDL 2.0 namespace from several WSDL 2.0
documents that define components for that namespace. The components
defined by a given WSDL 2.0 document consist of those whose
definitions are contained in the document and those that are
defined by any WSDL 2.0 documents that are included in it via the
include
element. The effect of the
include
element is cumulative so that if document A
includes document B and document B includes document C, then the
components defined by document A consist of those whose definitions
are contained in documents A, B, and C.
In contrast, the import
element does not define any
components. Instead, the import
element declares that
the components whose definitions are contained in a WSDL 2.0
document for a given WSDL 2.0 namespace refer to components that
belong to a different WSDL 2.0 namespace. If a WSDL 2.0 document
contains definitions of components that refer to other namespaces,
then those namespaces must be declared via an import
element. The import
element also has an optional
location
attribute that is a hint to the processor
where the definitions of the imported namespace can be found.
However, the processor may find the definitions by other means, for
example, by using a catalog.
After processing any include
elements and locating
the components that belong to any imported namespaces, the WSDL 2.0
component model for a WSDL 2.0 document will contain a set of
components that belong to the document's WSDL 2.0 namespace and any
imported namespaces. These components will refer to each other,
usually via QName references. A WSDL 2.0 document is invalid if any
component reference cannot be resolved, whether or not the
referenced component belongs to the same or a different
namespace.
We will cover a lot more about how to use WSDL 2.0 import and include in 3.1 Importing WSDL
The WSDL 2.0 types
element provides a mechanism for
enclosing message schemas in a WSDL 2.0 document. Because WSDL 2.0
directly supports schemas written in XML Schema [XML Schema: Structures], we will focus
here on the use of XML Schema to define message types. Schemas
written in other type definition languages must be defined using a
WSDL 2.0 language extension. For examples of other schema
languages, see the W3C notes on [Alternative Schema Languages Support]
.
There are two ways to indicate XML Schema message definitions
using the types
element. One way is to inline schema
definitions within xs:schema
elements that are
children of types
, as we have already seen. The other
way is to use xs:import
directly under
types
. It is perfectly reasonable to use both ways in
one WSDL 2.0 document.
A WSDL 2.0 description
may only refer to XML Schema
components that are either imported nor inlined into that WSDL 2.0
description
. In other words, the use of
xs:import
and/or xs:schema
is a necessary
condition for making XML Schema components available to a WSDL 2.0
Description component.
The following XML syntax for the types
element
illustrates the use of xs:import
and
xs:schema
:
<description> <types> <documentation />* [ <xs:import namespace="xs:anyURI" schemaLocation="xs:anyURI"? /> | <xs:schema targetNamespace="xs:anyURI" /> | other extension elements ]* </types> </description>
We have already seen an example of using inlined schema definitions in section 2.1.3 Defining Message Types, so we will merely add a few additional points here.
When XML Schema is inlined directly in a WSDL 2.0 document, it
uses the existing top-level xs:schema
element defined
by XML Schema [XML Schema:
Structures] to do so, as though the schema had been
copied and pasted into the types
element. The schema
components defined in the inlined schema are then available to WSDL
2.0 for reference by QName (see WSDL 2.0 Part 1 [WSDL 2.0 Core] "QName
Resolution").
Although WSDL 2.0 provides a wsdl:import
mechanism
(described in the next section), an inlined XML schema may also use
XML Schema's native xs:import
and
xs:include
elements to refer to schemas either in
separate files or inlined in the same WSDL 2.0 document. However,
components inlined using xs:import
have different
visibility from those inlined using xs:include
:
xs:include
d components are available to WSDL 2.0 for
reference by QName, but xs:import
ed components are
not.
There are many cases where one would prefer importing schema
definitions from separate schema files instead of inlining them
directly under the types
element. One reason is
reusability of the schemas. Although WSDL 2.0 provides a
wsdl:import
mechanism, type and element declarations
inlined in a WSDL 2.0 document are NOT automatically made available
to the importing document, even though other WSDL 2.0 components
(such as Interfaces, Bindings, etc.) do become available.
Therefore, if one wishes to share schema documents across several
WSDL 2.0 documents, they should instead be placed in separate XML
Schema documents and imported into each WSDL 2.0 document using
xs:import
directly under types
.
Within the types
element, the type components
defined in any XML Schema namespaces that are imported via
xs:import
elements and any type components defined via
xs:schema
elements are available to the WSDL 2.0
components defined in the containing WSDL 2.0 documents. The type
components defined in xs:schema
elements may by
introduced via xs:include
elements. However, the XML
Schema namespaces declared via xs:import
elements
within xs:schema
elements do not become available to
the WSDL 2.0 components defined in the containing WSDL 2.0
document.
Here is an example of importing a schema. Assuming the message types in Example 2-3 are defined in a separate schema file named "http://greath.example.com/2004/schemas/resSvc.xsd" with a target namespace "http://greath.example.com/2004/schemas/resSvc", the schema definition can then be imported into the WSDL 2.0 as follows:
Example 2-8. Example of Importing Message Definitions
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace= "http://greath.example.com/2004/wsdl/resSvc" xmlns:tns= "http://greath.example.com/2004/wsdl/resSvc" xmlns:ghns = "http://greath.example.com/2004/schemas/resSvc" . . . > . . . <types> <xs:import namespace="http://greath.example.com/2004/schemas/resSvc" schemaLocation= "http://greath.example.com/2004/schemas/resSvc.xsd"/> </types> . . . </description>
So far we have briefly covered both WSDL import/include and
schema import/include. The following table summarizes the
similarities and differences between the WSDL 2.0 and XML Schema
include
and import
mechanisms. We will
talk a lot more about importing mechanisms in 3.1 Importing WSDL and
3.2 Importing
Schemas
Mechanism | Object | Meaning |
---|---|---|
wsdl:import | WSDL 2.0 Namespace | Declare that WSDL 2.0 components refer to WSDL 2.0 components from a DIFFERENT targetNamespace. |
wsdl:include | WSDL 2.0 Document | Merge Interface, Binding and Service components from another WSDL 2.0 document that has the SAME targetNamespace. |
xs:import | XML Schema Namespace | Declare that XML Schema components refer to XML Schema components from a DIFFERENT targetNamespace. |
xs:include | XML Schema Document | Merge XML Schema components from another XML Schema document that has the SAME targetNamespace. |
We previously mentioned that a WSDL 2.0 interface is basically a
set of operations. However, there are some additional capabilities
that we have not yet covered. First, let's review the syntax for
the interface
element.
Below is the XML syntax summary of the interface
element, simplified by omitting optional
<documentation>
elements and
<feature>
and <property>
extension elements:
<description targetNamespace="xs:anyURI" > . . . <interface name="xs:NCName" extends="list of xs:QName"? styleDefault="list of xs:anyURI"? > <fault name="xs:NCName" element="xs:QName"? > </fault>* <operation name="xs:NCName" pattern="xs:anyURI" style="list of xs:anyURI"? wsdlx:safe="xs:boolean"? > <input messageLabel="xs:NCName"? element="union of xs:QName, xs:Token"? > </input>* <output messageLabel="xs:NCName"? element="union of xs:QName, xs:Token"? > </output>* <infault ref="xs:QName" messageLabel="xs:NCName"? > </infault>* <outfault ref="xs:QName" messageLabel="xs:NCName"? > </outfault>* </operation>* </interface>* . . . </description>
The interface
element has two optional attributes:
styleDefault
and extends
. The
styleDefault
attribute can be used to define a default
value for the style
attributes of all operations under
this interface (see WSDL 2.0 Part 1 "styleDefault
attribute information item"). The extends
attribute is for inheritance, and is explained next.
The optional extends
attribute allows an interface
to extend or inherit from one or more other interfaces. In such
cases the interface contains the operations of the interfaces it
extends, along with any operations it defines directly. Two things
about extending interfaces deserve some attention.
First, an inheritance loop (or infinite recursion) is prohibited: the interfaces that a given interface extends must NOT themselves extend that interface either directly or indirectly.
Second, we must explain what happens when operations from two different interfaces have the same target namespace and operation name. There are two cases: either the component models of the operations are the same, or they are different. If the component models are the same (per the component comparison algorithm defined in WSDL 2.0 Part 1 [WSDL 2.0 Core] " Equivalence of Components ") then they are considered to be the same operation, i.e., they are collapsed into a single operation, and the fact that they were included more than once is not considered an error. (For operations, component equivalence basically means that the two operations have the same set of attributes and descendents.) In the second case, if two operations have the same name in the same WSDL 2.0 target namespace but are not equivalent, then it is an error. For the above reason, it is considered good practice to ensure that all operations within the same target namespace are named uniquely.
Finally, since faults can also be defined as children of the
interface
element (as described in the following
sections), the same name-collision rules apply to those
constructs.
Let's say the GreatH hotel wants to maintain a standard message
log operation for all received messages. It wants this operation to
be reusable across the whole reservation system, so each service
will send out, for potential use of a logging service, the content
of each message it receives together with a timestamp and the
originator of the message. One way to meet such requirement is to
define the log operation in an interface which can be inherited by
other interfaces. Assuming a messageLog
element is
already defined in the ghns namespace with the required content,
the inheritance use case is illustrated in the following example.
As a result of the inheritance, the
reservationInterface
now contains two operations:
opCheckAvailability
and opLogMessage
Example 2-9. Interface Inheritance
<description ...> ... <interface name = "messageLogInterface" > <operation name="opLogMessage" pattern="http://www.w3.org/2005/08/wsdl/out-only"> <output messageLabel="out" element="ghns:messageLog" /> </operation> </interface> <interface name="reservationInterface" extends="tns:messageLogInterface" > <operation name="opCheckAvailability" pattern="http://www.w3.org/2005/08/wsdl/in-out" style="http://www.w3.org/2005/08/wsdl/style/iri" wsdlx:safe = "true"> <input messageLabel="In" element="ghns:checkAvailability" /> <output messageLabel="Out" element="ghns:checkAvailabilityResponse" /> <outfault ref="tns:invalidDataFault" messageLabel="Out"/> </operation> </interface> ... </description>
Now let's have a look at the element children of
interface
, beginning with fault
.
The fault
element is used to declare faults that
may occur during execution of operations of an interface. They are
declared directly under interface
, and referenced from
operations where they apply, in order to permit reuse across
multiple operations.
Faults are very similar to messages and can be viewed as a special kind of message. Both faults and messages may carry a payload that is normally described by an element declaration. However, WSDL 2.0 treats faults and messages slightly differently. The messages of an operation directly refer to their element declaration, however the faults of an operation indirectly refer to their element declaration via a fault element that is defined on the interface.
The reason for defining faults at the interface level is to allow their reuse across multiple operations. This design is especially beneficial when bindings are defined, since in binding extensions like SOAP there is additional information that is associated with faults. In the case of SOAP, faults have codes and subcodes in addition to a payload. By defining faults at the interface level, common codes and subcodes can be associated with them, thereby ensuring consistency across all operations that use the faults
The fault
element has a required name
attribute that must be unique within the parent
interface
element, and permits it to be referenced
from operation declarations. The optional element
attribute can be used to indicate a schema for the content or
payload of the fault message. Its value should be the QName of a
global element defined in the types
section. Please
note that when other type systems are used to define the schema for
a fault message, additional attributes may need to be defined via
WSDL 2.0's attribute extension mechanism to allow the schema to be
associated with the fault.
As shown earlier, the operation
element is used to
indicate an operation supported by the containing interface. It
associates message schemas with a message exchange pattern (MEP),
in order to abstractly describe a simple interaction with a Web
service.
An operation
has two required attributes and one
optional attribute:
A required name
attribute, as seen already, which
must be unique within the interface.
A required pattern
attribute whose value must be an
absolute URI that identifies the desired MEP for the
operation
. MEPs are further explained in 2.4.4.3 Understanding Message Exchange
Patterns (MEPs).
An optional style
attribute whose value is a list
of absolute URIs. Each URI identifies a certain set of rules that
were followed in defining this operation
. It is an
error if a particular style is indicated, but the associated rules
are not followed. [WSDL 2.0
Adjuncts] defines a set of styles, including
RPC Style. The RPC style is selected when the style
is assigned the value http://www.w3.org/2005/08/wsdl/rpc. It places
restrictions for Remote Procedure Call-types of interactions.
IRI Style. The IRI style is selected when the style
is assigned the value http://www.w3.org/2005/08/wsdl/style/iri. It
places restrictions on message definitions so they may be
serialized into something like HTTP URL encoded.
The Multipart style. The Multipart style is selected when the
style
is assigned the value
http://www.w3.org/2005/08/wsdl/style/multipart. In the HTTP
binding, for XForm clients, a message must be defined following the
Multipart style and serialized as "Multipart/form-data".
You can find more details of these WSDL 2.0 predefined styles.
Section 4.4 RPC Style provides
an example of using the RPC style
. [WSDL 2.0 Adjuncts] provides examples for
the URI style and Multipart style.
Note that [WSDL 2.0
Adjuncts] provides a predefined extension for indicating
operation safety. The wsdlx:safe
global attribute
whose value is a boolean can be used with an operation to indicate
whether the operation is asserted to be "safe" (as defined in
Section 3.5 of the Web Architecture [Web
Architecture]) for clients to invoke. In essence, a safe
operation is any operation that does not give the client any new
obligations. For example, an operation that permits the client to
check prices on products typically would not obligate the client to
buy those products, and thus would be safe, whereas an operation
for purchasing products would obligate the client to pay for the
products that were ordered, and thus would not be safe.
An operation should be marked safe (by using the
wsdlx:safe
and by setting its value to "true") if it
meets the criteria for a safe interaction defined in Section 3.5 of
the Web Architecture [Web
Architecture], because this permits the infrastructure
to perform efficiency optimizations, such as pre-fetch, re-fetch
and caching.
The default value of this attribute is false. If it is false or is not set, then no assertion is made about the safety of the operation; thus the operation may or may not be safe.
An operation
will also have input
,
output
,infault
, and/or
outfault
element children that specify the ordinary
and fault message types to be used by that operation. The MEP
specified by the pattern
attribute determines which of
these elements should be included, since each MEP has placeholders
for the message types involved in its pattern.
Since operations were already discussed in 2.1.4 Defining an Interface, this section will merely comment on additional capabilities that were not previously explained.
The messageLabel
attribute of the
input
and output
elements is optional. It
is not necessary to explicitly set the messageLabel
when the MEP in use is one of the eight MEPs predefined in WSDL 2.0
Part 2 [WSDL 2.0 Adjuncts]
and it has only one message with a given direction.
The element
attribute of the input
and
output
elements is used to specify the message content
schema (aka payload schema) when the content model is defined using
XML Schema. As we have seen already, it can specify the QName of an
element schema that was defined in the types
section.
However, alternatively it can specify one of the following
tokens:
#any
The message content is any single element.
#none
There is no message content, i.e., the message payload is empty.
#other
The message content is described by a non-XML type system. Extension attributes specify the type.
The element
attribute is also optional. If it is
not specified, then the message content is described by a non-XML
type system.
Note that there are situations that the information conveyed in
the element
attribute is not sufficient for a service
implementation to uniquely identify an incoming message and
dispatch it to an appropriate operation. In such situations,
additional means may be required to aid identifying an incoming
message. See 5.1 Enabling Easy
Message Dispatch for more detail.
WSDL 2.0 message exchange patterns (MEPs) are used to define the sequence and cardinality of the abstract messages in an operation. By design, WSDL 2.0 MEPs are abstract. First of all, they abstract out specific message types. MEPs identify placeholders for messages, and placeholders are associated with specific message types when an operation is defined, which includes specifying which MEP to use for that operation. Secondly, unless explicitly stated otherwise, MEPs also abstract out binding-specific information like timing between messages, whether the pattern is synchronous or asynchronous, and whether the messages are sent over a single or multiple channels.
It's worth pointing out that WSDL 2.0 MEPs do not exhaustively describe the set of messages that may be exchanged between a service and other nodes. By some prior agreement, another node and/or the service may send other messages (to each other or to other nodes) that are not described by the MEP. For instance, even though an MEP may define a single message sent from a service to one other node, a service defined by that MEP may multicast that message to other nodes. To maximize reuse, WSDL 2.0 message exchange patterns identify a minimal contract between other parties and Web Services, and contain only information that is relevant to both the Web service and the client that engages that service.
A total of eight MEPs are defined in [WSDL 2.0 Adjuncts]. These MEPs should cover the most common use cases, but they are not meant to be an exhaustive list of MEPs that can ever be used by operations. More MEPs can be defined for particular application needs by interested parties. (See 2.4.4.3 Understanding Message Exchange Patterns (MEPs) )
For the eight MEPs defined by WSDL 2.0, some of them are variations of others based on how faults may be generated. For example, the In-Only pattern ("http://www.w3.org/2005/08/wsdl/in-only") consists of exactly one message received by a service from some other node. No fault can be generated. As a variation of In-Only, Robust In-Only pattern ("http://www.w3.org/2005/08/wsdl/robust-in-only") also consists of exactly one message received by a service, but in this case faults can be triggered by the message and must be delivered to the originator of the message. If there is no path to this node, the fault must be discarded. For details about the common fault generation models used by the eight WSDL 2.0 MEPs, see [WSDL 2.0 Adjuncts].
Depending on how the first message in the MEP is initiated, the eight WSDL 2.0 MEPs may be grouped into two groups: in-bound MEPs, for which the service receives the first message in the exchange, and out-bound MEPs, for which the service sends out the first message in the exchange. (Such grouping is not provided in the WSDL 2.0 specification and is presented here only for the purpose of easy reference in this primer).
A frequently asked question about out-bound MEPs is how a
service knows where to send the message. Services using out-bound
MEPs are typically part of large scale integration systems that
rely on mapping and routing facilities. In such systems, out-bound
MEPs are useful for specifying the functionality of a service
abstractly, including its requirements for potential customers,
while endpoint address information can be provided at deployment or
runtime by the underlying integration infrastructure. For example,
the GreatH hotel reservation system may require that every time a
customer interacts with the system to check availability, data
about the customer must be logged by a CRM system. At design time,
it's unknown which particular CRM system would be used together
with the reservation system. To address this requirement, we may
change the "reservationInterface" in Example 2-1 to include an out-bound
logInquiry operation. This logInquiry
operation
advertises to potential service clients that customer data will be
made available by the reservation service at run time. When the
reservation service is deployed to GreatH's IT landscape,
appropriate configuration time and run time infrastructure will
help determine which CRM system will get the customer data and log
it appropriately. It's worth noting that in addition to being used
by a CRM system for customer management purpose, the same data may
also be used by a system performance analysis tool for different
purpose. Providing an out-bound operation in the reservation
service enables loose coupling and so improves the overall GreatH
IT landscape's flexibility and scalability.
Example 2-10. Use of outbound MEPs
<description ...> ... <interface name="reservationInterface"> ... <operation name="opCheckAvailability" ... > <operation name="opLogInquiry" pattern="http://www.w3.org/2005/08/wsdl/out-only"> <output messageLabel="Out" element="ghns:customerData" /> </operation> </interface> ... </description>
Although the eight MEPs defined in WSDL 2.0 Part 2 [WSDL 2.0 Adjuncts] are intended to cover most use cases, WSDL 2.0 has designed this set to be extensible. This is why MEPs are identified by URIs rather than a fixed set of tokens.
For more about defining new MEPs, see 4.3 Defining New MEPs.
Bindings are used to supply protocol and encoding details that
specify how messages are to be sent or received. Each
binding
element uses a particular binding
extension to specify such information. WSDL 2.0 Part 2
[WSDL 2.0 Adjuncts] defines
several binding extensions that are typically used. However,
binding extensions that are not defined in WSDL 2.0 Part 2 can also
be used, provided that client and service toolkits support
them.
Binding information must be supplied for every operation in the interface that is used in an endpoint. However, if the desired binding extension provides suitable defaulting rules, then the information will only need to be explicitly supplied at the interface level, and the defaulting rules will implicitly propagate the information to the operations of the interface. For example, see the Default Binding Rules of SOAP binding extension in WSDL 2.0 Part 2 [WSDL 2.0 Adjuncts].
Since bindings are specified using extensions to the WSDL 2.0 language (i.e., binding extensions are not in the WSDL 2.0 namespace), the XML for expressing a binding will consist of a mixture of elements and attributes from WSDL 2.0 namespace and from the binding extension's namespace, using WSDL 2.0's open content model.
Here is a syntax summary for binding
, simplified by
omitting optional documentation
, feature
and property
elements. Bear in mind that this syntax
summary only shows the elements and attributes defined within the
WSDL 2.0 namespace. When an actual binding is defined, elements and
attributes from the namespace of the desired binding extension will
also be intermingled as required by that particular binding
extension.
<description targetNamespace="xs:anyURI" > . . . <binding name="xs:NCName" interface="xs:QName"? > <fault ref="xs:QName" > </fault>* <operation ref="xs:QName" > <input messageLabel="xs:NCName"? > </input>* <output messageLabel="xs:NCName"? > </output>* <infault ref="xs:QName" messageLabel="xs:NCName"? > </infault>* <outfault ref="xs:QName" messageLabel="xs:NCName"? > </outfault>* </operation>* </binding>* . . . </description>
The binding
syntax parallels the syntax of
interface
: each interface construct has a binding
counterpart. Despite this syntactic similarity, they are indeed
different constructs, since they are in different symbol spaces and
are designed for different purposes.
A binding can either be reusable (applicable to any interface) or non-reusable (specified for a particular interface). Non-reusable bindings may be specified at the granularity of the interface (assuming the binding extension provides suitable defaulting rules), or on a per-operation basis if needed. A non-reusable binding was demonstrated in 2.1.5 Defining a Binding.
To define a reusable binding, the binding
element
simply omits the interface
attribute and omits
specifying any operation-specific and fault-specific binding
details. Endpoints can later refer to a reusable binding in the
same manner as for a non-reusable binding. Thus, a reusable binding
becomes associated with a particular interface when it is
referenced from an endpoint, because an endpoint is part of a
service, and the service specifies a particular interface that it
implements. Since a reusable binding does not specify an interface,
reusable bindings cannot specify operation-specific details.
Therefore, reusable bindings can only be defined using binding
extensions that have suitable defaulting rules, such that the
binding information only needs to be explicitly supplied at the
interface level.
A binding fault
associates a concrete message
format with an abstract fault of an interface. It describes how
faults that occur within a message exchange of an operation will be
formatted, since the fault does not occur by itself. Rather, a
fault occurs as part of a message exchange specified by an
interface operation
and its binding counterpart, the
binding operation
.
A binding fault
has one required ref
attribute which is a reference, by QName, to an
interface
fault
. It identifies the
abstract interface fault
for which binding information
is being specified. Be aware that the value of ref
attribute of all the faults
under a
binding
must be unique. That is, one cannot define
multiple bindings for the same interface fault within a given
binding
.
A binding operation
describes a concrete binding of
an interface operation to a concrete message format. An interface
operation is uniquely identified by the WSDL 2.0 target namespace
of the interface and the name of the operation within that
interface, via the required ref
attribute of binding
operation
. As with faults, for each
operation
within a binding
, the value of
the ref
attribute must be unique.
The WSDL 2.0 SOAP Binding Extension (see WSDL 2.0 Part 2 [WSDL 2.0 Adjuncts]) was primarily designed to support the features of SOAP 1.2 [SOAP 1.2 Part 1: Messaging Framework]. However, for backwards compatibility, it also provides some support for SOAP 1.1 [SOAP 1.1].
An example using the WSDL 2.0 SOAP binding extension was already presented in 2.1.5 Defining a Binding, but some additional points are worth mentioning:
Because the same binding extension is used for both SOAP 1.2 and
SOAP 1.1, a wsoap:version
attribute is provided to
allow you to indicate which version of SOAP you want. If this
attribute is not specified, it defaults to SOAP 1.2.
The WSDL 2.0 SOAP binding extension defines a set of default rules, so that bindings can be specified at the interface level or at the operation level (or both), with the operation level taking precedence. However, it does not define default binding rules for faults. Thus, if a given interface defines any faults, then corresponding binding information must be explicitly provided for each such fault.
If HTTP is used as the underlying protocol, then the binding can (and should) control whether each operation will use HTTP GET or POST. (See 2.5.7 HTTP GET Versus POST: Which to Use?.)
Here is an example that illustrates both a SOAP 1.2 binding (as seen before) and a SOAP 1.1 binding.
Example 2-11. SOAP 1.2 and SOAP 1.1 Bindings
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace="http://greath.example.com/2004/wsdl/resSvc" xmlns:tns="http://greath.example.com/2004/wsdl/resSvc" xmlns:ghns="http://greath.example.com/2004/schemas/resSvc" xmlns:wsoap="http://www.w3.org/2005/08/wsdl/soap" xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:soap11="http://schemas.xmlsoap.org/soap/envelope/"> .... <!-- SOAP 1.2 Binding --> <binding name="reservationSOAPBinding" interface="tns:reservationInterface" type="http://www.w3.org/2005/08/wsdl/soap" wsoap:protocol="http://www.w3.org/2003/05/soap/bindings/HTTP"> <operation ref="tns:opCheckAvailability" wsoap:mep="http://www.w3.org/2003/05/soap/mep/request-response"/> <fault ref="tns:invalidDataFault" wsoap:code="soap:Sender"/> </binding> <!-- SOAP 1.1 Binding --> <binding name="reservationSOAP11Binding" interface="tns:reservationInterface" type="http://www.w3.org/2005/08/wsdl/soap" wsoap:version="1.1" wsoap:protocol="http://www.w3.org/2005/08/soap11/bindings/HTTP"> <operation ref="tns:opCheckAvailability"/> <fault ref="tns:invalidDataFault" wsoap:code="soap11:Client"/> </binding> <service name="reservationService" interface="tns:reservationInterface"> <!-- SOAP 1.2 End Point --> <endpoint name="reservationEndpoint" binding="tns:reservationSOAPBinding" address="http://greath.example.com/2004/reservation"/> <!-- SOAP 1.1 End Point --> <endpoint name="reservationEndpoint2" binding="tns:reservationSOAP11Binding" address="http://greath.example.com/2004/reservation"/> </service> </description>
Most lines in this example is the same as previously explained in 2.1.5 Defining a Binding, so we'll only point out lines that are demonstrating something new for SOAP 1.1 binding.
<description ...
xmlns:soap11="http://schemas.xmlsoap.org/soap/envelope/">
This is the namespace for terms defined within the SOAP 1.1 specification [SOAP 1.1].
<binding...wsoap:version="1.1"
This line indicates that this binding uses SOAP 1.1 [WSDL 2.0 SOAP 1.1 Binding], rather than SOAP 1.2.
wsoap:protocol="http://www.w3.org/2005/05/soap11/bindings/HTTP">
This line specifies that HTTP should be used as the underlying transmission protocol. See also 2.5.7 HTTP GET Versus POST: Which to Use?.
<operation
ref="tns:opCheckAvailability"/>
Note that wsoap:mep
is not applicable to SOAP 1.1
binding.
<fault...wsoap:code="soap11:Client"/>
This line specifies the SOAP 1.1 fault code that will be used in transmitting invalidDataFault.
In addition to the WSDL 2.0 SOAP binding extension described above, WSDL 2.0 Part 2 [WSDL 2.0 Adjuncts] defines a binding extension for HTTP 1.1 [IETF RFC 2616] and HTTPS [IETF RFC 2818], so that these protocols can be used natively to send and receive messages, without first encoding them in SOAP.
The HTTP binding extension provides many features to control:
Which HTTP operation will be used. (GET, PUT, POST, DELETE, and other HTTP operations are supported.)
Input, output and fault serialization
Transfer codings
Authentication requirements
Cookies
HTTP over TLS (https)
As with the WSDL 2.0 SOAP binding extension, the HTTP binding extension also provides defaulting rules to permit binding information to be specified at the interface level and used by default for each operation in the affected interface, however, defaulting rules are not provided for binding faults.
Here is an example of using the HTTP binding extension to check hotel room availability at GreatH.
Example 2-12. HTTP Binding Extension
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" . . . xmlns:whttp="http://www.w3.org/2005/08/wsdl/http" > . . . <binding name="reservationHTTPBinding" interface="tns:reservationInterface" type="http://www.w3.org/2005/08/wsdl/http" whttp:methodDefault="GET"> <operation ref="tns:opCheckAvailability" whttp:location="{checkInDate}" /> </binding> <service name="reservationService" interface="tns:reservationInterface"> <!-- HTTP 1.1 GET End Point --> <endpoint name="reservationEndpoint" binding="tns:reservationHTTPBinding" address="http://greath.example.com/2004/checkAvailability/"/> </service> . . . </description>
Most of this example is the same as previously explained in 2.1.5 Defining a Binding, so we'll only point out lines that are demonstrating something new for HTTP binding extension.
<description...xmlns:whttp="http://www.w3.org/2005/08/wsdl/http"
>
This defines the namespace prefix for elements and attributes defined by the WSDL 2.0 HTTP binding extension.
<binding...type="http://www.w3.org/2005/08/wsdl/http"
This declares the binding as being an HTTP binding.
whttp:methodDefault="GET">
The default method for operations in this interface will be HTTP GET.
whttp:location="{checkInDate}"
>
The whttp:location
attribute specifies a pattern
for serializing input message instance data into the path component
of the request URI. The default binding rules for HTTP specify that
the default input serialization for GET is
application/x-www-form-urlencoded
. Curly braces are
used to specify the name of a schema type in the input message
schema, which determines what input instance data will be inserted
into the path component of the request URI. The curly
brace-enclosed name will be replaced with instance data in
constructing the path component. Remaining input instance data (not
specified by whttp:location
) will either be serialized
into the query string portion of the URI or into the message body,
as follows: if a "/" is appended to a curly brace-enclosed type
name, then any remaining input message instance data will be
serialized into the message body. Otherwise it will be serialized
into query parameters.
Thus, in this example, each of the elements in the
tCheckAvailability
type will be serialized into the
query parameters. A sample resulting URI would therefore be
http://greath.example.com/2004/checkAvailability/5-5-5?checkOutDate=6-6-5&roomType=foo
.
Here is an alternate example that appends "/" to the type name in order to serialize the remaining instance data into the message body:
Example 2-13. Serializing a Subset of Types in the Path
. . . <operation ref="tns:opCheckAvailability" whttp:location="bycheckInDate/{checkInDate/}" > . . .
This would instead serialize to a request URI such as:
http://greath.example.com/2004/checkAvailability/bycheckInDate/5-5-5
.
The rest of the message content would go to the HTTP message
body.
When a binding using HTTP is specified for an operation, the WSDL 2.0 author must decide which HTTP method is appropriate to use -- usually a choice between GET and POST. In the context of the Web as a whole (rather than specifically Web services), the W3C Technical Architecture Group (TAG) has addressed the question of when it is appropriate to use GET, versus when to use POST, in a finding entitled URIs, Addressability, and the use of HTTP GET and POST ([W3C TAG Finding: Use of HTTP GET]). From the abstract:
". . . designers should adopt [GET] for safe operations such as simple queries. POST is appropriate for other types of applications where a user request has the potential to change the state of the resource (or of related resources). The finding explains how to choose between HTTP GET and POST for an application taking into account architectural, security, and practical considerations."
Recall that the concept of a safe operation was discussed in
2.4.4.1 Operation
Attributes. (Briefly, a safe operation is one that does not
cause the invoker to incur new obligations.) Although the
wsdlx:safe
attribute of an interface operation
indicates that the abstract operation is safe, it does not
automatically cause GET to be used at the HTTP level when the
binding is specified. The choice of GET or POST is determined at
the binding level:
If the WSDL 2.0 SOAP binding extension is used (2.5.5 The SOAP Binding Extension), with HTTP as the underlying transport protocol, then GET may be specified by setting:
wsoap:protocol="http://www.w3.org/2003/05/soap/bindings/HTTP"
on the binding
element (to indicate the use of HTTP
as the underlying protocol); and
wsoap:mep="http://www.w3.org/2003/05/soap/mep/soap-response/"
on the binding operation
element, which causes GET
to be used by default.
If the WSDL 2.0 HTTP binding extension is used directly (2.5.6 The HTTP Binding Extension), GET may be specified by setting either:
whttp:methodDefault="GET"
on the binding
element; or
whttp:method="GET"
on the binding operation
element, which overrides
whttp:methodDefault
if set on the binding
element; or
wsdlx:safe="true"
on the bound interface operation
. When the above
two items are not explicitly set, and when the bound interface
operation is marked safe, the HTTP Binding will by default set the
method to GET.
For example, in the GreatH interface definition shown in Example 2-4, the wsdlx:safe attribute is set to "true". The HTTP binding definition in Example 2-12 may take advantage of that and be simplified as below and still have the http method set to GET by default:
In some circumstances WSDL authors may want to split up a Web service description into two or more documents. For example, if a description is getting long or is being developed by several authors, then it is convenient to divide it into several parts. Another very important case is when you expect parts of the description to be reused in several contexts. Clearly it is undesirable to cut and paste sections of one document into another, since that is error prone and leads to maintenance problems. More importantly, you may need to reuse components that belong to a wsdl:targetNamespace that is different than that of the document you are writing, in which case the rules of WSDL 2.0 prevent you from simply cutting and pasting them into your document.
To solve these problems, WSDL 2.0 provides two mechanisms for
modularizing Web service description documents: import
and include
. This section discusses the import
mechanism and describes some typical cases where it may be
used.
The import
mechanism lets one refer to the
definitions of Web service components that belong to other
namespaces. To illustrate this, consider the GreatH hotel
reservation service. Suppose that the reservation service uses a
standard credit card validation service that is provided by a
financial services company. Furthermore, suppose that companies in
the financial services industry decided that it would be useful to
report errors in credit card validation using a common set of
faults, and have defined these faults in the following Web service
description:
Example 3-1. Standard Credit Card Validation Faults (credit-card-faults.wsdl)
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace="http://finance.example.com/CreditCards/wsdl" xmlns:tns="http://finance.example.com/CreditCards/wsdl" xmlns:cc="http://finance.example.com/CreditCards/xsd"> <documentation> This document describes standard faults for use by Web services that process credit cards. </documentation> <types> <xs:import xmlns:xs="http://www.w3.org/2001/XMLSchema" namespace="http://finance.example.com/CreditCardFaults/xsd" schemaLocation="credit-card-faults.xsd" /> </types> <interface name="creditCardFaults"> <fault name="cancelledCreditCard" element="cc:CancelledCreditCard"> <documentation>Thrown when the credit card has been cancelled.</documentation> </fault> <fault name="expiredCreditCard" element="cc:ExpiredCreditCard"> <documentation>Thrown when the credit card has expired.</documentation> </fault> <fault name="invalidCreditCardNumber" element="cc:InvalidCreditCardNumber"> <documentation>Thrown when the credit card number is invalid. This fault will occur if the wrong credit card type is specified. </documentation> </fault> <fault name="invalidExpirationDate" element="cc:InvalidExpirationDate"> <documentation>Thrown when the expiration date is invalid.</documentation> </fault> </interface> </description>
This example defines an interface,
creditCardFaults
, that contains four faults,
cancelledCreditCard
, expiredCreditCard
,
invalidCreditCardNumber
, and
invalidExpirationDate
. These components belong to the
namespace
http://finance.example.com/CreditCards/wsdl
.
Because these faults are defined in a different wsdl:targetNamespace than the one used by the GreatH Web service description, import must be used to make them available within the GreatH Web service description, as shown in the following example:
Example 3-2. Using the Standard Credit Card Validation Faults (use-credit-card-faults.wsdl)
<?xml version="1.0"?> <description targetNamespace="http://greath.example.com/2004/wsdl/resSvc" xmlns:ghns="http://greath.example.com/2004/schemas/resSvc" xmlns:cc="http://finance.example.com/CreditCards/wsdl" xmlns="http://www.w3.org/2005/08/wsdl" xmlns:xs="http://www.w3.org/2001/XMLSchema"> <documentation> Description: The definition of the reservation Web service of GreatH hotel. Author: Joe Somebody Date: 05/17/2004 </documentation> <import namespace="http://finance.example.com/CreditCards/wsdl" location="credit-card-faults.wsdl"/> . . . <interface name="reservation" extends="cc:creditCardFaults"> . . . <operation name="makeReservation" pattern="http://www.w3.org/2005/08/wsdl/in-out"> <input messageLabel="In" element="ghns:makeReservation" /> <output messageLabel="Out" element="ghns:makeReservationResponse" /> <outfault ref="invalidDataFault" messageLabel="Out" /> <outfault ref="cc:cancelledCreditCard" messageLabel="Out" /> <outfault ref="cc:expiredCreditCard" messageLabel="Out" /> <outfault ref="cc:invalidCreditCardNumber" messageLabel="Out" /> <outfault ref="cc:invalidExpirationDate" messageLabel="Out" /> </operation> </interface> </description>
The hotel reservation service declares that it is using
components from another namespace via the import
>
element. The import element has a required namespace
attribute that specifies the other namespace, and an optional
location
attribute that gives the processor a hint
where to find the description of the other namespace. The
reservation
interface extends the
creditCardFault
interface from the other namespace in
order to make the faults available in the reservation interface.
Finally, the makeReservation
operation refers to the
standard faults in its outfault
elements.
Another typical situation for using imports is to define a
standard interface that is to be implemented by many services. For
example, suppose the hotel industry decided that it was useful to
have a standard interface for making reservations. This interface
would belong to some industry association namespace, e.g.
http://hotels.example.com/reservations/wsdl
. Each
hotel that implemented the standard reservation service would
define a service in its own namespace, e.g.
http://greath.example.com/2004/wsdl/resSvc
. The
description of each service would import the
http://hotels.example.com/reservations/wsdl
namespace
and refer to the standard reservation interface in it.
WSDL 2.0 documents may contain one or more XML schemas defined
within the wsdl:types
element. This section
illustrates the correct way to refer to these schemas, both from
within the same document and from other documents.
In this example, we consider some GreatH Hotel Web services that
retrieve and update reservation details. The retrieval Web service
is defined in the retrieveDetails.wsdl
WSDL 2.0
document, along with a schema for the message format. The updating
Web service is defined in the updateDetails.wsdl
WSDL
2.0 document which imports the first document and refers to both
WSDL 2.0 and schema definitions contained in the imported
document.
Example 3-3 shows the
definition of the retrieval Web service in the
http://greath.example.com/2004/services/retrieveDetails
namespace. This WSDL 2.0 document also contains an inline schema
that describes the reservation detail in the
http://greath.example.com/2004/schemas/reservationDetails
namespace. This schema is visible to the
retrieveDetailsInterface
interface definition which
refers to it in the retrieve
operation's output
message.
Example 3-3. The Retrieve Reservation Details Web Service: retrieveDetails.wsdl
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace="http://greath.example.com/2004/services/retrieveDetails" xmlns:tns="http://greath.example.com/2004/services/retrieveDetails" xmlns:wdetails="http://greath.example.com/2004/schemas/reservationDetails" xmlns:xs="http://www.w3.org/2001/XMLSchema"> <documentation> This document describes the GreatH Retrieve Reservation Details Web service. </documentation> <types> <xs:schema xmlns="http://www.w3.org/2001/XMLSchema" targetNamespace="http://greath.example.com/2004/schemas/reservationDetails"> <xs:element name="reservationDetails"> <xs:complexType> <xs:sequence> <xs:element name="confirmationNumber" type="string" /> <xs:element name="checkInDate" type="date" /> <xs:element name="checkOutDate" type="date" /> <xs:element name="roomType" type="string" /> <xs:element name="smoking" type="boolean" /> </xs:sequence> </xs:complexType> </xs:element> </xs:schema> </types> <interface name="retrieveDetailsInterface"> <operation name="retrieve" pattern="http://www.w3.org/2005/08/wsdl/in-out"> <input messageLabel="In" element="#none" /> <output messageLabel="Out" element="wdetails:reservationDetails" /> </operation> </interface> </description>
Example 3-4 shows the
definition of the updating Web service in the
http://greath.example.com/2004/services/updateDetails
namespace. The updateDetailsInterface
interface
extends the retrieveDetailsInterface
interface.
However, the retrieveDetailsInterface
belongs to the
http://greath.example.com/2004/services/retrieveDetails
namespace, so updateDetails.wsdl
must import
retrieveDetails.wsdl
to make that namespace
visible.
The updateDetailsInterface
interface also uses the
reservationDetails
element definition that is
contained in the inline schema of the imported
retrieveDetails.wsdl
document. However, this schema is
not automatically visible within the
updateDetails.wsdl
document. To make it visible, the
updateDetails.wsdl
document must import the namespace
of the inline schema within the types
element using
the XML schema import
element.
In this example, the schemaLocation
attribute of
the import
element has been omitted. The
schemaLocation
attribute is a hint to the WSDL 2.0
processor that tells it where to look for the imported schema
namespace. However, the WSDL 2.0 processor has already processed
the retrieveDetails.wsdl
document which contains the
imported namespace in an inline schema so it should not need any
hints. However, this behavior depends on the implementation of the
processor and so cannot be relied on.
Although the WSDL 2.0 document may validly omit the
schemaLocation
attribute, it is a best practice to
either provide a reliable value for it or move the inline schema
into a separate document, say reservationDetails.xsd
,
and directly import it in the types
element of both
retrieveDetails.wsdl
and
updateDetails.wsdl
. In general, schemas that are
expected to be referenced from more than one WSDL 2.0 document
should be defined in a separate schema document rather than be
inlined.
Example 3-4. The Update Reservation Details Web Service: updateDetails.wsdl
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace="http://greath.example.com/2004/services/updateDetails" xmlns:tns="http://greath.example.com/2004/services/updateetails" xmlns:retrieve="http://greath.example.com/2004/services/retrieveDetails" xmlns:details="http://greath.example.com/2004/schemas/reservationDetails" xmlns:xs="http://www.w3.org/2001/XMLSchema"> <documentation> This document describes the GreatH Update Reservation Details Web service. </documentation> <import namespace="http://greath.example.com/2004/services/retrieveDetails" location="retrieveDetails.wsdl" /> <types> <xs:import namespace="http://greath.example.com/2004/schemas/reservationDetails" /> </types> <interface name="updateDetailsInterface" extends="retrieve:retrieveDetailsInterface"> <operation name="update" pattern="http://www.w3.org/2005/08/wsdl/in-out"> <input messageLabel="In" element="details:reservationDetails" /> <output messageLabel="Out" element="details:reservationDetails" /> </operation> </interface> </description>
A WSDL 2.0 document may define multiple inline schemas in its
types
element. The two or more schemas may have the
same target namespace provided that they do not define the same
elements or types. It is an error to define the same element or
type more than once, even if the definitions are identical.
Each namespace of an inline schema becomes visible to the Web
service definitions. However, the namespaces are not automatically
visible to the other inline schemas. Each inline schema must
explicitly import any other namespace it references. The
schemaLocation
attribute is not required in this case
since the WSDL 2.0 processor knows the location of each schema by
virtue of having processed the enclosing WSDL 2.0 document.
To illustrate this, consider Example 3-5 which contains two inline
schemas. The
http://greath.example.com/2004/schemas/reservationItems
namespace contains some elements for items that appear in the
reservation details. The
http://greath.example.com/2004/schemas/reservationDetails
namespace contains the reservationDetails
element
which refers to the item elements. The schema for the
http://greath.example.com/2004/schemas/reservationDetails
namespace contains an import
element that imports the
http://greath.example.com/2004/schemas/reservationItems
namespace. No schemaLocation
attribute is required for
this import since the schema is defined inline in the importing
document.
Example 3-5. Multiple Inline Schemas: retrieveItems.wsdl
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace="http://greath.example.com/2004/services/retrieveDetails" xmlns:tns="http://greath.example.com/2004/services/retrieveDetails" xmlns:wdetails="http://greath.example.com/2004/schemas/reservationDetails" xmlns:xs="http://www.w3.org/2001/XMLSchema"> <documentation> This document describes the GreatH Retrieve Reservation Details Web service. </documentation> <types> <xs:schema targetNamespace="http://greath.example.com/2004/schemas/reservationItems"> <xs:element name="confirmationNumber" type="string" /> <xs:element name="checkInDate" type="date" /> <xs:element name="checkOutDate" type="date" /> <xs:element name="roomType" type="string" /> <xs:element name="smoking" type="boolean" /> </xs:schema> <xs:schema targetNamespace="http://greath.example.com/2004/schemas/reservationDetails" xmlns:items="http://greath.example.com/2004/schemas/reservationItems"> <xs:import namespace="http://greath.example.com/2004/schemas/reservationItems" /> <xs:element name="reservationDetails"> <xs:complexType> <xs:sequence> <xs:element ref="items:confirmationNumber" /> <xs:element ref="items:checkInDate" /> <xs:element ref="items:checkOutDate" /> <xs:element ref="items:roomType" /> <xs:element ref="items:smoking" /> </xs:sequence> </xs:complexType> </xs:element> </xs:schema> </types> <interface name="retrieveDetailsInterface"> <operation name="retrieve" pattern="http://www.w3.org/2005/08/wsdl/in-out"> <input messageLabel="In" element="#none" /> <output messageLabel="Out" element="wdetails:reservationDetails" /> </operation> </interface> </description>
In the preceding examples, schemas were defined inline in WSDL
2.0 documents. This section discusses the correct way to specify a
schemaLocation
attribute on a schema
import
element to provide a processor with a hint for
locating these schemas.
Example 3-4 shows how one WSDL
2.0 document imports a schema defined in another, i.e. Example 3-3. Similarly, Example 3-5 shows how one schema in a
WSDL 2.0 document imports another schema defined in the same
document. In both of these examples, the
schemaLocation
attribute was omitted since the WSDL
2.0 processor was assumed to know how to locate the imported
schemas because they were part of the WSDL 2.0 documents being
processed. The schemaLocation
attribute can be used to
give the processor a URI reference that explicitly locates the
schemas. A URI reference is a URI plus an optional fragment
identifier that indicates part of the resource. For schemas, the
fragment should identify the schema
element. The
simplest way to accomplish this is to use the id
attribute, however XPointer (see [XPointer
Framework]) can also be used.
Example 3-6 shows the use of the
id
attribute. Both of the inline schemas have
id
attributes. The id of the
http://greath.example.com/2004/schemas/reservationItems
schema is items
and the id of the
http://greath.example.com/2004/schemas/reservationDetails
schema is details
. The import
element in
the
http://greath.example.com/2004/schemas/reservationDetails
schema uses the id of the
http://greath.example.com/2004/schemas/reservationItems
schema in the schemaLocation
attribute, i.e.
#items
.
Example 3-6. Using Ids in Inline Schemas: schemaIds.wsdl
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace="http://greath.example.com/2004/services/retrieveDetails" xmlns:tns="http://greath.example.com/2004/services/retrieveDetails" xmlns:wdetails="http://greath.example.com/2004/schemas/reservationDetails" xmlns:xs="http://www.w3.org/2001/XMLSchema"> <documentation> This document describes the GreatH Retrieve Reservation Details Web service. </documentation> <types> <xs:schema id="items" targetNamespace="http://greath.example.com/2004/schemas/reservationItems"> <xs:element name="confirmationNumber" type="string" /> <xs:element name="checkInDate" type="date" /> <xs:element name="checkOutDate" type="date" /> <xs:element name="roomType" type="string" /> <xs:element name="smoking" type="boolean" /> </xs:schema> <xs:schema id="details" targetNamespace="http://greath.example.com/2004/schemas/reservationDetails" xmlns:items="http://greath.example.com/2004/schemas/reservationItems"> <xs:import namespace="http://greath.example.com/2004/schemas/reservationItems" schemaLocation="#items" /> <xs:element name="reservationDetails"> <xs:complexType> <xs:sequence> <xs:element ref="items:confirmationNumber" /> <xs:element ref="items:checkInDate" /> <xs:element ref="items:checkOutDate" /> <xs:element ref="items:roomType" /> <xs:element ref="items:smoking" /> </xs:sequence> </xs:complexType> </xs:element> </xs:schema> </types> <interface name="retrieveDetailsInterface"> <operation name="retrieve" pattern="http://www.w3.org/2005/08/wsdl/in-out"> <input messageLabel="In" element="#none" /> <output messageLabel="Out" element="wdetails:reservationDetails" /> </operation> </interface> </description>
WSDL 2.0 provides two extensibility mechanisms: an open content model, which allows XML elements and attributes from other (non-WSDL 2.0) XML namespaces to be interspersed in a WSDL 2.0 document; and Features and Properties. Both mechanisms use URIs to identify the semantics of the extensions. For extension XML elements and attributes, the namespace URI of the extension element or attribute acts as an unambiguous name for the semantics of that extension. For Features and Properties, the Feature or Property is named by a URI.
In either case, the URI that identifies the semantics of an extension should be dereferenceable to a document that describes the semantics of that extension. As of this writing, there is no generally accepted standard for what kind of document that should be. However, the W3C TAG has been discussing the issue (see TAG issue namespaceDocument-8) and is likely to provide guidance at some point.
Extensions can either be required or optional.
An optional extension is one that the client may either
engage or ignore, entirely at its discretion, and is signaled by
attribute wsdl:required="false"
or the absence of the
wsdl:required
attribute (because it defaults to
false). Thus, a WSDL 2.0 processor, acting on behalf of the client,
that encounters an unknown optional extension can safely ignore it
and continue to process the WSDL 2.0 document. However, it is
important to stress that optional extensions are only optional to
the client -- not the service. A service must support all
optional and required extensions that it advertises in its WSDL 2.0
document.
A required extension is one that must be supported and
engaged by the client in order for the interaction to proceed
properly, and is signaled by attribute
wsdl:required="true"
. If a WSDL 2.0 processor, acting
on behalf of the client, encounters a required extension that it
does not recognize or does not support, then it cannot safely
continue to process the WSDL 2.0 document. In most practical cases,
this is likely to mean that the processor will require manual
intervention to deal with the extension. For example, a client
developer might manually provide an implementation for the required
extension to the WSDL 2.0 processor.
Editorial note: KevinL | 20050519 |
The section is subject to change. Pending on the resolution of the minority opinions filed about Feature and Property. |
After a few successful trials of the reservation service, GreatH decides that it is time to make the makeReservation operation secure, so that sensitive credit-card information is not being sent across the public network in a snoopable fashion. We will do this using the WSDL 2.0 Features and Properties mechanisms [WSDL 2.0 Core], which is modeled after the Features and Properties mechanism defined in SOAP 1.2 [SOAP 1.2 Part 1: Messaging Framework].
To facilitate presentation, this section will assume the
existence of a hypothetical security feature named
"http://features.example.com/2005/securityFeature
",
which defines, in the abstract, the idea of message
confidentiality. This feature has an associated property, named
"http://features.example.com/2005/securityFeature/securityLevel
",
which defines various safety levels (from 0 meaning clear text, all
the way through 10, involving highly complex cryptographic
algorithms with keys in the tens of thousands of bits). We also
assume that a SOAP module (for more about SOAP module, see SOAP1.2 spec
and 4.2.1 SOAP Modules),
named
"http://features.example.com/2005/modules/Security
",
has been defined, which implements the security feature described
above.
GreatH has chosen an abstract security feature which is standard in the fictitious hotels community, and has integrated both a SOAP module and a new secure HTTP binding into its infrastructure – both of which implement the security feature (the SOAP module does this inside the SOAP envelope using headers, and the secure binding does it at the transport layer). Now they'd like to advertise and control the usage of these extensions using WSDL 2.0.
The first step GreatH takes is to require the usage of the SOAP module in their normal SOAP/HTTP endpoint, which looks like this:
Example 4-1. Requiring a SOAP Module in an Endpoint
. . . <service name="reservationService" interface="tns:reservationInterface"> <endpoint name="reservationEndpoint" binding="tns:reservationSOAPBinding" address ="http://greath.example.com/2004/reservation"> <wsoap:module uri="http://features.example.com/2005/modules/Security" required="true"/> </endpoint> </service> . . .
This syntax indicates that a SOAP Module is required by this endpoint. This means that anyone using this endpoint must both understand the specification that the module URI references, and must use that specification when communicating with the endpoint in question, which typically means including appropriate SOAP headers on transmitted messages.
If the "required" attribute was not present, or if it was set to
"false
", then the <wsoap:module>
syntax would indicate optional the availability of the referenced
module, rather than a requirement to engage it, as explained in
4.1.1 Optional Versus
Required Extensions.
Since GreatH began the web service improvements, they have been talking to several travel agents. The possibility of making their simple hotel interface an industry standard amongst a consortium of hotels has come up, and as such they would like to enable specifying the requirement for the "makeReservation" operation to be secure at the interface level – in other words indicating that the operation must be secure, but without specifying exactly how that should concretely be achieved (to enable maximal reuse of the interface). The next example uses the WSDL 2.0 Feature element to indicate this.
Example 4-2. Declaring an Abstract Feature Requirement
. . . <interface name="reservationInterface"> <operation name="makeReservation"> <feature uri="http://features.example.com/2005/securityFeature" required="true"/> . . . [The rest of the operation is unchanged] . . . </operation> </interface> . . .
This declaration indicates that understanding of, and compliance with, the specified security feature is required for all uses of the "makeReservation" operation. The security feature is abstract, which means that although it defines semantics and a level of detail about its general operation, it expects a concrete component (like a SOAP module or binding) to actually realize the functionality.
By definition, if you understand a SOAP module, you understand which (if any) abstract features it implements. Therefore, since the security module in this example is defined as an implementation of the abstract security feature, we know that the use of this module satisfies the requirement to implement the feature. Therefore users of the HTTP endpoint shown above (with the required SOAP module) will be able to make use of it. GreatH also defines a new endpoint:
Example 4-3. A SOAP Binding Over a Secure HTTP Protocol
. . . <binding name="reservationSecureSOAPBinding" interface="tns:reservationInterface" type="http://www.w3.org/2005/08/wsdl/soap" wsoap:protocol="http://bindings.example.com/SOAPBindings/secureHTTP"> . .. </binding> . . . <service name="reservationService"> . . . <endpoint name="secureReservationEndpoint" binding="tns:reservationSecureSOAPBinding" address="https://greath.example.com/2004/secureReservation"/> </service> . . .
The user will have a choice as to which of the endpoints, and therefore which binding, is to be used, but they both satisfy the abstract feature requirement specified in the interface.
Note that it is not necessary to declare the abstract feature in order to use/require the SOAP module, or in order to use/require the secure binding. Abstract feature declarations serve purely to indicate requirements which must be fulfilled by more concrete components such as modules or bindings. In other words, the abstract feature declaration allows components such as interfaces to be reused without caring exactly which SOAP modules or bindings satisfy the feature.
So far we've discussed how to indicate the availability or the
"requiredness" of features and modules. Often it is not enough to
indicate that a particular extension is available/required: you
also need some way to control or parameterize aspects of its
behavior. This is achieved by the use of WSDL 2.0
properties. Each feature, SOAP module, or SOAP binding may
express a variety of properties in its specification.
These properties are very much like variables in a programming
language. If GreatH would like to indicate that the
securityLevel
property should be 5 for the
"makeReservation" operation, it would look like this:
Example 4-4. Defining a Property
. . . <interface name="reservationInterface"> <operation name="makeReservation"> <property uri="http://features.example.com/2005/securityFeature/securityLevel"> <value>5</value> </property> . . . [rest of operation definition] . . . </operation> </interface> . . .
The property
element specifies which property is to
be set. By setting the value
element, a toolkit
processing this WSDL 2.0 document is informed that the
securityLevel property must be set to 5. The particular meanings of
any such values are up to the implementations of the
modules/bindings that use them. The property
element
can be placed at many different levels in a WSDL 2.0 document (see
"Property Composition Model" section in WSDL 2.0 Part 1
[WSDL 2.0 Core]).
It is also possible to provide a constraint on the value space for a given property. This allows the author of the WSDL 2.0 document to indicate that several valid values for the property are possible for a given scope, limiting the value space already described in the specification that defined the property. Let's extend our example to make this clearer.
The security feature specification defines securityLevel as an integer with values between 1 and 10, each of which indicates, according to the spec, a progressively higher level of security. The GreatH service authors, having read the relevant specifications, have decided that any security level between 3 and 7 will be supported by their infrastructure. Levels less than 3 are deemed unsafe for GreatH's purposes, and levels greater than 7 require too much in the way of resources to make it worthwhile. We can express this in WSDL 2.0 as follows:
Example 4-5. Defining Property Constraints
. . . <types> <schema> <simpleType name="securityLevelConstraint"> <restriction base="xs:int"> <min 3, max 7> <!-- check schema for syntax --> </restriction> </simpleType> </schema> </types> . . . <property uri="http://features.example.com/2005/securityFeature/securityLevel"> <constraint type="tns:securityLevelConstraint"> </property> . . .
First we define, in the types
section, an XML
Schema restriction type over integers with minimum and maximum
values, per our discussion above. Then instead of using the
value
element inside property
, we use
constraint
and refer to the restriction type. This
informs the implementation that the property must have the
appropriate values. This information might be useful to a
deployment user interface, for example, which might allow an
administrator to set this value with a slider when deploying the
service.
As we mentioned in 2.4.4.3 Understanding Message Exchange Patterns (MEPs), even though the 8 MEPs defined by WSDL 2.0 are intended to cover most of the common use cases, there are situations that require new MEPs to be defined. In this section, we will explain how new MEPs can be defined to address special business requirements.
Following the wild success of its reservation service, GreatH discovered that it could radically increase tourist interest by supplying information on weather conditions, both to travel agents and to the general touring public. This produced a challenge for the service implementers: how could this information be supplied to interested parties without requiring knowledge of web service technology specifically, and of computers generally? At issue was the desire to provide asynchronous updates to unsophisticated customers without incurring onerous overheads for technical support.
The solution adopted was to create a standard mailing list, and to make available a small cross-platform web service client (actually, a subscriber) that could be installed on any computer with POP or IMAP access to a mailbox. The mailbox, once signed up for the mailing list, could either be processed as "dedicated" (to the GreatH weather service; travel agents did this) or as "general purpose" (in which case the application would only examine those emails that contained Subject headers associated with the service). This required development of a binding to email, which is out of scope for this example, but the resulting WSDL 2.0 was otherwise quite straightforward.
Note: the email binding in use here supports publish/subscribe, by supporting the robust-out-only MEP as well as the client/server style in-out used for subscribing and unsubscribing. Details of this binding would require a document as long as the primer, so play along.
Example 4-6. Weather Notification Service (Initial)
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace="http://greath.example.com/2004/wsdl/weathSvc.wsdl" xmlns:tns="http://greath.example.com/2004/wsdl/weathSvc.wsdl" xmlns:wsoap="http://www.w3.org/2005/08/wsdl/soap" xmlns:email="http://www.example.com/webservices/email" > <types> . . . </types> <interface name="weatherInterface"> <operation name="opSubscribeWeather" pattern="http://www.w3.org/2005/08/wsdl/in-out"> <input element=". . ." /> <output element=". . ." /> </operation> <operation name="opUnsubscribeWeather" pattern="http://www.w3.org/2005/08/wsdl/in-out"> <output element=". . ." /> <input element=". . ." /> </operation> <operation name="opNotifyWeather" pattern="http://www.w3.org/2005/08/wsdl/robust-out-only"> <output element=". . ." /> </operation> </interface> <binding name="weatherMailingListBinding" interface="tns:weatherInterface type="http://www.w3.org/2005/08/wsdl/soap" wsoap:protocol="http://www.example.com/bindings/email"> . . . </binding> <service name="weatherService" interface="tns:weatherInterface"> <endpoint name="greatHWeatherList" binding="tns:weatherMailingListBinding" address="mailto:weather-owner@greath.example.com" /> </service> </description>
Note: in the example, the messageLabels of all input and output elements have been elided, as they are not necessary to disambiguate (but note that the order of input and output elements is not significant).
Unfortunately, the service was soon highjacked for the purpose of annoyment. Repeatedly, hotels in less salubrious climes, and the victims of various natural climactic disasters (hurricanes, tornadoes) found themselves signed up to receive material full of incomprehensible pointy brackets. They complained to GreatH, who complained to their service designers.
Applying public key infrastructure to solving the problem was immediately rejected as too complex and too heavyweight. Analysis showed that the problem was simply to verify that the address requesting information actually wanted that information. Consequently, a new message exchange pattern was defined.
This pattern consists of two or more messages in order as follows:
A message:
indicated by a Message Label component whose message label is "Request" and direction is "in"
received from some node N1
A message:
indicated by a Message Label component whose message label is "Challenge" and direction is "out"
sent to some node N2 (which may be the same node as N1)
An optional message:
indicated by a Message Label component whose message label is "Confirmation" and direction is "in"
received from node N2
An optional message:
indicated by a Message Label component whose message label is "Response" and direction is "out"
sent to node N2
This pattern uses the rule Message Triggers Fault.
An operation using this message exchange pattern has a pattern property with the value "http://www.example.com/webservices/meps/confirmed-challenge".
Once the MEP had been defined (and the email binding specification appropriately modified to indicate that this was a supported MEP), the service was redefined and redeployed. Only the changed operations are shown in the excerpt below.
Example 4-7. Weather Notification Service (Revised)
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace="http://greath.example.com/2004/wsdl/weathSvc.wsdl" xmlns:tns="http://greath.example.com/2004/wsdl/weathSvc.wsdl" xmlns:wsoap="http://www.w3.org/2005/08/wsdl/soap" xmlns:email="http://www.example.com/webservices/email" > . . . <interface name="weatherInterface"> <operation name="opSubscribeWeather" pattern="http://www.example.com/webservices/meps/confirmed-challenge"> <input messageLabel="Request" element=". . ." /> <output messageLabel="Challenge" element=". . ." /> <input messageLabel="Confirmation" element=". . ." /> <output messageLabel="Response" element=". . ." /> </operation> <operation name="opUnsubscribeWeather" pattern="http://www.example.com/webservices/meps/confirmed-challenge"> <output messageLabel="Challenge" element=". . ." /> <output messageLabel="Response" element=". . ." /> <input messageLabel="Confirmation" element=". . ." /> <input messageLabel="Request" element=". . ." /> </operation> . . . </interface> . . . </description>
Note: in the second example, the input and output examples are not in the sequence in which they occur in the pattern; this illustrates that the sequence is not significant. Note, however, that for this pattern, the messageLabel attribute is required on every input and output element.
Section 2.4.4.1 Operation
Attributes mentioned that the (optional) style
attribute of an interface operation is used to indicate that the
operation conforms to a particular pre-defined operation style, or
set of constraints. Actually, if desired the style
attribute can hold a list of URIs, indicating that the operation
simultaneously conforms to multiple styles.
Operation styles are named using URIs, in order to be unambiguous while still permitted new styles to be defined without requiring updates to the WSDL 2.0 language. WSDL 2.0 Part 2 [WSDL 2.0 Adjuncts] defines three such operation styles; one of these is the RPC Style (RPC Style).
The RPC Style is designed to facilitate programming language bindings to WSDL 2.0 constructs. It allows a WSDL 2.0 interface operation to be easily mapped to a method or function signature, such as a method signature in Java(TM) or C#. RPC Style is restricted to operations that use the In-Out or In-Only MEPs (see 2.4.4.3 Understanding Message Exchange Patterns (MEPs)).
A WSDL 2.0 document makes use of the RPC Style in an interface
operation by first defining the operation in conformance with all
of the RPC Style rules, and then setting that operation's
style
attribute to include the URI that identifies the
RPC Style, thus asserting that the operation does indeed conform to
the RPC Style. These rules permit the input and output message
schemas to map conveniently to inputs and outputs of a method
signature. Roughly, input elements map to input parameters, output
elements map to output parameters, and elements that appear both in
the input and output message schemas map to input/output
parameters. WSDL 2.0 Part 2 section "RPC
Style" provides full details of the mapping rules and
requirements.
The RPC Style also permits the full signature of the intended
mapping to be indicated explicitly, using the
wrpc:signature
attribute defined in WSDL 2.0 Part 2
section "wrpc:signature
Extension". This is an (optional) extension to the WSDL 2.0
language whose value designates how input and output message schema
elements map to input and output parameters in the method
signature.
The example below illustrates how RPC Style may be used to
designate a signature. This example is a modified version of the
GreatH reservation service. In particular, the
interface
and types
sections have been
modified to specify and conform to the RPC Style.
Example 4-8. Specifying RPC Style
. . . <types> <xs:element name="checkAvailability"> <xs:complexType> <xs:sequence> <xs:element name="checkInDate" type="xs:date"/> <xs:element name="checkOutDate" type="xs:date"/> <xs:element name="roomType" type="xs:string"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="checkAvailabilityResponse"> <xs:complexType> <xs:sequence> <xs:element name="roomType" type="xs:string"/> <xs:element name="rateType" type="xs:string"/> <xs:element name="rate" type="xs:double"/> </xs:sequence> </xs:complexType> </xs:element> . . . </types> <interface name = "reservationInterface" > <operation name="checkAvailability" pattern="http://www.w3.org/2005/08/wsdl/in-out" style="http://www.w3.org/2005/08/wsdl/rpc" wrpc:signature= "checkInDate #in checkOutDate #in roomType #inout rateType #out rate #return"> <input messageLabel="In" element="tns:checkAvailability" /> <output messageLabel="Out" element="tns:checkAvailabilityResponse" /> </operation> . . . </interface> . . .
Note that the interface operation's name
"checkAvailability
", is the same as the localPart of
the input element's QName, "tns:checkAvailability
".
This is one of the requirements of the RPC Style. The name of the
operation is used as the name of the method in a language binding,
subject to further mapping restrictions specific to the target
programming language. In this case, the name of the method would be
"checkAvailability
".
The local children elements of the input element and output
element designate the parameters and the return type for a method
call. Note that the elements checkInDate
,
checkOutDate
are input parameters, however the element
roomType
is an in-out parameter, as it appears both as
a local element child of both input and output elements. This
indicates that the reservation system may change the room type
requested based on availability.
The reservation service also returns a rate type for the reservation, such as "rack rate". The return value for the method is designated as the "rate" element.
Based on the value of the wrpc:signature
attribute,
the method signature would be obtained following the order of the
parameters. A sample mapping is provided below for the Java(TM)
language. This example was created using JAX RPC 1.1
[JAX RPC 1.1] for mapping simple
types to Java types and designated inout and output parameters by
using Holder classes.
Example 4-9. Sample Java(TM) Signature for RPC Style
public interface reservationInterface extends Remote{ double checkAvailability(java.util.calendar checkInDate, java.util.calendar checkOutDate, StringHolder roomType, StringHolder rateType) throws RemoteException; . . . }
Programming languages may further specify how faults are mapped to language constructs and their scopes, such as Exceptions, but they are not specific to RPC style.
Unlike WSDL 1.1 which defines a MIME binding for attachments support, WSDL 2.0 supports MIME attachments via the SOAP Message Transmission Optimization Mechanism (MTOM) [SOAP MTOM]. This section shows how MTOM may be engaged in the WSDL 2.0 SOAP binding extension.
We will modify the CheckAvailability
operation of
the GreatH Hotel Reservation Service (Example 2-1) to return not only the room
rate, but images of the room and the floorplan. This will involve
modifying the checkAvailabilityResponse
data structure
to include binary data representing these two images, indicated by
the xs:base64Binary
data type. Here is an example:
Example 4-10. XML Schema with Optimizable Elements
. . . <xs:element name="checkAvailabilityResponse"> <xs:sequence> <xs:element name="rate" type="xs:double"/> <xs:element name="photo" type="xmime:base64Binary" xmime:expectedContentType="image/jpeg image/png" /> <xs:element name="floorplan" xmime:expectedContentType="image/svg"> <xs:simpleContent> <xs:restriction base="xs:base64Binary"> <xs:attribute ref="xmime:contentType" fixed="image/svg" /> </xs:restriction> </xs:simpleContent> </xs:element> </xs:sequence> </xs:element> . . .
Note the use of the xmime:expectedContentType
and
xmime:contentType
attributes to declare the expected
media type of the encoded data and to allow the client to indicate
the type at runtime, respectively. These attributes are defined in
[Describing Media Content of Binary
Data in XML].
A checkAvailabilityResponse
message conforming to
this schema might look like this:
Example 4-11. Non-optimized SOAP Message with Embedded Binary Data
<soap:Envelope xmlns:soap='http://www.w3.org/2003/05/soap-envelope' xmlns:xmime='http://www.w3.org/2005/05/xmlmime'> <soap:Body> <g:checkAvailabilityResponse xmlns:g="http://greath.example.com/2004/schemas/resSvc"> <g:rate>129.95</g:rate> <g:photo xmime:contentType='image/png'>/aWKKapGGyQ=</g:photo> <g:floorplan xmime:contentType="image/svg">Faa7vROi2VQ=</g:floorplan> </g:checkAvailabilityResponse> </soap:Body> </soap:Envelope>
While this (non-optimized) message satisfies the schema definition, a service may choose to allow or require that the binary data be sent in an optimized format using the Message Transmission and Optimization Mechanism (MTOM). The use of this feature by the WSDL 2.0 SOAP binding extension is indicated as follows:
Example 4-12. Specifying MTOM in a WSDL 2.0 Binding
. . . <binding name="reservationSOAPBinding" interface="tns:reservationInterface" type="http://www.w3.org/2005/08/wsdl/soap" wsoap:protocol="http://www.w3.org/2003/05/soap/bindings/HTTP"> <operation ref="tns:opCheckAvailability" wsoap:mep="http://www.w3.org/2003/05/soap/mep/request-response"> <input name="checkAvailability" /> <output name="checkAvailabilityResponse"> <feature uri="http://www.w3.org/2004/08/soap/features/http-optimization" required="true" /> </output> </operation> . . . </binding> . . .
The HTTP Message Transmission Optimization (MTOM) feature is
engaged using the feature
element. Note that the
attribute required="true"
on the feature declaration
indicates that the message must be encoded using the HTTP
Optimization feature. If the attribute were
required="false"
(or this attribute were absent), it
would indicate that the use of MTOM is optional for this service:
the service accepts either MTOM-encoded messages, or the embedded
base64Binary data directly in the SOAP Body, and the client is free
to send either form of message.
The example above shows MTOM enabled for a specific message
within an operation. Placing the feature declaration as a child of
operation
would require (or enable if
required="false"
) MTOM support for all the messages in
that operation. Placing the feature declaration as a child of
binding
would require (or enable if
required="false"
) MTOM support for all the operations
in that interface.
This section covers various topics that may fall outside the scope of WSDL 2.0, but shall provide useful background and best practice guidances that may be useful when authoring a WSDL 2.0 document or implementing the WSDL 2.0 specification.
It is desirable for a message recipient to have the capability to uniquely identify a message in order to handle it correctly. The capability of identifying a message is typically used for dispatching purposes within an implementation of a web service. Therefore, WSDL authors are recommended to take disambiguating of messages that are defined in a description into consideration when they develop descriptions of their services.
The context that a Web service may be deployed plays an important role in choosing an appropriate way to disambiguate and identify messages. In a typical deployment, an endpoint address may host a single service that is described by a WSDL service element. In this case, when XSD is used, assigning unique qualified names of global element declarations as inputs within the interface that describes the service would be sufficient to disambiguate the messages that are received. However, when endpoint address hosts multiple services, in essence supports several WSDL descriptions, the desire to disambiguate messages should considered within the context of all the deployed services, not only within a single interface.
As explained in 2.4.4.1
Operation Attributes, when XSD is used as the type system,
a few special tokens can be used for the element
attributes. Uniquely identifying a message may become very
difficult when:
any of these input elements within an interface has a value of “#any”; or
more than one of these input elements (see below) has a value of “#none”; or
the qualified names of the global element declarations that are specified as input elements are NOT unique when considered together.
If any of the three cases above arise, then one of the following two alternatives can be used within the context of a single WSDL service by WSDL authors:
Feature. The service or the interface element contains a Feature element declaration, having a required attribute with a value of true. The feature unambiguously identifies the mechanism that a message sender is required to support in order to enable the message recipient to unambiguously determine the message received.
Extension. The interface element contains an extension element (i.e., an element that is not in the http://www.w3.org/2005/08/wsdl namespace), having a wsdl:required attribute with a value of "true". The extension element unambiguously identifies the mechanism that a message sender is required to support in order to enable the message recipient to unambiguously determine the message received.
In addition, WS-Addressing [WS-Addressing] specification already provides a disambiguation mechanism. It defines a required [action] property whose value is always present in a message delivery. The value of the action property can be used to disambiguate the message by the receiver and there is a well defined way to associate actions to messages in WS-Addressing specifications. Further, WS-Addressing also provides an appropriate default action value that identifies each message uniquely.
A WSDL 2.0 document describes a set of messages that a Web service may send and receive. In essence, it describes a language for interacting with that service. However it is possible for a Web service to exchange other messages beyond those described in a particular WSDL 2.0 document. Often this circumstance occurs following an evolution of the client and/or service, and thus an evolution of the interaction language.
How best to manage the evolution (versioning) of Web based systems is, at the time of writing, the subject of a wide ranging debate. However, there are three activities within the W3C that are directly relevant to versioning of Web services description:
The Technical Architecture Group (TAG) has published guidance on the extensibility and versioning of data formats in its Web Architecture document [Web Architecture]. There is also a more wide ranging draft finding on Versioning and Extensibility [W3C TAG Finding: Versioning]. Both of these works build upon the technical note on Web Architecture: Extensible Languages [WebArch: Extensible Languages].
The XML Schema Working Group is collecting a series of use cases for schema versioning as a part of the Schema 1.1 activity. See XML Schema Versioning Use Cases [XML Schema: Versioning Use-Cases].
The Semantic Web Best Practices and Deployments Working Group is examining how vocabularies may evolve. See [SW VocabManagementNote]
Editorial note: PaulD | 20050706 |
This section may be subject to change dependent upon the outcome of the WSDL Last Call Issue LC124, which discusses support compatible evolution of messages described using XML Schema 1.0. |
While incomplete, these activities all agree in one important respect: that versioning is difficult, but you should anticipate and plan for change.
The draft finding on Versioning and Extensibility details two key approaches to versioning:
compatible evolution; and
big bang.
In compatible evolution, designers are expected to limit changes to those that are either backward or forward compatible, or both:
The receiver behaves correctly if it receives a message in an older version of the interaction language.
The receiver behaves correctly if it receives a message in a newer version of the interaction language.
Since Web services and their clients both send and receive messages, these concepts can apply to both parties. However, since WSDL 2.0 is service-centric, we will focus on the case of service evolution.
There are three critical areas in which a service described in WSDL 2.0 my evolve:
The service now also supports additional binding. In compatible evolution, this should be a safe addition, given that adding a new binding should not impact any existing interactions using another transport.
An interface supports new operations. Again, in compatible evolution this is usually safe, given that adding an additional operation to an abstract interface should not impact any existing interactions.
The message bodies may include additional data. How the message
contents may change within a description depends to a large extent
upon the type system being used to describe the message contents.
RelaxNG [RELAX NG] has good
support for describing vocabularies that ignore unknown XML, as
does OWL/RDF. XML Schema 1.0 has limited support for extending the
description of a message via the xs:any
and
xs:anyAttribute
constructs. XML Schema 1.1 has been
chartered to provide "changes necessary to provide better support
for versioning of schemas", and it is anticipated that this may
include improved support for more "open content" and therefore
better support for compatible evolution of messages.
The protocol used to exchange messages may provide mechanisms for exchanging data outside of the message body. In the case of SOAP, the WSDL 2.0 binding provides the ability to describe application data to be exchanged as headers. The SOAP processing model has a very good extensibility model with unknown headers being ignored by a receiver by default. There is also a mechanism whereby headers which are required as a part of an incompatible change may be marked with a 'mustUnderstand' flag. Passing additional items as headers may be the only way to compatibly evolve messages with fixed bodies.
The big bang approach to versioning is the simplest to currently represent in WSDL 2.0. In this approach, any change to a WSDL 2.0 document implies a change to the document's namespace, a change to the interface implies a new interface namespace and a change to the message contents is communicated using a new message namespace. This approach has particular benefits where an agent may quickly tell if a service has changed by simply comparing the namespace value.
Compatible changes are far more easily managed than incompatible ones:
With a compatible change the service need only support the latest version of a service. A client may continue to use a service adjusting to new version of the interface description at a time of its choosing.
With an incompatible change, the client receives a new version of the interface description and is expected to adjust to the new interface before old interface is terminated. Either the service will need to continue to support both versions of the interface during the hand over period, or the service and the clients are coordinated to change at the same time. An alternative is for the client to continue until it encounters an error, at which point it uses the new version of the interface.
It is feasible to combine the "compatible evolution" and "big bang" approaches in a variety of different ways. For example, the namespace could be changed when message descriptions are changed, but the namespace could stay the same when new operations are added.
While the big bang approach is currently the easiest to implement in WSDL 2.0, it can lead to a large number of cloned interfaces that become difficult to manage, thus making the compatible approach preferable to many for widely distributed systems. In the end, the choice of a versioning strategy for Web services described in WSDL 2.0 is left as an exercise to the reader.
The following example demonstrates how content may be extended with additional content. The reservation service is changed to a newer version that can accept an optional number of guests parameter. The service provider wants existing clients to continue to be able to use the service. The author adds the element into the schema as an optional element.
Example 5-1. XML Schema with Optional Elements
<xs:complexType name="tCheckAvailability"> <xs:sequence> <xs:element name="checkInDate" type="xs:date"/> <xs:element name="checkOutDate" type="xs:date"/> <xs:element name="roomType" type="xs:string"/> <xs:element name="numberOfGuests" type="xs:integer" minOccurs="0"/> <xs:any namespace="##other" processContents="lax"/> </xs:sequence> </xs:complexType>
The author has the choice of keeping the same namespace or using a different namespace for the additional content and the existing content. In this scenario, it is a compatible change and the author decides to keep the same namespace. This allows existing clients to interact with a new service, and it allows newer clients to interact with older services.
Another option is to add the extension as a header block. This is accomplished by defining an element for the extension and adding a header element that references the element into the binding operation as child of the input.
Example 5-2. Additional optional elements added to a SOAP header
<xs:element name="NumberOfGuests" type="tNumberOfGuests"/> <xs:complexType name="tNumberOfGuests"> <xs:sequence> <xs:element name="numberOfGuests" type="xs:integer" minOccurs="0"/> </xs:sequence> </xs:complexType> <binding name="reservationSOAPBinding" interface="tns:reservationInterface" type="http://www.w3.org/2005/08/wsdl/soap" wsoap:protocol="http://www.w3.org/2003/05/soap/bindings/HTTP"> <operation ref="tns:opCheckAvailability"> <input> <wsoap:header element="tns:NumberOfGuests"/> </input> </operation> ... </binding>
It is also possible for the header to be marked with soap:mustUnderstand set to true. The HTTP Binding has similar functionality though without a mustUnderstand attribute.
This following example demonstrates an extension with additional content. The reservation service requires a number of guests parameter. The service provider wants existing clients to be unable to use the service. The author adds the element into the schema as a mandatory element.
Example 5-3. Additional Mandatory Elements in Content
<xs:complexType name="tCheckAvailabilityV2"> <xs:sequence> <xs:element name="checkInDate" type="xs:date"/> <xs:element name="checkOutDate" type="xs:date"/> <xs:element name="roomType" type="xs:string"/> <xs:element name="numberOfGuests" type="xs:integer"/> <xs:any namespace="##other" processContents="lax"/> </xs:sequence> </xs:complexType>
The author has the choice of keeping the same namespace or using a different namespace for the additional content and the existing content. In this scenario, it is an incompatible change and the author decides to use a new name but the same namespace. This type is then used in the interface operation, and then binding and service endpoints.
Section 2.4.2 Interface Inheritance shows another type of versioning or extension, where the reservationInterface extends the MessageLogInterface. By definition of interface inheritance, a client that understands just the MessageLogInterface will continue to work with the reservationInterface, that it is backwards compatible.
Often mandatory operations are added to an interface. The Hotel service decides to add an operation to the reservation service which is a confirmation. The Hotel service requires that all clients upgrade to the new interface to use the service. They have a variety of options for indicating that the old interface is deprecated.
By the definition of interface inheritance, they cannot use interface inheritance for defining the extension.
Example 5-4. Additional Mandatory Operation Added to the Interface
<interface name="reservationWithConfirmation" extends="cc:creditCardFaults"> ... <operation name="makeReservation"> <input messageLabel="In" element="ghns:makeReservation" /> <output messageLabel="Out" element="ghns:makeReservationResponse" /> <outfault ref="invalidDataFault" messageLabel="Out" /> <outfault ref="cc:cancelledCreditCard" messageLabel="Out" /> <outfault ref="cc:expiredCreditCard" messageLabel="Out" /> <outfault ref="cc:invalidCreditCardNumber" messageLabel="Out" /> <outfault ref="cc:invalidExpirationDate" messageLabel="Out" /> </operation> <operation name="confirmReservation"> <input messageLabel="In" element="ghns:makeReservationResponse" /> <output messageLabel="Out" element="ghns:confirmReservationResponse" /> <outfault ref="expiredReservation" messageLabel="Out" /> </operation> </interface>
This interface cannot be bound and deployed at the existing URI and indicate incompatibility, as the service will still accept the makeReservation request. Changing the name of the interface from reservation to reservationWithConfirmation or changing the name of the operation from makeReservation to makeReservationV2 does not affect the messages that are exchanged. Thus it can't be used as a mechanism for indicating incompatibility. To indicate incompatibility, a change must be made to something that appears in the message. For a SOAP over HTTP request, the list is roughly the URI, the SOAP Action HTTP Header, or the Message content.
To indicate incompatibility, the URI of the Hotel Endpoint can be changed and messages send to the old Endpoint return a Fault.
The SOAP Action can be set for the makeReservation request, and making it different than the earlier version should indicate incompatibility.
Example 5-5. Indicating Incompatibility by changing the SOAP Action
<binding name="reservationSOAPBinding" interface="tns:reservationInterface" type="http://www.w3.org/2005/08/wsdl/soap" wsoap:protocol="http://www.w3.org/2003/05/soap/bindings/HTTP"> <operation ref="tns:makeReservation" wsoap:action="tns:makeReservationV2"/> . . .
Note that this mechanism is applicable on a per-binding basis. The SOAP HTTP Binding provides for setting Action, but other bindings may not provide such a facility.
The namespace or name of the makeReservation element can be changed, and then the interface and bindings changed. To indicate incompatibility, requests using the old makeReservation Qname should probably return a fault. The new interface, with a changed makeReservation, is:
Example 5-6. Indicating incompatibility by changing the element content
<xs:element name="ghns2:makeReservation" type="ghns:tmakeReservation"/> <interface . . .> <operation name="makeReservation"> <input messageLabel="In" element="ghns2:makeReservation" /> </interface>
The binding and service endpoints require no change.
Finally, the service could also provide an interface for ghns:makeReservation that only returns a fault.
Hyperlinking is one of the defining characteristics of the Web. The ability to navigate from one Web page to another is extremely useful. It is therefore natural to apply this capability to Web services. This section describes service references and endpoint references, which are the Web service analogs of document hyperlinks.
A service reference is an element or attribute that contains one
or more endpoint references for a service. An endpoint reference is
an element or attribute that contains the address of a Web service
endpoint. If the interface or binding that the endpoint implements
is known at description time, it may be useful to add this
information to the WSDL 2.0 document that describes the Web
service. This is accomplished by using the
wsdlx:interface
or wsdlx:binding
attribute to annotate the XML Schema component that defines the
message.
One may wonder, from a Web architectural point of view, why anything more than a URI would be needed to reference a Web service. Indeed, a service reference does make use of one or more URIs to indicate the endpoint addresses of a service. However, it may also include additional metadata about that service, such as the WSDL 2.0 interface and binding that the service supports.
Service and endpoint references will be illustrated by expanding the GreatH example already discussed.
When designing a Web application it is natural to give each important concept a URI. In the GreatH hotel reservation system, the important concepts are reservations, so we begin our design by assigning a URI to each reservation. Since each reservation has a unique confirmation number, e.g OMX736, we create a URI for each reservation by appending the confirmation number to a base URI, e.g. http://greath.example.com/2004/reservation/OMX736. This URI will be the endpoint address for a Reservation Details Web service that can retrieve and update the state of a reservation. Example 5-7 shows the format of the reservation detail.
Example 5-7. Detail for Reservation OMX736
<?xml version="1.0" encoding="UTF-8"?> <reservationDetails xmlns="http://greath.example.com/2004/schemas/reservationDetails"> <confirmationNumber>OMX736</confirmationNumber> <checkInDate>2005-06-01</checkInDate> <checkOutDate>2005-06-03</checkOutDate> <roomType>single</roomType> <smoking>false</smoking> </reservationDetails>
The Reservation Details Web service provides operations for
retrieving and updating the detail for a reservation. Example 5-8 shows the description
for this Web service. Note that there is no service
element in this description since the set of reservations is
dynamic. Instead, the endpoints for the reservations will be
returned by querying the Reservation List Web service.
Example 5-8. The Reservation Details Web Service Description: reservationDetails.wsdl
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace="http://greath.example.com/2004/services/reservationDetails" xmlns:tns="http://greath.example.com/2004/services/reservationDetails" xmlns:wdetails="http://greath.example.com/2004/schemas/reservationDetails" xmlns:wsoap="http://www.w3.org/2005/08/wsdl/soap" xmlns:xs="http://www.w3.org/2001/XMLSchema"> <documentation> This document describes the GreatH Reservation Details Web services. Use these services to retrieve or update reservation details. Each reservation has its own service and endpoint. To obtain the service reference for a reservation, make a request to the GreatH Reservation List Web service. See reservationList.wsdl for a description of the Reservation List Web service. </documentation> <types> <xs:import namespace="http://greath.example.com/2004/schemas/reservationDetails" schemaLocation="reservationDetails.xsd" /> </types> <interface name="reservationDetailsInterface"> <operation name="retrieve" pattern="http://www.w3.org/2005/08/wsdl/in-out"> <input messageLabel="In" element="#none" /> <output messageLabel="Out" element="wdetails:reservationDetails" /> </operation> <operation name="update" pattern="http://www.w3.org/2005/08/wsdl/in-out"> <input messageLabel="In" element="wdetails:reservationDetails" /> <output messageLabel="Out" element="wdetails:reservationDetails" /> </operation> </interface> <binding name="reservationDetailsSOAPBinding" interface="tns:reservationDetailsInterface" type="http://www.w3.org/2005/08/wsdl/soap" wsoap:protocol="http://www.w3.org/2003/05/soap/bindings/HTTP"> <operation ref="tns:retrieve" wsoap:mep="http://www.w3.org/2003/05/soap/mep/request-response" /> <operation ref="tns:update" wsoap:mep="http://www.w3.org/2003/05/soap/mep/request-response" /> </binding> </description>
Example 5-9 shows the XML schema elements that are used in this Web service.
Example 5-9. The Reservation Details Web Service XML Schema: reservationDetails.xsd
<?xml version="1.0" encoding="UTF-8"?> <schema xmlns="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" targetNamespace="http://greath.example.com/2004/schemas/reservationDetails" xmlns:tns="http://greath.example.com/2004/schemas/reservationDetails" xmlns:wdetails="http://greath.example.com/2004/services/reservationDetails" xmlns:wsdli="http://www.w3.org/2005/08/wsdl-instance" xmlns:wsdlx="http://www.w3.org/2005/08/wsdl-extensions" wsdli:wsdlLocation="http://greath.example.com/2004/services/reservationDetails reservationDetails.wsdl"> <element name="confirmationNumber" type="string" /> <element name="checkInDate" type="date" /> <element name="checkOutDate" type="date" /> <element name="reservationDetails"> <complexType> <sequence> <element ref="tns:confirmationNumber" /> <element ref="tns:checkInDate" /> <element ref="tns:checkOutDate" /> <element name="roomType" type="string" /> <element name="smoking" type="boolean" /> </sequence> </complexType> </element> <simpleType name="reservationDetailsSOAPEndpointType" wsdlx:binding="wdetails:reservationDetailsSOAPBinding"> <restriction base="anyURI"/> </simpleType> <element name="reservationDetailsSOAPEndpoint" type="tns:reservationDetailsSOAPEndpointType" /> <element name="reservationDetailsService"> <annotation> <documentation> This element contains references to the Reservation Details Web Service endpoints for this reservation. </documentation> </annotation> <complexType> <sequence> <element name="soap" type="tns:reservationDetailsSOAPEndpointType"/> <element name="secure-soap" type="tns:reservationDetailsSOAPEndpointType"/> </sequence> </complexType> </element> </schema>
This XML schema contains the usual definitions for the elements
that appear in the messages of the Web service. For example, the
reservationDetails
element is used in the messages of
the retrieve
and update
operations. In
addition, the schema defines the simple type
reservationDetailsSOAPEndpointType
which is based on
xs:anyURI
and has the annotation wsdlx:binding =
"wdetails:reservationDetailsSOAPBinding"
which means that
the URI is the address of a Reservation Details Web service
endpoint that implements the
wdetails:reservationDetailsSOAPBinding
binding. Note
that the wsdli:wsdlLocation
attribute is used to
define the location of the WSDL 2.0 document that defines the
wdetails:reservationDetailsSOAPBinding
binding. This
annotated simple type is used to define the
reservationDetailsSOAPEndpoint
element which will be
used in the Reservation List service.
Since the set of reservations changes as reservations are made and cancelled, the Reservation Detail endpoints are not described in a fixed WSDL 2.0 document. Instead they are returned as endpoint references in response to requests made on a Reservation List Web service. The endpoint for the Reservation List service will be http://greath.example.com/2004/reservationList.
Example 5-10 shows the format of the response from the Reservation List service.
Example 5-10. Response from the Reservation List Web Service
<?xml version="1.0" encoding="UTF-8"?> <reservationList xmlns="http://greath.example.com/2004/schemas/reservationList" xmlns:details="http://greath.example.com/2004/schemas/reservationDetails"> <reservation> <details:confirmationNumber>HSG635</details:confirmationNumber> <details:checkInDate>2005-06-27</details:checkInDate> <details:checkOutDate>2005-06-28</details:checkOutDate> <details:reservationDetailsSOAPEndpoint> http://greath.example.com/2004/reservation/HSG635 </details:reservationDetailsSOAPEndpoint> </reservation> <reservation> <details:confirmationNumber>OMX736</details:confirmationNumber> <details:checkInDate>2005-06-01</details:checkInDate> <details:checkOutDate>2005-06-03</details:checkOutDate> <details:reservationDetailsSOAPEndpoint> http://greath.example.com/2004/reservation/OMX736 </details:reservationDetailsSOAPEndpoint> </reservation> <reservation> <details:confirmationNumber>WUH663</details:confirmationNumber> <details:checkInDate>2005-06-11</details:checkInDate> <details:checkOutDate>2005-06-15</details:checkOutDate> <details:reservationDetailsSOAPEndpoint> http://greath.example.com/2004/reservation/WUH663 </details:reservationDetailsSOAPEndpoint> </reservation> </reservationList>
Here, the
<details:reservationDetailsSOAPEndpoint>
elements contain endpoint references to the Reservation Details Web
services for the reservations HSG635, OMX736, and WUH663.
Example 5-11 shows the description of the Reservation List Web service. Note that it contains operations to retrieve the entire list and to query for a list of reservations by confirmation number, check-in date, and check-out date. In each case, the operation returns a list of reservations.
Example 5-11. The Reservation List Web Service Description: reservationList.wsdl
<?xml version="1.0" encoding="utf-8" ?> <description xmlns="http://www.w3.org/2005/08/wsdl" targetNamespace="http://greath.example.com/2004/services/reservationList" xmlns:tns="http://greath.example.com/2004/services/reservationList" xmlns:details="http://greath.example.com/2004/schemas/reservationDetails" xmlns:list="http://greath.example.com/2004/schemas/reservationList" xmlns:wsoap="http://www.w3.org/2005/08/wsdl/soap" xmlns:xs="http://www.w3.org/2001/XMLSchema"> <documentation> This document describes the GreatH Reservation List Web services. Use this service to retrieve lists of reservations based on a variety of search criteria. </documentation> <types> <xs:import namespace="http://greath.example.com/2004/schemas/reservationDetails" schemaLocation="reservationDetails.xsd" /> <xs:import namespace="http://greath.example.com/2004/schemas/reservationList" schemaLocation="reservationList.xsd" /> </types> <interface name="reservationListInterface"> <operation name="retrieve" pattern="http://www.w3.org/2005/08/wsdl/in-out"> <input messageLabel="In" element="#none" /> <output messageLabel="Out" element="list:reservationList" /> </operation> <operation name="retrieveByConfirmationNumber" pattern="http://www.w3.org/2005/08/wsdl/in-out"> <input messageLabel="In" element="details:confirmationNumber" /> <output messageLabel="Out" element="list:reservationList" /> </operation> <operation name="retrieveByCheckInDate" pattern="http://www.w3.org/2005/08/wsdl/in-out"> <input messageLabel="In" element="details:checkInDate" /> <output messageLabel="Out" element="list:reservationList" /> </operation> <operation name="retrieveByCheckOutDate" pattern="http://www.w3.org/2005/08/wsdl/in-out"> <input messageLabel="In" element="details:checkOutDate" /> <output messageLabel="Out" element="list:reservationList" /> </operation> </interface> <binding name="reservationListSOAPBinding" interface="tns:reservationListInterface" type="http://www.w3.org/2005/08/wsdl/soap" wsoap:protocol="http://www.w3.org/2003/05/soap/bindings/HTTP"> <operation ref="tns:retrieve" wsoap:mep="http://www.w3.org/2003/05/soap/mep/request-response" /> <operation ref="tns:retrieveByConfirmationNumber" wsoap:mep="http://www.w3.org/2003/05/soap/mep/request-response" /> <operation ref="tns:retrieveByCheckInDate" wsoap:mep="http://www.w3.org/2003/05/soap/mep/request-response" /> <operation ref="tns:retrieveByCheckOutDate" wsoap:mep="http://www.w3.org/2003/05/soap/mep/request-response" /> </binding> <service name="reservationListService" interface="tns:reservationListInterface"> <endpoint name="reservationListEndpoint" binding="tns:reservationListSOAPBinding" address="http://greath.example.com/2004/reservationList" /> </service> </description>
Example 5-12 shows the schema for the messages used in the Reservation List Web service.
Example 5-12. The Reservation List Schema: reservationList.xsd
<?xml version="1.0" encoding="UTF-8"?> <schema xmlns="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" targetNamespace="http://greath.example.com/2004/schemas/reservationList" xmlns:tns="http://greath.example.com/2004/schemas/reservationList" xmlns:details="http://greath.example.com/2004/schemas/reservationDetails" xmlns:wsdli="http://www.w3.org/2005/08/wsdl-instance"> <import namespace="http://www.w3.org/2005/08/wsdl-instance" /> <import namespace="http://greath.example.com/2004/schemas/reservationDetails" schemaLocation="reservationDetails.xsd" /> <element name="reservation"> <annotation> <documentation> A reservation contains the confirmation number, check-in and check-out dates, and a reference to a Reservation Details Web service. </documentation> </annotation> <complexType> <sequence> <element ref="details:confirmationNumber" /> <element ref="details:checkInDate" /> <element ref="details:checkOutDate" /> <element ref="details:reservationDetailsSOAPEndpoint" /> </sequence> </complexType> </element> <element name="reservationList"> <annotation> <documentation> A reservation list contains a sequence of zero or more reservations. </documentation> </annotation> <complexType> <sequence> <element ref="tns:reservation" minOccurs="0" maxOccurs="unbounded"> </element> </sequence> <attribute ref="wsdli:wsdlLocation" /> </complexType> </element> </schema>
In the preceding example, there was a single endpoint associated
with each Reservation Detail Web service. Suppose GreatH hotel
decided to provide a second, secure endpoint. In this case, service
references would be used to collect together the endpoints for each
reservation. The reservationDetails.xsd schema defines the
reservationDetailsService
element for this purpose. It
contains the nested elements soap
and
secure-soap
which are each of type
reservationDetailsSOAPEndpointType
and therefore
contain the address of an endpoint that implements the
wdetails:reservationDetailsSOAPBinding
binding.
Example 5-13 shows an example of a message that contains a service reference for reservation HGS635. Note that the service contains two endpoints, one of which provides secure access to the Reservation Details Web service.
Example 5-13. A Service Reference to the Reservation Details Web Service
<?xml version="1.0" encoding="UTF-8"?> <details:reservationDetailsService xmlns:details="http://greath.example.com/2004/schemas/reservationDetails" <details:soap> http://greath.example.com/2004/reservation/HSG635 </details:soap> <details:secure-soap> https://greath.example.com/2004/reservation/HSG635 </details:secure-soap> </details:reservationDetailsService>
This section presents a variation on the example in 5.3.1 The Reservation Details Web Service. It illustrates the use of HTTP transfer operations, GET and PUT, to retrieve and update GreatH hotel reservation details using the Representational State Transfer (REST) architectural style described by Roy Fielding [REST]. REST is a distillation of the architectural properties that Dr. Fielding identified as being vital to the Web's robustness and enormous scalability.
Since each reservation in our example will have a distinct URI, the Reservation Details Web service can be offered using HTTP GET and HTTP PUT. The binding would be modified as follows:
Example 5-14. Reservation Details Web Service Using HTTP Transfer
. . . <binding name="reservationDetailsHTTPBinding" type="http://www.w3.org/2005/08/wsdl/http" interface="tns:reservationDetailsInterface" > <operation ref="tns:retrieve" whttp:method="GET" /> <operation ref="tns:update" whttp:method="PUT" /> </binding> . . .
As with the example in 5.3.1 The Reservation Details Web Service, service and endpoint elements are not provided because the Reservation List Web service provides the endpoints.
This section continues the REST-style example of 5.3.3 Reservation Details Web Service Using HTTP Transfer by modifying the example of 5.3.2 The Reservation List Web Service to use HTTP GET.
The SOAP version of the Reservation List Web service above offers four different search operations. These can also be expressed as various parameters in a URI used by HTTP GET:
Example 5-15. Reservation List Web Service Using HTTP GET
. . . <binding name="reservationListHTTPBinding" type="http://www.w3.org/2005/08/wsdl/http" interface="tns:reservationListInterface" whttp:methodDefault="GET"> <operation ref="tns:retrieve" whttp:location="" /> <operation ref="tns:retrieveByConfirmationNumber" whttp:location="reservationList/ConfirmationNumber/{confirmationNumber/}" /> <operation ref="tns:retrieveByCheckInDate" whttp:location="reservationList/CheckInDate/{checkInDate/}" /> <operation ref="tns:retrieveByCheckOutDate" whttp:location="reservationList/CheckOutDate/{checkOutDate/}" /> </binding> . . . <service . . . > <endpoint name="reservationListEndpoint" binding="tns:reservationListHTTPBinding" address="http://greath.example.com/2004/reservationList" /> . . . </service> . . .
A retrieval by Confirmation Number URI would look like:
http://greath.example.com/2004/reservationList/ConfirmationNumber/HSG635
.
Alternatively, a single query type may be provided. This query type is a sequence of optional items. Any items in the sequence are serialized into the URI query string. A query sequence for any of ConfirmationNumber, checkInDate, checkOutDate would look like this:
Example 5-16. Query Sequence Using a Single Query Type
<element name="reservationQuery"> <annotation> <documentation> A reservation contains the confirmation number, check-in and check-out dates, and a reference to a Reservation Details Web service. </documentation> </annotation> <complexType> <sequence> <element ref="details:confirmationNumber" minOccurs="0"/> <element ref="details:checkInDate" minOccurs="0"/>/> <element ref="details:checkOutDate" minOccurs="0"/>/> </sequence> </sequence> </complexType> </element>
The WSDL 2.0 service that offers this type serialized as a parameter would look like this:
Example 5-17. WSDL 2.0 for Using a Single Query Type
. . . <interface name="reservationListInterfaceWithQuery"> <operation name="retrieveByReservationQuery" pattern="http://www.w3.org/2005/08/wsdl/in-out"> <input messageLabel="In" element="details:ReservationQuery" /> <output messageLabel="Out" element="list:reservationList" /> </operation> </interface> <binding name="reservationListQueryHTTPBinding" type="http://www.w3.org/2005/08/wsdl/http" interface="tns:reservationListInterfaceWithQuery" whttp:methodDefault="GET"> <operation ref="tns:retrieveByReservationQuery" whttp:location="reservationList/{ReservationQuery}}" /> </binding> . . . <endpoint name="reservationListEndpoint" binding="tns:reservationListHTTPBinding" address="http://greath.example.com/2004/reservationList" /> . . .
Various URIs would be:
http://greath.example.com/2004/reservationList/ReservationQuery?confirmationNumber=HSG635
http://greath.example.com/2004/reservationList/ReservationQuery?checkInDate=06-06-05
.
It is important to observe that using the URI serialization can result in very flexible queries and few operations. The previous discrete SOAP operations are collapsed into one "parameterized" operation.
Suppose a Web service wishes to expose two different interfaces:
a customer interface for its regular users, and a management
interface for its operator. A wsdl:service
specifies
only one wsdl:interface, so to achieve the desired effect the
service provider would somehow need to indicate a relationship
between two services. How can a service provider indicate a
relationship between services? Potential strategies include:
Declare both interfaces in the same wsdl:description element. Although WSDL 2.0 does not ascribe any particular significance to the fact that two wsdl:services are declared within the same wsdl:description, an application or toolkit could interpret this to mean that they are related in some way.
Declare both interfaces in the same wsdl:targetNamespace. Again, although WSDL 2.0 does not ascribe any particular significance to the fact that two wsdl:services are declared within the same wsdl:targetNamespace, an application or toolkit could interpret this to mean that they are related in some way.
Add an extension to WSDL 2.0 that links together all services that are related in this way. WSDL 2.0's open content model permits extension elements from other namespaces to appear in a WSDL 2.0 document.
Declare them in completely separate WSDL 2.0 documents, but
use the same endpoint address for both. I.e., declare a
wsdl:interface
and wsdl:service
for the
customer interface in one WSDL 2.0 document, and a
wsdl:interface
and wsdl:service
for the
management interface in a different WSDL 2.0 document, but use the
same endpoint address for both. (By "different WSDL 2.0 document"
we mean that both documents are never included or imported into the
same WSDL 2.0 descriptions component.) Although this approach may
work in some circumstances, it means that the same endpoint address
would be used for two different purposes, which is apt to cause
confusion or ambiguity. Furthermore, it is contrary to the Web
architectural principle that different URIs should be used to
identify different Web resources. (See the Web Architecture
[Web Architecture] section on
URI
collision.)
Use inheritance to combine the customer interface and management interface into a single, larger wsdl:interface. Of course, this reduces modularity and means that the management interface becomes exposed to the customers, which is not good.
Bear in mind that since the above strategies step outside of the WSDL 2.0 language specifies (and are therefore neither endorsed nor forbidden by the WSDL 2.0 specification) the WSDL 2.0 specification cannot define or standardize their semantics.
The desire to express relationships between services is also relevant to Web service versioning, discussed next.
Editorial note: KevinL | 20050429 |
This section might be removed - pending on the availability of the RDF mapping note. |
WSDL 2.0 is a language designed primarily with XML syntax. While XML is almost universally understood, it has several issues:
The ability to compose two XML documents into one depends on the languages of those documents. WSDL 2.0 does not permit Web service descriptions in different targetNamespaces to be merged into a single (physical) XML document.
The ability to extend XML languages with other XML languages depends on the languages again. WSDL 2.0 is extremely extensible, but the meaning of every single extension in WSDL 2.0 must be defined explicitly. Putting a piece of XMI (XML format for UML) into a WSDL 2.0 document may have different meaning from putting it into an XHTML document. Therefore XML-based extensibility has very high cost if many languages are involved.
Similarly, extending another XML language with pieces of WSDL 2.0, while possible, has to be defined for all the possible destinations. Putting a WSDL 2.0 interface element into a UDDI registry may mean a different thing from putting that interface element into an XHTML document.
Finally, the meaning of a portion of a WSDL 2.0 document is not defined by the WSDL 2.0 specification. While an interface element could form a single XML document, it is not a WSDL 2.0 document, so its meaning is largely undefined.
Applications that require such levels of composability (or decomposability) are increasingly being based on RDF [RDF], a graph-based knowledge representation language, and Web Ontology Language (OWL) [OWL], which can be thought of as an advanced schema language for RDF. Effectively, a WSDL 2.0 document represented in RDF can be more easily extended with arbitrary RDF assertions and the WSDL 2.0 information can be more easily associated with arbitrary other knowledge.
WSDL 2.0: Mapping to RDF @@bibref@@ describes how WSDL 2.0 constructs can be expressed in RDF using classes of resources (described with an ontology expressed in OWL) and assertions over individual resources. As RDF represents knowledge using resources and relationships between them, we need to turn WSDL 2.0 concepts into this model. This is done as follows.
First, all components in WSDL 2.0 (like Interfaces, Operations, Bindings, Services, Endpoints etc., including extensions) are turned into resources identified with the appropriate URIs created according to @@Appendix C@@.
Further, things are represented as resources:
Element declarations gathered from XML Schema (or similarly, other components from other type systems)
Message content models
Message exchange patterns (the URI identifying the MEP is the URI of the resource)
Operation styles (similarly to MEPs, the URI of an operation style is the URI of the resource)
All the resources above are given the appropriate types using rdf:type statements (an interface will belong to the class Interface and an operation within an interface will belong to the class InterfaceOperation, for example).
All relationships in WSDL 2.0 (like an Operation belonging to an
Interface and having a given operation style) are turned into RDF
statements using appropriate properties, such as
operation
and operationStyle
.
It is a common misperception to equate either the target
namespace of an XML Schema or the value of the xmlns
attribute in XML instances with the location of the corresponding
schema. Even though namespaces are URIs, and URIs may be locations,
and it may be possible to retrieve a schema from such a location,
this does not mean that the retrieved schema is the only
schema that is associated with that namespace. There can be
multiple schemas associated with a particular namespace, and it is
up to a processor of XML to determine which one to use in a
particular processing context. The WSDL 2.0 specification provides
the processing context here via the import
mechanism,
which is based on XML Schema's term for the similar concept.
Throughout this document there are fully qualified URIs used in WSDL 2.0 and XSD examples. In some cases, fully qualified URIs were used simply to illustrate the referencing concepts. However, the use of relative URIs is allowed and warranted in many cases. For information on processing relative URIs, see RFC2396.
In general, when a WSDL 2.0 document is published for use by others, it should only contain URIs that are globally unique. This is usually done by allocating them under a domain name that is controlled by the issuer. For example, the W3C allocates namespace URIs under its base domain name, w3.org.
However, it is sometimes desirable to make up a temporary URI for an entity, for use during development, but not make the URI globally unique for all time and have it "mean" that version of the entity (schema, WSDL 2.0 document, etc.). Reserved Top Level DNS Names [IETF RFC 2606] specifies some URI base names that are reserved for use for this type of behavior. For example, the base URI "http://example.org/" can be used to construct a temporary URI without any unique association to an entity. This means that two people or programs could choose to simultaneously use the temporary URI " http://example.org/userSchema" for two completely different schemas. As long as the scope of use of these URIs does not intersect, then they would be unique enough. However, it is not recommended that " http://example.org/" be used as a base for stable, fixed entities.
This document is the work of the W3C Web Service Description Working Group.
Members of the Working Group are (at the time of writing, and by alphabetical order): Allen Brookes (Rogue Wave Softwave), Dave Chappell (Sonic Software), Helen Chen (Agfa-Gevaert N. V.), Roberto Chinnici (Sun Microsystems), Kendall Clark (University of Maryland), Ugo Corda (SeeBeyond), Glen Daniels (Sonic Software), Paul Downey (British Telecommunications), Youenn Fablet (Canon), Hugo Haas (W3C), Tom Jordahl (Macromedia), Anish Karmarkar (Oracle Corporation), Jacek Kopecky (DERI Innsbruck at the Leopold-Franzens-Universität Innsbruck, Austria), Amelia Lewis (TIBCO Software, Inc.), Michael Liddy (Education.au Ltd.), Kevin Canyang Liu (SAP AG), Jonathan Marsh (Microsoft Corporation), Josephine Micallef (SAIC - Telcordia Technologies), Jeff Mischkinsky (Oracle Corporation), Dale Moberg (Cyclone Commerce), Jean-Jacques Moreau (Canon), Mark Nottingham (BEA Systems, Inc.), David Orchard (BEA Systems, Inc.), Bijan Parsia (University of Maryland), Tony Rogers (Computer Associates), Arthur Ryman (IBM), Adi Sakala (IONA Technologies), Asir Vedamuthu (Microsoft Corporation), Sanjiva Weerawarana (Independent), Ümit Yalçınalp (SAP AG).
Previous members were: Lily Liu (webMethods, Inc.), Don Wright (Lexmark), Joyce Yang (Oracle Corporation), Daniel Schutzer (Citigroup), Dave Solo (Citigroup), Stefano Pogliani (Sun Microsystems), William Stumbo (Xerox), Stephen White (SeeBeyond), Barbara Zengler (DaimlerChrysler Research and Technology), Tim Finin (University of Maryland), Laurent De Teneuille (L'Echangeur), Johan Pauhlsson (L'Echangeur), Mark Jones (AT&T), Steve Lind (AT&T), Sandra Swearingen (U.S. Department of Defense, U.S. Air Force), Philippe Le Hégaret (W3C), Jim Hendler (University of Maryland), Dietmar Gaertner (Software AG), Michael Champion (Software AG), Don Mullen (TIBCO Software, Inc.), Steve Graham (Global Grid Forum), Steve Tuecke (Global Grid Forum), Michael Mahan (Nokia), Bryan Thompson (Hicks & Associates), Ingo Melzer (DaimlerChrysler Research and Technology), Sandeep Kumar (Cisco Systems), Alan Davies (SeeBeyond), Jacek Kopecky (Systinet), Mike Ballantyne (Electronic Data Systems), Mike Davoren (W. W. Grainger), Dan Kulp (IONA Technologies), Mike McHugh (W. W. Grainger), Michael Mealling (Verisign), Waqar Sadiq (Electronic Data Systems), Yaron Goland (BEA Systems, Inc.), Ümit Yalçınalp (Oracle Corporation), Peter Madziak (Agfa-Gevaert N. V.), Jeffrey Schlimmer (Microsoft Corporation), Hao He (The Thomson Corporation), Erik Ackerman (Lexmark), Jerry Thrasher (Lexmark), Prasad Yendluri (webMethods, Inc.), William Vambenepe (Hewlett-Packard Company), David Booth (W3C), Sanjiva Weerawarana (IBM), Charlton Barreto (webMethods, Inc.), Asir Vedamuthu (webMethods, Inc.), Igor Sedukhin (Computer Associates), Martin Gudgin (Microsoft Corporation).
The people who have contributed to discussions on www-ws-desc@w3.org are also gratefully acknowledged.