The World Wide Web is a network-spanning information space of
resources interconnected by links. This information space is the
basis of, and is shared by, a number of information systems. Within
each of these systems, agents (people and software) retrieve,
create, display, analyze, and reason about resources.
Web architecture includes the definition of the information
space in terms of identification and representation of its
contents, and of the protocols that support the interaction of
agents in an information system making use of the space. Web
architecture is influenced by social requirements and software
engineering principles, leading to design choices that constrain
the behavior of systems using the Web in order to achieve desired
properties of the shared information space: efficiency,
scalability, and the potential for indefinite growth across
languages, cultures, and media. This document reflects the three
bases of Web architecture: identification, interaction, and
representation.
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 the 9 December 2003 Last Call Working Draft of
"Architecture of the World Wide Web, First Edition." The Last Call
review period ends 5 March 2004, at 23:59 ET. Please send Last Call
review comments on this document before that date to the public W3C
TAG mailing list public-webarch-comments@w3.org (archive). Last Call Working Draft status is
described in section 7.4.2 of the W3C Process Document.
This document has been developed by W3C's Technical
Architecture Group (TAG) (charter).
The TAG decided unanimously to advance to Last Call at their 4 Dec
2003 teleconference (minutes). A complete list of changes
to this document since the first public Working Draft is available
on the Web.
The TAG
charter describes a process for issue resolution by the TAG. In
accordance with those provisions, the TAG maintains a running
issues list. The First Edition of "Architecture of the World
Wide Web" does not address every issue that the TAG has accepted
since it began work in January 2002. The TAG has selected a subset
of issues that the First Edition does address to the satisfaction
of the TAG; those issues are identified in the TAG's issues list.
The TAG intends to address the remaining (and future) issues after
publication of the First Edition as a Recommendation.
This document uses the concepts and terms regarding URIs as
defined in draft-fielding-uri-rfc2396bis-03, preferring them to
those defined in RFC 2396. The IETF Internet Draft draft-fielding-uri-rfc2396bis-03 is expected to
obsolete RFC 2396, which is the current URI standard. The
TAG is tracking the evolution of
draft-fielding-uri-rfc2396bis-03.
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." The latest information regarding patent disclosures
related to this document is available on the Web.
The World Wide Web (WWW, or simply Web) is an
information space in which the items of interest, referred to as resources, are
identified by global identifiers called Uniform Resource
Identifiers (URIs).
A travel
scenario is used throughout this document to illustrate typical
behavior of Web
agents — people or software (on behalf of a person,
entity, or process) acting on this information space. Software
agents include servers, proxies, spiders, browsers, and multimedia
players.
Story
While planning a trip to Mexico, Nadia reads "Oaxaca weather
information: 'http://weather.example.com/oaxaca'" in a glossy
travel magazine. Nadia has enough experience with the Web to
recognize that "http://weather.example.com/oaxaca" is a URI. Given
the context in which the URI appears, she expects that it allows
her to access weather information. When Nadia enters the URI into
her browser:
- The browser performs an information retrieval action in
accordance with its configured behavior for resources identified
via the "http" URI scheme.
- The authority responsible for "weather.example.com" provides
information in a response to the retrieval request.
- The browser displays the retrieved information, which includes
hypertext links to other
information. Nadia can follow these hypertext links to retrieve
additional information.
This scenario illustrates the three architectural bases of the
Web that are discussed in this document:
- Identification. Each
resource is identified by
a URI. In this travel scenario, the resource is about the weather
in Oaxaca and the URI is "http://weather.example.com/oaxaca".
- Interaction. Protocols
define the syntax and semantics of messages exchanged by agents
over a network. Web agents communicate information about the state
of a resource through the exchange of representations. In the travel scenario, Nadia (by
clicking on a hypertext link)
tells her browser to request a representation of the resource
identified by the URI in the hypertext link. The browser sends an
HTTP GET request to the server at "weather.example.com". The server
responds with a representation that includes XHTML data and the
Internet Media Type "application/xml+xhtml".
- Formats. Representations
are built from a non-exclusive set of data formats, used separately
or in combination (including XHTML, CSS, PNG, XLink, RDF/XML, SVG,
and SMIL animation). In this scenario, the representation data is
XHTML. While interpreting the XHTML representation data, the
browser retrieves and displays weather maps identified by URIs
within the XHTML.
The following illustration shows the relationship between
identifier, resource, and representation.
This document describes the properties we desire of the Web and
the design choices that have been made to achieve them.
This document promotes re-use of existing standards when
suitable, and gives guidance on how to innovate in a manner
consistent with the Web architecture.
The terms MUST, MUST NOT, SHOULD, SHOULD NOT, and MAY are used
in the good practice notes, principles, etc. in accordance with RFC
2119 [RFC2119]. However, this
document does not include conformance provisions for at least these
reasons:
- Conforming software is expected to be so diverse that it would
not be useful to be able to refer to the class of conforming
software agents.
- Some of the good practice notes concern people; specifications
generally define conformance for software, not people.
- The addition a conformance section is not likely to increase
the utility of the document.
This document is intended to inform discussions about issues of
Web architecture. The intended audience for this document
includes:
- Participants in W3C Activities; i.e., developers of Web
technologies and specifications in W3C
- Other groups and individuals developing technologies to be
integrated into the Web
- Implementers of W3C specifications
- Web content authors and publishers
Readers will benefit from familiarity with the Requests for
Comments (RFC) series from the IETF, some of which
define pieces of the architecture discussed in this document.
This document presents the general architecture of the Web.
Other groups inside and outside W3C also address specialized
aspects of Web architecture, including accessibility,
internationalization, device independence, and Web Services. The
section on Architectural
Specifications includes references.
This document strikes a balance between brevity and precision
while including illustrative examples. TAG
findings are informational documents that complement the
current document by providing more detail about selected topics.
This document includes some important material from the findings.
Since the findings evolve independently, this document also
includes references to approved TAG findings. For other TAG issues
covered by this document but without an approved finding,
references are to entries in the TAG issues
list.
The important points of this document are categorized as
follows:
- Constraint
- An architectural constraint is a restriction in behavior or
interaction within the system. Constraints may be imposed for
technical, policy, or other reasons.
- Design
Choice
- In the design of the Web, some design choices, like the names
of the <p> and <li> elements in HTML, or the choice of
the colon character in URIs, are somewhat arbitrary; if
<par>, <elt>, or
*
had been chosen
instead, the large-scale result would, most likely, have been the
same. Other design choices are more fundamental; these are the
focus of this document.
- Good
practice
- Good practice — by software developers, content authors,
site managers, users, and specification writers — increases
the value of the Web.
- Principle
- An architectural principle is a fundamental rule that applies
to a large number of situations and variables. Architectural
principles include "separation of concerns", "generic interface",
"self-descriptive syntax," "visible semantics," "network effect"
(Metcalfe's Law), and Amdahl's Law: "The speed of a system is
determined by its slowest component."
- Property
- Architectural properties include both the functional properties
achieved by the system, such as accessibility and global scope, and
non-functional properties, such as relative ease of evolution,
re-usability of components, efficiency, and dynamic
extensibility.
This categorization is derived from Roy Fielding's work on
"Representational State Transfer" [REST]. Authors of protocol specifications in
particular should invest time in understanding the REST model and
consider the role to which of its principles could guide their
design: statelessness, clear assignment of roles to parties,
uniform address space, and a limited, uniform set of verbs.
A number of general architecture principles apply to across all
three bases of Web architecture.
Identification, interaction, and representation are orthogonal
(or, "independent", or "loosely coupled") concepts: an identifier
can be assigned without knowing what representations are available,
agents can interact with any identifier, and representations can
change without regard to the identifiers or interactions that may
dereference them.
Orthogonality in specifications facilitates a flexible design
that can evolve over time. The fact, for example, that the an image
can be identified using a URI without needing any information about
the representation of that image allowed PNG and SVG to evolve
independent of the specifications that define image elements.
Orthogonal abstractions deserve orthogonal specifications.
Specifications should clearly indicate those features that
simultaneously access information from otherwise orthogonal
abstractions. For example a specification should draw attention to
a feature that requires information from both the header and the
body of a message.
Although the HTTP, HTML, and URI specifications are orthogonal
for the most part, they are not completely orthogonal. Experience
demonstrates that where they are not orthogonal, problems have
arisen:
- The HTML specification includes a protocol extension of sorts:
it specifies how a user agent sends HTML form data to a server (as
a URI query string). The design works reasonably well, although
there are limitations related to internationalization (see the TAG
finding "URIs, Addressability, and the use of HTTP GET and
POST") and the query string design impinges on the
server design. Developers (for example, of [CGI] applications) might have an easier time
finding the specification if it were published separately and then
cited from the HTTP, URI, and HTML specifications.
- The HTML specification allows content providers to instruct
HTTP servers to build response headers from
META
element instances. This is an abstraction violation; the developer
community deserves to be able to find all HTTP headers from the
HTTP specification (including any associated extension registries
and specification updates per IETF process). Perhaps as a result,
this feature of the HTML specification is not widely deployed.
Furthermore, this design has led to confusion in user agent
development. The HTML specification states that META
in conjunction with http-equiv
is intended for HTTP
servers, but many HTML user agents interpret
http-equiv='refresh'
as a client-side
instruction.
- Some authors use the
META
/http-equiv
approach to declare the character encoding scheme of an HTML
document. By design, this is a hint that an HTTP server should emit
a corresponding "Content-Type" header field. In practice, the use
of the hint in servers is not widely deployed. Furthermore, many
user agents use this information to override the "Content-Type"
header sent by the server. This works against the principle of authoritative
representation metadata.
The information in the Web and the technologies used to
represent that information change over time. Some examples of
successful technologies designed to allow change while minimizing
disruption include:
- the fact that URI schemes are independently specified,
- the use of an open set of Internet media types in mail and HTTP
to specify document interpretation,
- the separation of the generic XML grammar and the open set of
XML namespaces for element and attribute names,
- Extensibility models in Cascading Style Sheets (CSS), XSLT 1.0,
and SOAP
- user agent plug-ins
The following applies to languages, in particular the
specifications of data formats, of message formats, and URIs.
Note: This document does not distinguish in any
formal way the terms "format" and "language." Context has
determined which term is used.
Language
subset: one language is a subset (or, "profile") of a
second language if any document in the first language is also a
valid document in the second language and has the same
interpretation in the second language.
Language extension: one
language is an extension of a second language if the second is a language subset of the
first (thus, the extension is a superset). Clearly, creating an
extension language is better for interoperability than creating an
incompatible language.
Ideally, many instances of a superset language can be safely and
usefully processed as though they were in the subset language.
Languages that exhibit this property are said to be "extensible."
Language designers can facilitate extensibility by defining how
implementations must handle unknown extensions -- for example, that
they be ignored (in some way) or should be considered errors.
For example, from early on in the Web, HTML agents followed the
convention of ignoring unknown elements. This choice left room for
innovation (i.e., non-standard elements) and encouraged the
deployment of HTML. However, interoperability problems arose as
well. In this type of environment, there is an inevitable tension
between interoperability in the short term and the desire for
extensibility. Experience shows that designs that strike the right
balance between allowing change and preserving interoperability are
more likely to thrive and are less likely to disrupt the Web
community. Orthogonal
specifications help reduce the risk of disruption.
For further discussion, see the section on versioning and extensibility.
See also TAG issue xmlProfiles-29.
Errors occur in networked information systems. The manner in
which they are dealt with depends on application context. A user agent
acts on behalf of the user and therefore is expected to help the
user understand the nature of errors, and possibly overcome them.
User agents that correct errors without the consent of the user are
not acting on the user's behalf.
Principle: Error recovery
Silent recovery from error is harmful.
To promote interoperability, specifications should set
expectations about behavior in the face of known error conditions.
Experience has led to the following observations about
error-handling approaches.
- Protocol designers should provide enough information about the
error condition so that a an agent can address the error condition.
For instance, an HTTP 404 message ("resource not found") is useful
because it allows user agents to present relevant information to
users, enabling them to contact the author of the representation
that included the (broken) link.
- Experience with the cost of building a user agent to handle the
diverse forms of ill-formed HTML content convinced the authors of
the XML specification to require that agents fail deterministically
upon encountering ill-formed content. Because users are unlikely to
tolerate such failures, this design choice has pressured all
parties into respecting XML's constraints, to the benefit of
all.
- An agent that encounters unrecognized content may handle it in
a number of ways, including as an error; see also the section on extensibility and
versioning.
- Error behavior that is appropriate for a person may not be
appropriate for software. People are capable of exercising
judgement in ways that software applications generally cannot. An
informal error response may suffice for a person but not for a
processor.
See the TAG issues contentTypeOverride-24 and errorHandling-20.
The Web follows Internet tradition in that its important
interfaces are defined in terms of protocols, by specifying the
syntax, semantics, and sequence of the messages interchanged. The
technology shared among Web agents lasts longer than the agents
themselves.
It is common for programmers working with the Web to write code
that generates and parses these messages directly. It is less
common, but not unusual, for end users to have direct exposure to
these messages. This leads to the well-known "view source" effect,
whereby users gain expertise in the workings of the systems by
direct exposure to the underlying protocols.
Parties who wish to communicate must agree upon a shared set of
identifiers and on their meanings. The ability to use common
identifiers across communities motivates global identifiers in Web
architecture. Thus, Uniform
Resource Identifiers ([URI], currently being revised) which are global
identifiers in the context of the Web, are central to Web
architecture.
Constraint: Identify with URIs
The identification mechanism for the Web is
the URI.
A URI must be assigned to a resource in order for agents to be
able to refer to the resource. It follows that a resource should be
assigned a URI if a third party might reasonably want to link to
it, make or refute assertions about it, retrieve or cache a
representation of it, include all or part of it by reference into
another representation, annotate it, or perform other operations on
it.
When a representation uses a URI (instead of a local
identifier) as an identifier, then it gains great power from the
vastness of the choice of resources to which it can refer. The
phrase the "network effect" describes the fact that the usefulness
of the technology is dependent on the size of the deployed Web.
Resources exist before URIs; a resource may be identified by
zero URIs. However, there are many benefits to assigning a URI to a
resource, including linking, bookmarking, caching, and indexing by
search engines. Designers should expect that it will prove useful
to be able to share a URI across applications, even if that utility
is not initially evident.
The scope of a URI is global; the resource identified by a URI
does not depend on the context in which the URI appears (see also
the section about URIs in
other roles). Of course, what an agent does with a URI may
vary. The TAG finding "URIs, Addressability, and the use of HTTP GET and
POST" discusses additional benefits and considerations
of URI addressability.
Principle: URI assignment
A resource owner SHOULD assign a URI to each
resource that others will expect to refer to.
This principle dates back at least as far as Douglas Engelbart's
seminal work on open hypertext systems; see section Every Object Addressable in [Eng90].
Web architecture allows resource owners to assign more than one
URI to a resource.
Constraint: URI
uniqueness
Web architecture does not constrain a Web
resource to be identified by a single URI.
Thus, URIs that are not identical (character for character) do
not necessarily refer to different resources. The most
straightforward way of establishing that two parties are referring
to the same Web resource is to compare, as character strings, the
URIs they are using. URI equivalence is discussed in section 6 of
[URI]
Good practice: URI aliases
Resource owners should not create arbitrarily
different URIs for the same resource.
URI producers should be conservative about the number of
different URIs they produce for the same resource. For example, the
parties responsible for weather.example.com should not use both
"http://weather.example.com/Oaxaca" and
"http://weather.example.com/oaxaca" to refer to the same resource;
agents will not detect the equivalence relationship by following
specifications. On the other hand, there may be good reasons for
creating similar-looking URIs. For instance, one might reasonably
create URIs that begin with "http://www.example.com/tempo" and
"http://www.example.com/tiempo" to provide access to resources by
users who speak Italian and Spanish.
Likewise, URI consumers should ensure URI consistency. For
instance, when transcribing a URI, agents should not gratuitously
escape characters. The term "character" refers to URI characters as
defined in section 2 of [URI].
Good practice: Consistent URI usage
If a URI has been assigned to a resource,
agents SHOULD refer to the resource using the same URI, character
for character.
Applications may apply rules beyond basic string comparison that
are licensed by specifications to reduce the risk of false
negatives and positives. For example, for "http" URIs, the
authority component is case-insensitive. Agents that reach
conclusions based on comparisons that are not licensed by relevant
specifications take responsibility for any problems that result.
Agents should not assume, for example, that
"http://weather.example.com/Oaxaca" and
"http://weather.example.com/oaxaca" identify the same resource,
since none of the specifications involved states that the path part
of an "http" URI is case-insensitive.
See section 6 [URI] for more
information about comparing URIs and reducing the risk of false
negatives and positives. See the section on future directions for
approaches other than string comparison that may allow different
parties to assert that
two URIs identify the same resource.
The requirement for URIs to be unambiguous demands that different agents do not
assign the same URI to different resources. URI scheme specifications assure this using a
variety of techniques, including:
- Hierarchical delegation of authority. This approach,
exemplified by the "http" and "mailto" schemes, allows the
assignment of a part of URI space to one party, reassignment of a
piece of that space to another, and so forth.
- Random numbers. The generation of a fairly large random number,
used in the "uuid" scheme, reduces the risk of ambiguity to a
calculated small risk.
- Checksums. The generation of a URI as a checksum based on a
data object has similar properties to the random number approach.
This is the approach taken by the "md5" scheme.
- Combination of approaches. The "mid" and "cid" schemes combine
some of the above approaches.
The approach taken for the "http" URI scheme follows the pattern
whereby the Internet community delegates authority, via the IANA
URI scheme registry [IANASchemes] and the DNS, over a set of URIs with
a common prefix to one particular owner. One consequence of this
approach is the Web's heavy reliance on the central DNS
registry.
Whatever the techniques used, except for the checksum case, the
agent has a unique relationship with the URI, called URI
ownership. The phrase "authority responsible for a URI"
is synonymous with "URI owner" in this document.
The social implications of URI ownership are not discussed here.
However, the success or failure of these different approaches
depends on the extent to which there is consensus in the Internet
community on abiding by the defining specifications. The concept of
URI ownership is especially visible in the case of the HTTP
protocol, which enables the URI owner to serve authoritative
representations of a resource. In this case, the HTTP origin
server (defined in [RFC2616])
is the agent acting on behalf of the URI owner.
Just as a shared vocabulary has tangible value, the ambiguous
use of terms imposes a cost in communication. URI
ambiguity refers to the use of the same URI to refer to
more than one distinct resource.
Good practice: URI
ambiguity
Avoid URI ambiguity.
URI ambiguity should not be confused with ambiguity in natural
language. The English statement "'http://www.example.com/moby'
identifies 'Moby Dick'" is ambiguous because one could understand
the phrase "Moby Dick" to refer to distinct resources: a particular
printing of this work, or the work itself in an abstract sense, or
the fictional white whale, or a particular copy of the book on the
shelves of a library (via the Web interface of the library's online
catalog), or the record in the library's electronic catalog which
contains the metadata about the work, or the Gutenberg project's online version.
In Web architecture, URIs identify resources. Outside the bounds
of Web architecture specifications, URIs can be useful for other
purposes, for example, as database keys. For instance, the
organizers of a conference might use "mailto:nadia@example.com" to
refer to Nadia. While this usage is not licensed by Web
architecture specifications, in the context of the conference, all
parties may agree to that local policy and understand one another.
Certain properties of URIs, such as their potential for uniqueness,
make them appealing as general-purpose identifiers. In the Web
architecture, "mailto:nadia@example.com" identifies an Internet
mailbox; that is what is licensed by the "mailto" URI scheme
specification. The fact that the URI serves other purposes in
non-Web contexts does not lead to URI ambiguity. URI ambiguity
arises a URI is used to identify two different Web
resources.
In the URI "http://weather.example.com/", the "http" that
appears before the colon (":") names a URI scheme. Each URI scheme
has a normative specification that explains how identifiers are
assigned within that scheme. The URI syntax is thus a federated and
extensible naming mechanism wherein each scheme's specification may
further restrict the syntax and semantics of identifiers within
that scheme.
Examples of URIs from various schemes include:
- mailto:joe@example.org
- ftp://example.org/aDirectory/aFile
- news:comp.infosystems.www
- tel:+1-816-555-1212
- ldap://ldap.example.org/c=GB?objectClass?one
- urn:oasis:names:tc:entity:xmlns:xml:catalog
While the Web architecture allows the definition of new schemes,
introducing a new scheme is costly. Many aspects of URI processing
are scheme-dependent, and a significant amount of deployed software
already processes URIs of well-known schemes. Introducing a new URI
scheme requires the development and deployment not only of client
software to handle the scheme, but also of ancillary agents such as
gateways, proxies, and caches. See [RFC2718] for other considerations and costs
related to URI scheme design.
Because of these costs, if a URI scheme exists that meets the
needs of an application, designers should use it rather than invent
one.
Good practice: New URI schemes
Authors of specifications SHOULD NOT introduce
a new URI scheme when an existing scheme provides the desired
properties of identifiers and their relation to resources.
Consider our travel
scenario: should the authority providing information about the
weather in Oaxaca register a new URI scheme "weather" for the
identification of resources related to the weather? They might then
publish URIs such as "weather://travel.example.com/oaxaca". When a
software agent dereferences such a URI, if what really happens is
that HTTP GET is invoked to retrieve a representation of the
resource, then an "http" URI would have sufficed.
If the motivation behind registering a new scheme is to allow a
software agent to launch a particular application when retrieving a
representation, such dispatching can be accomplished at lower
expense via Internet Media Types. When designing a new data format,
the appropriate mechanism to promote its deployment on the Web is
the Internet Media Type.
Note that even if an agent cannot process representation data in
an unknown format, it can at least retrieve it. The data may
contain enough information to allow a user or user agent to make
some use of it. When an agent does not handle a new URI scheme, it
cannot retrieve a representation.
The Internet Assigned Numbers Authority
(IANA) maintains a registry [IANASchemes] of mappings
between URI scheme names and scheme specifications. For instance,
the IANA registry indicates that the "http" scheme is defined in
[RFC2616]. The process for
registering a new URI scheme is defined in [RFC2717].
The use of unregistered URI schemes is discouraged for a number
of reasons:
- There is no generally accepted way to locate the scheme
specification.
- Someone else may be using the scheme for other purposes.
- One should not expect that general-purpose software will do
anything useful with URIs of this scheme; the network effect is
lost.
Note: Some URI scheme specifications (such as
the "ftp" URI scheme specification) use the term "designate" where
the current document uses "identify."
TAG issue siteData-36 is about expropriation of naming
authority.
It is tempting to guess the nature of a resource by inspection
of a URI that identifies it. However, the Web is designed so that
agents communicate resource state through representations, not
identifiers. In general, one cannot determine the Internet Media
Type of representations of a resource by inspecting a URI for that
resource. For example, the ".html" at the end of
"http://example.com/page.html" provides no guarantee that
representations of the identified resource will be served with the
Internet Media Type "text/html". The HTTP protocol does not
constrain the Internet Media Type based on the path component of
the URI; the server is free to return a representation in PNG or
any other data format for that URI.
Resource state may evolve over time. Requiring resource owners
to change URIs to reflect resource state would lead to a
significant number of broken links. For robustness, Web
architecture promotes independence between an identifier and the
identified resource.
Good practice: URI
opacity
Agents making use of URIs MUST NOT attempt to
infer properties of the referenced resource except as licensed by
relevant specifications.
The example URI used in the travel scenario
("http://weather.example.com/oaxaca") suggests that the identified
resource has something to do with the weather in Oaxaca. A site
reporting the weather in Oaxaca could just as easily be identified
by the URI "http://vjc.example.com/315". And the URI
"http://weather.example.com/vancouver" might identify the resource
"my photo album."
On the other hand, the URI "mailto:joe@example.com" indicates
that the URI refers to a mailbox. The "mailto" URI scheme
specification authorizes agents to infer that URIs of this form
identify Internet mailboxes.
In some cases, relevant technical specifications license URI
assignment authorities to publish assignment policies. For more
information about URI opacity, see TAG issue metaDataInURI-31.
Story
When navigating within the XHTML data that Nadia receives as a
representation of the resource identified by
"http://weather.example.com/oaxaca", Nadia finds that the URI
"http://weather.example.com/oaxaca#tom" refers to information about
tomorrow's weather in Oaxaca. This URI includes the fragment
identifier "tom" (the string after the "#").
The fragment
identifier of a URI allows indirect identification of a
secondary resource by
reference to a primary resource and additional information. The
secondary resource may be some portion or subset of the primary
resource, some view on representations of the primary resource, or
some other resource. The interpretation of fragment identifiers is
discussed in the section on media types and fragment identifier semantics.
See TAG issues abstractComponentRefs-37 and DerivedResources-43.
There remain open questions regarding identifiers on the Web.
The following sections identify a few areas of future work in the
Web community.
The integration of internationalized identifiers (i.e., composed
of characters beyond those allowed by [URI]) into the Web architecture is an important
and open issue. See TAG issue IRIEverywhere-27 for discussion about work going
on in this area.
Emerging Semantic Web technologies, including the "Web Ontology
Language (OWL)" [OWL10], define
RDF [RDF10] properties such as
sameAs
to assert that two URIs identify the same
resource or functionalProperty
to imply it.
Communication between agents over a network about resources
involves URIs, messages, and data.
Story
Nadia follows a hypertext link labeled "satellite image"
expecting to retrieve a satellite photo of the Oaxaca region. The
link to the satellite image is an XHTML link encoded as
<a href="http://example.com/satimage/oaxaca">satellite image</a>
.
Nadia's browser analyzes the URI and determines that its scheme is "http". The browser
configuration determines how it locates the identified information,
which might be via a cache of prior retrieval actions, by
contacting an intermediary (such as a proxy server), or by direct
access to the server identified by the URI. In this example, the
browser opens a network connection to port 80 on the server at
"example.com" and sends a "GET" message as specified by the HTTP
protocol, requesting a representation of the resource identified by
"/satimage/oaxaca".
The server sends a response message to the browser, once again
according to the HTTP protocol. The message consists of several
headers and a JPEG image. The browser reads the headers, learns
from the "Content-Type" field that the Internet Media Type of the
representation is "image/jpeg", reads the sequence of octets that
comprises the representation data, and renders the image.
This section describes the architectural principles and
constraints regarding interactions between agents, including such
topics as network protocols and interaction styles, along with
interactions between the Web as a system and the people that make
use of it. The fact that the Web is a highly distributed system
affects architectural constraints and assumptions about
interactions.
Note: The Web Architecture does not require a
formal definition of the commonly used phrase "on the Web."
Informally, a resource is "on the Web" when it has a URI and an
agent can use the URI to retrieve a representation of it using
network protocols (given appropriate access privileges, network
connectivity, etc.). See the related TAG issue httpRange-14.
Agents may use a URI to access the referenced resource; this is
called dereferencing the URI. Access
may take many forms, including retrieving a representation of
resource state (for instance, by using HTTP GET or HEAD), modifying
the state of the resource (for instance, by using HTTP POST or
PUT), and deleting the resource (for instance, by using HTTP
DELETE).
There may be more than one way to access a resource for a given
URI; application context determines which access mechanism an agent
uses. For instance, a browser might use HTTP GET to retrieve a
representation of a resource, whereas a link checker might use HTTP
HEAD on the same URI simply to establish whether a representation
is available. Some URI schemes set expectations about available
access mechanisms, others (such as the URN scheme [RFC 2141]) do not. Section 1.2.2
of [URI] discusses the separation
of identification and interaction in more detail. For more
information about relationships between multiple access mechanisms
and URI addressability, see the TAG finding "URIs, Addressability, and the use of HTTP GET and
POST".
Although many URI schemes
are named after protocols, this does not imply that use of such a
URI will result in access to the resource via the named protocol.
Even when an agent uses a URI to retrieve a representation, that
access might be through gateways, proxies, caches, and name
resolution services that are independent of the protocol associated
with the scheme name.
Dereferencing a URI generally involves a succession of steps as
described in multiple independent specifications and implemented by
the agent. The following example illustrates the series of
specifications that are involved when a user instructs a user agent
to follow a hypertext link
that is part of an SVG document. In this example, the URI is
"http://weather.example.com/oaxaca" and the application context
calls for the user agent to retrieve and render a representation of
the identified resource.
- Since the URI is part of a hypertext link in an SVG document,
the first relevant specification is the SVG 1.1 Recommendation [SVG11]. Section 17.1
of this specification imports the link semantics defined in XLink
1.0 [XLink10]: "The remote
resource (the destination for the link) is defined by a URI
specified by the XLink href attribute on the 'a' element." The SVG
specification goes on to state that interpretation of an
a
element involves retrieving a representation of a
resource, identified by the href
attribute in the
XLink namespace: "By activating these links (by clicking with the
mouse, through keyboard input, voice commands, etc.), users may
visit these resources."
- The XLink 1.0 [XLink10]
specification, which defines the
href
attribute in
section 5.4, states that "The value of the href attribute must be a
URI reference as defined in [IETF RFC 2396], or must result in a
URI reference after the escaping procedure described below is
applied."
- The URI specification [URI]
states that "Each URI begins with a scheme name that refers to a
specification for assigning identifiers within that scheme." The
URI scheme name in this example is "http".
- [IANASchemes] states
that the "http" scheme is defined by the HTTP/1.1 specification
(RFC 2616 [RFC2616], section
3.2.2).
- In this SVG context, the agent constructs an HTTP GET request
(per section 9.3 of [RFC2616])
to retrieve the representation.
- Section 6 of [RFC2616]
defines how the server constructs a corresponding response message,
including the 'Content-Type' field.
- Section 1.4 of [RFC2616]
states "HTTP communication usually takes place over TCP/IP
connections." This example does not address that step in the
process, or other steps such as Domain Name System
(DNS) resolution.
- The agent interprets the returned representation according to
the data format specification that corresponds to the
representation's Internet Media Type (the value of the HTTP
'Content-Type') in the relevant IANA registry [MEDIATYPEREG].
The Web's protocols (including HTTP, FTP, SOAP, NNTP, and SMTP)
are based on the exchange of messages. A message may include representation
data as well as metadata about the resource (such as the
"Alternates" and "Vary" HTTP headers), the representation, and the
message (such as the "Transfer-encoding" HTTP header). A message
may even include metadata about the message metadata (for
message-integrity checks, for instance).
Two important classes of message are those that request a
representation of a resource, and those that return the result of
such a request. Such a response message (for example, a response to
an HTTP GET) includes a representation of the state
of the resource. A representation is an octet sequence that
consists logically of two parts:
- Representation data,
electronic data about resource state, expressed in one or more formats used separately or in
combination, and
- Representation
metadata. One important piece of metadata is the Internet Media Type,
discussed below.
Agents use representations to modify as well as retrieve
resource state. Note that even though the response to an HTTP POST
request may contain the above types of data, the response to an
HTTP POST request is not necessarily a representation of the state
of the resource identified in the POST request.
The Internet Media Type [RFC2046]) of a representation determines which
data format specification(s) provide the authoritative
interpretation of the representation data (including fragment identifier syntax
and semantics, if any). The IANA registry [MEDIATYPEREG] maps media
types to data formats.
See the TAG finding "Internet Media Type registration, consistency of
use" for more information about media type
registration.
Story
In one of his XHTML pages, Dirk links to an image that Nadia has
published on the Web. He creates a hypertext link with <a
href="http://www.example.com/images/nadia#hat">Nadia's
hat</a>
. Nadia serves an SVG representation of the
image (with Internet Media Type "image/svg+xml"), so the
authoritative interpretation of the fragment identifier "hat"
depends on the SVG specification.
Per [URI], in order to know the
authoritative interpretation of a fragment identifier, one must
dereference the URI containing the fragment identifier. The
Internet Media Type of the retrieved representation specifies the
authoritative interpretation of the fragment identifier. Thus, in
the case of Dirk and Nadia, the authoritative interpretation
depends on the SVG specification, not the XHTML specification
(i.e., the context where the URI appears).
Given a URI "U#F", and a representation retrieved by
dereferencing URI "U", the (secondary) resource identified by "U#F" is
determined by interpreting "F" according to the specification
associated with the Internet Media Type of the representation.
Interpretation of the fragment identifier during a retrieval
action is performed solely by the agent; the fragment identifier is
not passed to other systems during the process of retrieval. This
means that some intermediaries in the Web architecture (such as
proxies) have no interaction with fragment identifiers and that
redirection (in HTTP [RFC2616],
for example) does not account for them.
Note that one can use a URI with a fragment identifier even if
one does not have a representation available for interpreting the
fragment identifier (one can compare two such URIs, for example).
Parties that draw conclusions about the interpretation of a
fragment identifier without retrieving a representation do so at
their own risk; such interpretations are not authoritative.
Story
Dirk informs Nadia that he would also like her to make her
images available in formats other than SVG. For the same resource,
Nadia makes available a PNG image as well. Dirk's user agent and
Nadia's server negotiate so that the user agent retrieves a
suitable representation. Which specification specifies the
authoritative interpretation of the "hat" fragment identifier, the
PNG specification or the SVG specification?
For a given resource, an agent may have the choice between
representation data in more than one data format (through HTTP
content negotiation, for example). Since different data formats may
define different fragment identifier semantics, it is important to
note that by design, the secondary resource identified by a URI
with a fragment identifier is expected to be the same across all
representations. Thus, if a fragment has defined semantics in any
one representation, the fragment is identified for all of them,
even though a particular data format may not be able to represent
it.
Suppose, for example, that the authority responsible for
"http://weather.example.com/oaxaca/map#zicatela" provides
representations of the resource identified by
http://weather.example.com/oaxaca/map using three image formats:
SVG, PNG, and JPEG/JFIF. The SVG specification defines semantics
for fragment identifiers while the other specifications do not. It
is not considered an error that only one of the data formats
specifies semantics for the fragment identifier. Because the Web is
a distributed system in which formats and agents are deployed in a
non-uniform manner, the architecture allows this sort of
discrepancy. This design allows authors to take advantage of new
data formats while still ensuring reasonable backward-compatibility
for users whose agents do not yet implement them.
On the other hand, it is considered an error if the semantics of
the fragment identifiers used in two representations of a secondary
resource are inconsistent.
Good practice: Fragment identifier consistency
A resource owner who creates a URI with a
fragment identifier and who uses content negotiation to serve
multiple representations of the identified resource SHOULD NOT
serve representations with inconsistent fragment identifier
semantics.
Inconsistent fragment identifier semantics are one potential
source of URI
ambiguity.
See related TAG issues httpRange-14 and RDFinXHTML-35.
Successful communication between two parties using a piece of
information relies on shared understanding of the meaning of the
information. Arbitrary numbers of independent parties can identify
and communicate about a Web resource. To give these parties the
confidence that they are all talking about the same thing when they
refer to "the resource identified by the following URI ..." the
design choice for the Web is, in general, that the owner of a resource assigns
the authoritative interpretation of representations of the
resource.
In our travel scenario, the
authority responsible for "weather.example.com" has license to
create representations of this resource. Which representation(s)
Nadia receives depends on a number of factors, including:
- Whether the authority responsible for "weather.example.com"
responds to requests at all;
- Whether the authority responsible for "weather.example.com"
makes available one or more representations for the resource
identified by "http://weather.example.com/oaxaca";
- Whether Nadia has access privileges to such representations
(see the section on linking and
access control);
- If the authority responsible for "weather.example.com" has
provided more than one representation (in different formats such as
HTML, PNG, or RDF, or in different languages such as English and
Spanish), the resulting representation may depend on negotiation
between the user agent and server that occurs as part of the HTTP
transaction.
- When Nadia made the request. Since the weather in Oaxaca
changes, Nadia should expect that representations will change over
time.
See TAG issues contentTypeOverride-24 and rdfURIMeaning-39.
Inconsistencies between the data format of representation data
and assigned representation metadata do occur. Examples that have
been observed in practice include:
- The actual character encoding of a representation is
inconsistent with the charset parameter in the representation
metadata.
- The namespace of the root element of XML representation data is
inconsistent with the value of the 'Content-Type' field in HTTP
headers.
User agents should detect such inconsistencies but should not
resolve them without involving the user.
Principle: Authoritative server
metadata
User agents MUST NOT silently ignore
authoritative server metadata.
Thus, for example, if the parties responsible for
"weather.example.com" mistakenly label the satellite photo of
Oaxaca as "image/gif" instead of "image/jpeg", and if Nadia's
browser detects a problem, Nadia's browser must not silently ignore
the problem and render the JPEG image. Nadia's browser can notify
Nadia of the problem or notify Nadia and take corrective action. Of
course, user agent designers should not ignore usability issues
when handling this type of error; notification may be discreet, and
handling may be tuned to meet the user's preferences. See the TAG
finding "Client handling of MIME headers" for more
in-depth discussion and examples.
Furthermore, server managers can help reduce the risk of error
through careful assignment of representation metadata (especially
that which applies across representations). The section on media types for XML
presents an example of reducing the risk of error by providing no
metadata about character encoding when serving XML.
Story
Nadia decides to book a vacation to Oaxaca at
"booking.example.com." She enters data into a series of online
forms and is ultimately asked for credit card information to
purchase the airline tickets. She provides this information in
another form. When she presses the "Purchase" button, her browser
opens another network connection to the server at
"booking.example.com" and sends a message composed of form data
using the POST method. Note that this is not a safe interaction; Nadia
wishes to change the state of the system by exchanging money for
airline tickets.
The server reads the POST request, and after performing the
booking transaction returns a message to Nadia's browser that
contains a representation of the results of Nadia's request. The
representation data is in XHTML so that it can be saved or printed
out for Nadia's records. Note that neither the data transmitted
with the POST nor the data received in the response necessarily
correspond to any resource named by a URI.
Nadia's retrieval of weather information (an example of a
read-only query or lookup) qualifies as a "safe" interaction; a safe
interaction is one where the agent does not incur any
obligation beyond the interaction. An agent may incur an obligation
through other means (such as by signing a contract). If an agent
does not have an obligation before a safe interaction, it does not
have that obligation afterwards.
Other Web interactions resemble orders more than queries. These
unsafe interactions may
cause a change to the state of a resource and the user may be held
responsible for the consequences of these interactions. Unsafe
interactions include subscribing to a newsletter, posting to a
list, or modifying a database.
Safe interactions are important because these are interactions
where users can browse with confidence and where agents (including
search engines and browsers that pre-cache data for the user) can
follow links safely. Users (or agents acting on their behalf) do
not commit themselves to anything by querying a resource or
following a link.
Principle: Safe retrieval
Agents do not incur obligations by retrieving
a representation.
For instance, it is incorrect to publish a link that, when
followed, subscribes a user to a mailing list. Remember that search
engines may follow such links.
For more information about safe and unsafe operations using HTTP
GET and POST, and handling security concerns around the use of HTTP
GET, see the TAG finding "URIs, Addressability, and the use of HTTP GET and
POST".
Story
Nadia pays for her airline tickets online (through a POST
interaction as described above). She receives a Web page with
confirmation information and wishes to bookmark it so that she can
refer to it when she calculates her expenses. Although Nadia can
print out the results, or save them to a file, she cannot bookmark
the results. In fact, neither the POST request, which expresses her
commitment to pay, nor the airline company's response, which
expresses its acknowledgment and its own commitment, can be
referenced by URIs.
It is a breakdown of the Web architecture if agents cannot use
URIs to reconstruct a "paper trail" of transactions, i.e., to refer
to receipts and other evidence of accepting an obligation. Indeed,
each electronic mail message includes a unique message identifier,
one reason why email is so useful for managing accountability
(since, for example, email can be copied to public archives). On
the other hand, HTTP servers and deployed user agents do not
generally keep records of POST transactions, making it difficult
for all parties to reconstruct a series of transactions.
There are mechanisms in HTTP, not widely deployed, to remedy
this situation. HTTP servers can assign a URI to the results of a
POST transaction using the "Content-Location" header (described in
section 14.14 of [RFC2616]),
and allow authorized parties to retrieve a record of the
transaction thereafter via this URI (the value of URI persistence is
apparent in this case). User agents can provide an interface for
managing transactions where the user agent has incurred an
obligation on behalf of the user.
Story
Since Nadia finds the Oaxaca weather site useful, she emails a
review to her friend Dirk recommending that he check out
'http://weather.example.com/oaxaca'. Dirk clicks on the link in the
email he receives and is surprised to see his browser display a
page about auto insurance. Dirk confirms the URI with Nadia, and
they both conclude that the resource is unreliable. Although the
managers of Oaxaca have chosen the Web as a communication medium,
they have lost two customers due to ineffective resource
management.
The usefulness of a resource depends on good management by its
owner. As is the case with many human interactions, confident
interactions with a resource depend on stability and
predictability. The value of a URI increases with the
predictability of interactions using that URI. Avoiding unnecessary
URI aliases is one aspect
of proper resource management.
Good practice: Consistent
representation
Publishers of a URI SHOULD provide
representations of the identified resource consistently and
predictably.
This section discusses important aspects of representation
management.
The authority responsible for a resource may supply zero or more
representations of a resource. The authority is also responsible
for accepting or rejecting requests to modify a resource, for
example, by configuring a server to accept or reject HTTP PUT data
based on Internet Media Type, validity constraints, or other
constraints.
Good practice: Available representation
Publishers of a URI SHOULD provide
representations of the identified resource.
There are strong social expectations that once a URI identifies
a particular resource, it should continue indefinitely to refer to
that resource; this is called URI persistence. URI
persistence is a matter of policy and commitment on the part of
authorities servicing URIs. The choice of a particular URI scheme
provides no guarantee that those URIs will be persistent or that
they will not be persistent.
Since representations are used to communicate resource state,
persistence is directly affected by how well representations are
served. Service breakdowns include:
- Inconsistent representations served. Note the difference
between a resource owner changing representations predictably in
light of the nature of the resource (the changing weather of
Oaxaca) and the owner changing representations arbitrarily.
- Improper use of content negotiation, such as serving two images
as equivalent through HTTP content negotiation, where one image
represents a square and the other a circle.
HTTP [RFC2616] has been
designed to help manage URIs. For example, HTTP redirection (using
the 3xx response codes) permits servers to tell an agent that
further action needs to be taken by the agent in order to fulfill
the request (for example, the resource has been assigned a new
URI). In addition, content negotiation also promotes consistency,
as a site manager is not required to define new URIs when adding
support for a new format specification. Protocols that do not
support content negotiation (such as FTP) require a new identifier
when a new data format is introduced.
For more discussion about URI persistence, see [Cool].
It is reasonable to limit access to a resource (for commercial
or security reasons, for example), but it is unreasonable to
prohibit others from merely identifying the resource.
As an analogy: The owners of a building might have a policy that
the public may only enter the building via the main front door, and
only during business hours. People who work in the building and who
make deliveries to it might use other doors as appropriate. Such a
policy would be enforced by a combination of security personnel and
mechanical devices such as locks and pass-cards. One would not
enforce this policy by hiding some of the building entrances, nor
by requesting legislation requiring the use of the front door and
forbidding anyone to reveal the fact that there are other doors to
the building.
Story
Nadia and Dirk both subscribe to the "weather.example.com"
newsletter. Nadia wishes to point out an article of particular
interest to Dirk, using a URI. The authority responsible for
"weather.example.com" can offer newsletter subscribers such as
Nadia and Dirk the benefits of URIs (such as bookmarking and
linking) and still limit access to the newsletter to authorized
parties.
The Web provides several mechanisms to control access to
resources; these mechanisms do not rely on hiding or suppressing
URIs for those resources. For more information, see the TAG finding
"'Deep Linking' in the World Wide Web".
There remain open questions regarding Web interactions. The TAG
expects future versions of this document to address in more detail
the relationship between the architecture described herein, Web Services,
the Semantic
Web, peer-to-peer systems (including Freenet, MLdonkey, and NNTP [RFC977]), instant messaging systems (including [XMPP]), and voice-over-ip (including
RTSP [RFC2326]).
A data format (including XHTML, CSS, PNG, XLink, RDF/XML, and
SMIL animation) specifies the interpretation of representation data.
The first data format used on the Web was HTML. Since then, data
formats have grown in number. The Web architecture does not
constrain which data formats content providers can use. This
flexibility is important because there is constant evolution in
applications, resulting in new data formats and refinements of
existing formats. Although the Web architecture allows for the
deployment of new data formats, the creation and deployment of new
formats (and agents able to handle them) is expensive. Thus, before
inventing a new data format, designers should carefully consider
re-using one that is already available.
For a data format to be usefully interoperable between two
parties, the parties must have a shared understanding of its syntax
and semantics. This is not to imply that a sender of data
can count on constraining its treatment by a receiver; simply that
making good use of a data format requires knowledge of its
designers' intentions. Below we describe some characteristics of a
data format make it easier to integrate into the Web architecture.
This document does not address generally beneficial characteristics
of a specification such as readability, simplicity, attention to
programmer goals, attention to user needs, accessibility, and
internationalization. The section on architectural specifications includes references
to additional format specification guidelines.
A textual data format is one in which the data is specified as a
sequence of characters. HTML, Internet e-mail, and all XML-based formats are textual.
In modern textual data formats, the characters are usually taken
from the Unicode repertoire [UNICODE].
Binary data formats are those in which portions of the data are
encoded for direct use by computer processors, for example
thirty-two bit little-endian two's-complement and sixty-four bit
IEEE double-precision floating-point. The portions of data so
represented include numeric values, pointers, and compressed data
of all sorts.
In principle, all data can be represented using textual
formats.
The trade-offs between binary and textual data formats are
complex and application-dependent. Binary formats can be
substantially more compact, particularly for complex pointer-rich
data structures. Also, they can be consumed more rapidly by agents
in those cases where they can be loaded into memory and used with
little or no conversion.
Textual formats are usually more portable and interoperable.
Textual formats also have the considerable advantage that they can
be directly read and understood by human beings. This can simplify
the tasks of creating and maintaining software, and allow the
direct intervention of humans in the processing chain without
recourse to tools more complex than the ubiquitous text editor.
Finally, it simplifies the necessary human task of learning about
new data formats (the "view source" effect).
It is important to emphasize that intuition as to such matters
as data size and processing speed are not a reliable guide in data
format design; quantitative studies are essential to a correct
understanding of the trade-offs. Therefore, data format
specification authors should make a considered choice between
binary and textual format design.
Note: Text (i.e., a sequence of characters from
a repertoire) is distinct from serving data with a media type
beginning with "text/". Although XML-based formats are textual,
many such formats are not primarily comprised of phrases in natural
language. See the section on media types for XML for issues that arise when
"text/" is used in conjunction with an XML-based format.
See TAG issue binaryXML-30.
Extensibility and versioning are strategies to help manage the
natural evolution of information on the Web and technologies used
to represent that information.
For more information on about versioning strategies and agent
behavior in the face of unrecognized extensions, see TAG issue XMLVersioning-41 and "Web Architecture: Extensible
Languages" [EXTLANG].
There is typically a (long) transition period during which
multiple versions of a format, protocol, or agent are
simultaneously in use.
Good practice: Version
information
Format designers SHOULD provide for version
information in language instances.
Story
Nadia and Dirk are designing an XML data format to encode data
about the film industry. They provide for extensibility by using
XML namespaces and creating a schema that allows the inclusion, in
certain places, of elements from any namespace. When they revise
their format, Nadia proposes a new optional "lang" attribute on the
"film" element. Dirk feels that such a change requires them to
assign a new namespace name, which might require changes to
deployed software. Nadia explains to Dirk that their choice of
extensibility strategy in conjunction with their namespace policy
allows certain changes that do not affect conformance of existing
content and software, and thus no change to the namespace
identifier is required. They chose this policy to help them meet
their goals of reducing the cost of change.
Dirk and Nadia have chosen a particular namespace change policy
that allows them to avoid changing the namespace name whenever they
make changes that do not affect conformance of deployed content and
software. They might have chosen a different policy, for example
that any new element or attribute has to belong to a namespace
other than the original one. Whatever the chosen policy, it should
set clear expectations for users of the format.
Good practice: Namespace policy
Format designers SHOULD document change
policies for XML namespaces.
As an example of a change policy designed to reflect the
variable stability of a namespace, consider the W3C namespace
policy for documents on the W3C Recommendation track. The
policy sets expectations that the Working Group responsible for the
namespace may modify it in any way until a certain point in the
process ("Candidate Recommendation") at which point W3C constrains
the set possible changes to the namespace in order to promote
stable implementations.
Note that since namespace names are URIs, the party (if any)
responsible for a namespace URI has the authority to decide the
namespace change policy.
Designers can facilitate the transition process by making
careful choices about extensibility during the design of a language
or protocol specification.
Good practice: Extensibility
mechanisms
Language designers SHOULD provide mechanisms
that allow any party to create extensions that do not interfere
with conformance to the original specification.
Application needs determine the most appropriate extension
strategy for a specification. For example, applications designed to
operate in closed environments may allow specification authors to
define a versioning strategy that would be impractical at the scale
of the Web. As part of defining an extensibility mechanism, a
specification should set expectations about agent behavior in the
face of unrecognized extensions.
Good practice: Unknown extensions
Language designers SHOULD specify agent
behavior in the face of unrecognized extensions.
Two strategies have emerged as being particularly useful:
- "Must ignore": The agent ignores any content it does not
recognize.
- "Must understand": The agent treats unrecognized markup as an
error condition.
A powerful design approach is for the language to allow either
form of extension, but to distinguish explicitly between them in
the syntax.
Additional strategies include prompting the user for more input,
automatically retrieving data from available links, and falling
back to default behavior. More complex strategies are also
possible, including mixing strategies. For instance, a language can
include mechanisms for overriding standard behavior. Thus, a data
format can specify "must ignore" semantics but also allow people to
create extensions that override that semantics in light of
application needs (for instance, with "must understand" semantics
for a particular extension).
Extensibility is not free. Providing hooks for extensibility is
one of many requirements to be factored into the costs of language
design. Experience suggests that the long term benefits of
extensibility generally outweigh the costs.
Many modern data format specifications include mechanisms for
composition. For example:
- It is possible to embed text comments in some image formats,
such as JPEG/JFIF. Although these comments are embedded in the
containing data, they have little or no effect on the display of
the image.
- There are container formats such as SOAP which fully expect to
be composed from multiple namespaces but which provide an overall
semantic relationship of message envelope and payload.
- RDF allows well-defined mixing of vocabularies, and allows text
and XML to be used as a data type values within a statement having
clearly defined semantics.
These relationships can be mixed and nested arbitrarily. In
principle, a SOAP message can contain a JPEG image that contains an
RDF comment which refers to a vocabulary of terms for describing
the image.
Note however, that for general XML there is no semantic model
that defines the interactions within XML documents with elements
and/or attributes from a variety of namespaces. Each application
must define how namespaces interact and what effect the namespace
of an element has on the element's ancestors, siblings, and
descendants.
See TAG issues mixedUIXMLNamespace-33, xmlFunctions-34, and RDFinXHTML-35.
The Web is a heterogeneous environment where a wide variety of
agents provide access to content to users with a wide variety of
capabilities. It is good practice for authors to create content
that can reach the widest possible audience, including users with
graphical desktop computers, hand-held devices and cell phones,
users with disabilities who may require speech synthesizers, and
devices not yet imagined. Furthermore, authors cannot predict in
some cases how an agent will display or process their content.
Experience shows that the allowing authors to separate content,
presentation, and interaction concerns promotes reuse and
device-independence (see [DIPRINCIPLES]); this follows from the principle of orthogonal of
specifications.
Good practice: Separation of content, presentation,
interaction
Language designers SHOULD design formats that
allow authors to separate content from presentation and interaction
concerns.
Note that when content, presentation, and interaction are
separated by design, agents need to recombine them. There is a
recombination spectrum, with "client does all" at one end and
"server does all" at the other. There are advantages to each:
recombination on the server allows the server to send out generally
smaller amounts of data that can be tailored to specific devices
(such as mobile phones). However, such data will not be readily
reusable by other clients and may not allow client-side agents to
perform useful tasks unanticipated by the author. When a client
does the work of recombination, content is likely to be more
reusable by a broader audience and more robust. However, such data
may be of greater size and may require more computation by the
client.
Of course, it may not always be desirable to reach the widest
possible audience. Application context may require a very specific
display (for a legally-binding transaction, for example). Also,
digital signature technology, access control, and other technologies are
appropriate for controlling access to content.
Some data formats are designed to describe presentation
(including SVG and XSL Formatting Objects). Data formats such as
these demonstrate that one can only separate content from
presentation (or interaction) so far; at some point it becomes
necessary to talk about presentation. Per the principle of
orthogonal specifications, these data formats should only
address presentation issues.
See the TAG issues formattingProperties-19 and contentPresentation-26.
A defining characteristic of the Web is that it allows embedded
references to other Web resources via URIs. The simplicity of
creating links using absolute URIs (<a
href="http://www.example.com/foo">
) and relative URI
references (<a href="foo">
and <a
href="foo#anchor">
) is partly (perhaps largely)
responsible for the birth of the hypertext Web as we know it
today.
When one resource (representation) refers to another resource
with a URI, this constitutes a link between the two resources.
Additional metadata may also form part of the link (see [XLink10], for example).
Good practice: Link
mechanisms
Language designers SHOULD provide mechanisms
for identifying links to other resources and to portions of
representation data (via fragment identifiers).
Good practice: Web linking
Language designers SHOULD provide mechanisms
that allow Web-wide linking, not just internal document
linking.
Good practice: Generic URIs
Language designers SHOULD allow authors to use
URIs without constraining them to a limited set of URI schemes.
What agents do with a hypertext link is not constrained by Web
architecture and may depend on application context. Users of the
hypertext links expect to be able to navigate links among
representations. Data formats that do not allow authors to create
hypertext links lead to the creation of "terminal nodes" on the
Web.
Good practice: Hypertext links
Language designers SHOULD incorporate hypertext
links into a data format if hypertext is the expected user
interface paradigm.
Links are commonly expressed using URI references (defined in section 4.2
of [URI]), which may be combined
with a base URI to yield a usable URI. Section 5.1 of [URI] explains different mechanisms for
establishing a base URI for a resource and establishes a precedence
among the various mechanisms. For instance, the base URI may be a
URI for the resource, or specified in a representation (see the
base
elements provided by HTML and XML, and the HTTP
'Content-Location' header). See also the section on links in XML.
Agents resolve a URI reference before using the resulting URI to
interact with another agent. URI references help in content
management by allowing authors to design a representation locally,
i.e., without concern for which global identifier may later be used
to refer to the associated resource.
Many data formats are XML-based, that is to say they
conform to the syntax rules defined in the XML specification [XML10]. This section discusses
issues that are specific to such formats. Anyone seeking guidance
in this area is urged to consult the "Guidelines For the Use of XML
in IETF Protocols" [IETFXML],
which contains a thorough discussion of the considerations that
govern whether or not XML ought to be used, as well as specific
guidelines on how it ought to be used. While it is directed at
Internet applications with specific reference to protocols, the
discussion is generally applicable to Web scenarios as well.
The discussion here should be seen as ancillary to the content
of [IETFXML]. Refer also to
"XML Accessibility Guidelines" [XAG] for help designing XML formats that lower
barriers to Web accessibility for people with disabilities.
XML defines textual data formats that are naturally suited to
describing data objects which are hierarchical and processed in a
chosen sequence. It is widely, but not universally, applicable for
data format specifications; an audio or video format, for example,
is unlikely to be well suited to expression in XML. Design
constraints that would suggest the use of XML include:
- Requirement for a hierarchical structure.
- The data's usefulness should outlive the tools currently used
to process it (though obviously XML can be used for short-term
needs as well).
- Ability to support internationalization in a self-describing
way that makes confusion over coding options unlikely.
- Early detection of encoding errors with no requirement to "work
around" such errors.
- A high proportion of human-readable textual content.
- Potential composition of the data format with other XML-encoded
formats.
Sophisticated linking mechanisms have been invented for XML
formats. XPointer allows links to address content that does not
have an explicit, named anchor. XLink is an appropriate
specification for representing links in hypertext XML applications. XLink allows links to
have multiple ends and to be expressed either inline or in "link
bases" stored external to any or all of the resources identified by
the links it contains.
Designers of XML-based formats should consider using XLink and,
for defining fragment identifier syntax, using the XPointer
framework and XPointer element() Schemes.
See TAG issue xlinkScope-23.
Story
The authority responsible for "weather.example.com" realizes
that it can provide more interesting representations by creating
instances that consist of elements defined in different XML-based formats, such as
XHTML, SVG, and MathML.
How do the application designers ensure that there are no naming
conflicts when they combine elements from different formats (for
example, suppose that the "p" element is defined in two or more XML
formats)? "Namespaces in XML" [XMLNS] provides a mechanism for establishing a
globally unique name that can be understood in any context.
Language specification designers that declare namespaces thus
provide a global context for instances of the data format.
Establishing this global context allows those instances (and
portions thereof) to be re-used and combined in novel ways not yet
imagined. Failure to provide a namespace makes such re-use more
difficult, perhaps impractical in some cases.
Good practice: Namespace
adoption
Language designers who create new XML
vocabularies SHOULD place all element names and global attribute
names in a namespace.
Attributes are always scoped by the element on which they
appear. An attribute that is "global," that is, one that might
meaningfully appear on different elements, including elements in
other namespaces, should be explicitly placed in a namespace. Local
attributes, ones associated with only a particular element, need
not be included in a namespace since their meaning will always be
clear from the context provided by that element.
The type
attribute from W3C XML Schema is an
example of a global attribute. It can be used by authors of any
vocabulary to make an assertion about the type of the element on
which it appears. The type
attribute occurs in the W3C
XML Schema namespace and must always be fully qualified. The
frame
attribute on an HTML table is an example of a
local attribute. There is no value in placing that attribute in a
namespace since the attribute is unlikely to be useful on an
element other than an HTML table.
Applications that rely on DTD processing must impose additional
constraints on the use of namespaces. DTDs perform validation based
on the lexical form of the element and attribute names in the
document. This makes prefixes syntactically significant in ways
that are not anticipated by [XMLNS].
Story
Nadia receives a representation data from "weather.example.com"
in an unfamiliar data format. She knows enough about XML to
recognize which XML namespace the elements belong to. Since the
namespace is identified by the URI
"http://weather.example.com/2003/format", she asks her browser to
retrieve a representation of the namespace via that URI. Nadia is
requesting the namespace document.
Nadia gets back some useful data that allows her to learn more
about the data format. Nadia's browser may also be able to perform
some operations automatically (i.e., unattended by a human
overseer) given data that has been optimized for software agents.
For example, her browser might, on Nadia's behalf, download
additional agents to process and render the format.
There are many reasons to provide information about a namespace.
A person might want to:
- understand its purpose,
- learn how to use the markup vocabulary in the namespace,
- find out who controls it,
- request authority to access schemas or collateral material
about it, or
- report a bug or situation that could be considered an error in
some collateral material.
A processor might want to:
- retrieve a schema, for validation,
- retrieve a style sheet, for presentation, or
- retrieve ontologies, for making inferences.
In general, there is no established best practice for creating a
namespace document. Application expectations will influence what
data format or formats are used to create a namespace document.
Application expectations will also influence whether relevant
information appears in the namespace document itself or is
referenced from it.
Good practice: Namespace
documents
Resource owners who publish an XML namespace
name SHOULD make available material intended for people to read and
material optimized for software agents in order to meet the needs
of those who will use the namespace vocabulary.
For example, the following are examples of formats used to
create namespace documents: [OWL10], [RDDL],
[XMLSCHEMA], and [XHTML11]. Each of these formats
meets different requirements described above for satisfying the
needs of an agent that wants more information about the namespace.
Note, however, issues related to fragment identifiers and multiple representations
if content negotiation is used with namespace documents.
See TAG issues namespaceDocument-8 and abstractComponentRefs-37.
Section 3 of "Namespaces in XML" [XMLNS] provides a syntactic construct known as a
QName for the compact expression of qualified names in XML
documents. A qualified name is a pair consisting of a URI, which
names a namespace, and a local name placed within that namespace.
"Namespaces in XML" provides for the use of QNames as names for XML
elements and attributes.
Other specifications, starting with [XSLT10], have employed the idea of using QNames in
contexts other than element and attribute names, for example in
attribute values and in element content. However, general XML
processors cannot recognize QNames as such when they are used in
attribute values and in element content; they are indistinguishable
from URIs. Experience has also revealed other limitations to
QNames, such as losing namespace bindings after XML
canonicalization.
Good practice: QNames
Indistinguishable from URIs
Specifications that use QNames to represent
URI/local-name pairs SHOULD NOT allow both forms in attribute
values or element content where they would be indistinguishable
from URIs.
For more information, see the TAG finding "Using QNames as Identifiers in
Content".
Because QNames are compact, some specifications have adopted the
same syntax as a means of identifying Web resources. Though
convenient as a shorthand notation, this usage has a cost. There is
no single, accepted way to convert a QName into a URI or
vice-versa. Although QNames are convenient, they do not replace the
URI as the identification mechanism of the Web. The use of QNames
to identify Web resources without providing a mapping to URIs is
inconsistent with Web architecture.
Good practice: QName Mapping
Language designers who use QNames as
identifiers of Web resources MUST provide a mapping to URIs.
For examples of QName-to-URI mappings, see [RDF10]. See also TAG issues rdfmsQnameUriMapping-6, qnameAsId-18, and abstractComponentRefs-37.
Consider the following fragment of XML: <section
name="foo">
. Does the section
element have
the ID "foo"? One cannot answer this question by examining the
element and its attributes alone. In XML, the quality of "being an
ID" is associated with the type of the attribute, not its name.
Finding the IDs in a document requires additional processing.
- Processing the document with a processor that recognizes DTD
attribute list declarations (in the external or internal subset)
might reveal a declaration that identifies the name attribute as an
ID. Note: This processing is not necessarily part
of validation. A non-validating, DTD-aware processor can perform ID
assignment.
- Processing the document with a W3C XML Schema might reveal an
element declaration that identifies the name attribute as an
xs:ID
.
- In practice, processing the document with another schema
language, such as RELAX NG [RELAXNG], might reveal the attributes of type ID.
Many modern specifications begin processing XML at the Infoset [INFOSET] level and do not specify
normatively how an Infoset is constructed. For those
specifications, any process that establishes the ID type in the
Infoset (and Post Schema Validation Infoset
(PSVI) defined in [XMLSCHEMA]) may usefully identify the attributes
of type ID.
To further complicate matters, DTDs establish the ID type in the
Infoset whereas W3C XML Schema produces a PSVI but does not modify
the original Infoset. This leaves open the possibility that a
processor might only look in the Infoset and consequently would
fail to recognize schema-assigned IDs.
See TAG issue xmlIDSemantics-32.
RFC 3023 defines the Internet Media Types "application/xml" and
"text/xml", and describes a convention whereby XML-based data
formats use Internet Media Types with a "+xml" suffix, for example
"image/svg+xml".
These Internet Media Types create two problems: First, for data
identified as "text/*", Web intermediaries are allowed to
"transcode", i.e., convert one character encoding to another.
Transcoding may make the self-description false or may cause the
document to be not well-formed.
Good practice: XML and "text/*"
In general, server managers SHOULD NOT assign
Internet Media Types beginning with "text/" to XML
representations.
Second, representations whose Internet Media Types begin with
"text/" are required, unless the charset
parameter is
specified, to be considered to be encoded in US-ASCII. Since the
syntax of XML is designed to make documents self-describing, it is
good practice to omit the charset
parameter, and since
XML is very often not encoded in US-ASCII, the use of "text/"
Internet Media Types effectively precludes this good practice.
Good practice: XML and character
encodings
In general, server managers SHOULD NOT specify
the character encoding for XML data in protocol headers since the
data is self-describing.
The section on media
types and fragment identifier semantics discusses the
interpretation of fragment identifiers. Designers of an XML-based
data format specification should define the semantics of fragment
identifiers in that format. The XPointer Framework [XPTRFR] provides a interoperable
starting point.
When the media type assigned to representation data is
"application/xml", there are no semantics defined for fragment
identifiers, and authors should not make use of fragment
identifiers in such data. The same is true if the assigned media
type has the suffix "+xml" (defined in "XML Media Types" [RFC3023]), and the data format
specification does not specify fragment identifier semantics. In
short, just knowing that content is XML does not provide
information about fragment identifier semantics.
Many people assume that the fragment identifier
#abc
, when referring to XML data, identifies the
element in the document with the ID "abc". However, there is no
normative support for this assumption.
See TAG issue fragmentInXML-28.