Web Services Choreography Requirements

3.1.1.3.1 Variation 1

The travel agency might use a choreography definition internally as a documented model for its entire reservation booking process. The choreography definition language would need to present a multi-party global view of the reservation choreography, depicting the communication amongst the travel agent and all its various partners. Further, the definition language would need to support the addition of annotation or comments to the various elements of a choreography, in order to fully describe the behavior of the global choreography.

Requirements for variation 1

1. There MUST be a mechanism for adding free form annotation or comments to a choreography description.

2. A CDL must support the description of a multi-party global model.

3.1.1.3.2 Variation 2

The travel agent might assist new service providers to join its "network" by providing them with a choreography description outlining their communication responsibilities. The choreography definition language should support the construction of a global model (see above) and allow choreographies to reference one another, thus providing composition.

For instance, the travel agent would share a "credit check" choreography with a new credit bureau, and the travel agent's internal global choreography would simply include an instruction to "call credit check" choreography. The global choreography could be composed of calls to many such nested choreographies. This has the added benefit of making the travel agent's global choreography modular and easier to understand.

Optionally, the travel agent could share the entire global choreography with the new credit bureau, withholding nested choreographies that details its interaction with other parties, such as airlines and rental car companies. This takes advantage of hierarchical composition to provide information hiding in contexts where partial information loss is appropriate.

Requirements for variation 2

1. A CDL must facilitate decomposition to enable choreography descriptions to reference each other.

2. Some level of validation of a hierarchically composed choreography definition should be possible even when some or all nested choreographies are missing, in order to support information hiding by the use of hierarchical composition.

3. Choreographies must be uniquely named.

3.1.1.3.3 Variation 3

In order to protect against out-of-sequence messaging, a choreography participant might use a choreography definition to provide runtime validation of message typing and sequencing. When messages arrive they are checked for appropriate sequencing and, if the check fails, an exception is issued.

A car rental agency might wish to protect its backend systems from potential corruption by out-of-sequence messages; for instance, a reservation being cancelled before it's ever placed. The corresponding action might be to respond to the offending document with an error message.

This runtime validation and exception generation is the responsibility of each participant; however, the choreography definition language must not preclude the implementation of such a feature.

Requirements for variation 3

1. Failure to adhere exception MAY be generated by a choreography participant.

3.1.1.3.4 Variation 4

In some cases, there will be system or infrastructure errors. For example, when confirming the travel itinerary, it may happen that one or more of the parties fails and are not immediately re-contactable. Depending on the urgency and importance of the travel services offered by the crashed parties, some form of recovery or correction may be required. In some cases it may be necessary to wait for the crashed parties to become active again, and to replay the messages back to them from some pre-defined checkpoint. The ability to demarcate such checkpoints is required in order to support this sort of scenario.

Requirements for variation 4

1. Needs exception handling to be able to express observable actions to take on receipt of an exception message.

2. Needs timeouts to be able to express time to live on reservations.

3. A CDL shall facilitate the demarcation of observable actional behavior in order to define collaboration boundaries and provide guidance to the underlying infrastructure (for example, recovery, replay messages etc.).

4. Need to be able to model retries and timeouts and the ability to choose different paths based on which timeout expires first.

3.1.1.3.5 Variation 5

When interacting with the various parties, a number of application errors could occur. For example, between being offered services to a certain destination and confirming those services, it is entirely possible that the destination is withdrawn from sale for some travel advisory reason. When the client goes to book such services this should result in a set of error messages, that signify some erroneous state. Depending on how the actual web services are defined, these errors may simply be specific WSDL operations, or may be defined as WSDL faults; which is chosen is a web service design issue.

Requirements for variation 5

1. A CDL shall support the description of choreography specific errors.

2. A CDL shall support the description of WSDL faults.

3.1.1.3.6 Variation 6

In this variation of the Travel Agent use case most of the main carriers (airlines) have in place a robust messaging infrastructure that delivers high quality robust connections that ensure guaranteed delivery. However the rise of budget airlines means that a new class of carriers have become part of the overall offering from travel agents where the messing infrastructure is no where near as reliable.

Fortunately the same choreography description can be used for both classes of carrier and the only thing that needs to change on both sides of the connection is for the participants to bind to the underlying messaging protocol available.

Also in this variation each carrier has different correlation keys that are used to match messages that belong to a specific transaction. The choreography that is used across the various carrier types is left unchanged with only the binding to correlation different across them. For each instance of a choreography the choreography description, on instantiation with a participant, bind the appropriate correlation mechanism in to the choreography so that it may be followed. Binding on a per participant basis enables the choreography to use whatever correlation mechanism is required between any two participants.

Requirements for variation 6

1. There should be a means to describe the differing QoS as applied to messaging.

2. There should be a means to describe the differing correlation mechanisms as applied to messaging.

3.1.2.3.1 Variation 1

In this variation of the Quote Request use case a new manufacturer wishes to participate in the overall supply chain. They manufacture a component in a new way that makes them very competitive with their rivals and the suppliers certainly wish them to participate.

In order for them to participate they look for tools that can be used in conjunction with the existing CDL descriptions that the suppliers use in order to generate the web services required. In this case they use their preffered IDE with a new W3C CDL plug-in. The tool imports the CDL description and generates the necessary Java code skeletons for the relevant Web Services. This enables the new manufacturer to build the necessary web services to participate in the supply chain at a much lower entry cost that would have been possible otherwise.

Having created the necessary Web Services, the CDL IDE plug-in uses the CDL document to generate test messages that are then played into the newly created Web Services. This further cuts the cost of this work and ensures that the Web Services conform to the contractual definition laid down by the CDL document that is in use by the existing suppliers.

In some cases the initial document may need to be modified to add a new partner. However, the new document may introduce problems into the design. Using CDL IDE plug-in, the document desgin can be checked for various properties. Thus, the designers may be able to statictaly detect problematic areas such as livelocks, deedlocks and leaks.

Requirements for variation 1

1. A CDL description may be used in the generation of code skeletons (for example, may be WS-BPEL, Java, C# or others).

2. A CDL description may be used in the generation of test messages.

3. A CDL tool may need the capability to check for correctness properties in order to ensure the robustness of an implementation.

3.1.2.3.2 Variation 2

A variation of this is that modified quotes from a buyer are an agreement to order based on the modified quote. Or the supplier may, in response to the buyer require that there to be more than one manufacturer that can meet the quote.

Requirements for variation 2

1. Needs the support of conditional paths.

2. Needs the support of sequence.