Web Services Choreography Description Language Overview 24th November2004 Steve Ross-Talbot Chief Scientist, Enigmatec Corporation Ltd Chair W3C Web Services Activity Co-chair W3C Web Services Choreography

Agenda What is Choreography? What is CDL? Why would I use the CDL? How would I use the CDL? What are the components of CDL? BPEL and CDL The CDL approach Summary

What is Choreography? A working group in W3C, that is tasked with defining a language for describing peer to peer interactions of services from a neutral perspective. Based on a formalized description of external observable behavior across domains Current status – Requirements document (published March 2004) – Model Overview document (published April 2004), – 1st Working Draft of the CDL specification (published April 2004), – Latest Working Draft of CDL (published Sept 2004) – Next draft will form the basis of a Last Call Working Draft

What is CDL? CDL is the Choreography Description Language It is a language that can be used to describe collaboration protocols of cooperating [Web] Service participants in which – Services act as peers – Interactions may be long-lived and stateful A CDL-based description is a multi-participant contract that describes, from a neutral or global viewpoint, the common observable behavior of the collaborating Service participants

Why would I use the CDL? You would use the CDL to create – More robust Services because they can be validated statically and at runtime against a choreography description, – To ensure effective interoperability of Services, which is guaranteed because Services will have to conform to a common behavioral multi-party contract specified in the CDL, – To reduce the cost of implementing Services by ensuring conformance to expected behaviour described in the CDL. This in turn can be used to guide and structure testing and so reduce the overall time to deployment of a Service. – To formally encode agreed multi-party business protocols such as fpML, FIX, SWIFT and TWIST so that those parties that use these protocols can be sure of conformance across parties.

How would I use the CDL? You would use the CDL through a validating design tool, perhaps as an Eclipse plug-in. You would describe your multi-party contract in terms of a global model and it would be rendered in CDL. You would use additional CDL-based tools to generate: – Skeletal code (in Java, C#, BPEL etc) that ensures behavior of a service conforms to the CDL description, – Test programs that generate appropriate messages based on the CDL description, You would use additional CDL-based tools to provide runtime verification of services against their expected behavior as defined in the CDL description. You would use CDL to describe existing multi-party protocol in terms of their behavior and import the necessary information types (data types i.e. fpML + CDL)

Components of the CDL Package – Information - data, – Participants - service locations that implement a set of roles, – Roles - service end points (WSDL), – Relationships - association between roles – Channel types – communication channel definitions – Choreography - the multi-party behavioral contract details

Components of the CDL (2) Choreography – Composable unit that describes the behaviour between the defined participants based on the roles that they play and the channels that they use to interact. Channel instances – Represents the communication 'pipe' that is established between two roles. The channel type, associated with the instance, may permit an instance of the channel to transfer other channel instances as valid messages (i.e. channel passing). Interaction – Describes a communication between two participants along a channel instance (e.g. req/resp, oneway req, etc).

Components of the CDL (3) Workunits – A workunit represents a grouping construct for a set of contained activities – A workunit can define a guard condition, to make these activities conditional – A workunit can express a repetition condition, to indicate whether the activities should be repeated – A workunit can be used for synchronization based the variables used in the guard condition. If variables used in the condition are not currently set, then the evaluation of the condition would suspend until all the information was available.

Components of the CDL (4) Activities – Sequence - describes a sequence of activities – Parallel - describes a parallel set of activities – Choice – describes a mutually exclusive set of activities – Perform - describes an enclosed choreography to be peformed – Variable assignment - described the assignment of variable information

Components of the CDL (5) Exceptions – a special kind of workunit associated with a choreography that is enabled when an exception is detected. Finalizers – A finalizer defines a set of activities. When a 'performed' choreography completes successfully, it can enable one or more finalizers. One of these finalizers can then be selected by the performing choreography to complete the work associated with the 'performed' choreography (e.g. Confirm or Cancel). This mechanism can be used to provide a typical compensation model, as well as support a range of coordination models.

Components of the CDL (6)

BPEL and CDL BPEL – Orchestration implies a centralized control mechanism. – Recursive Web Service Composition. – Executable language. – Requires Web Services. CDL – Choreography has no centralized control. Instead control is shared between domains. – Description language. – Does not need Web Services but is targeted to deliver over them. – Can be used to generate BPEL and so complimentary.

Approach Based on simple contract-like mechanisms – Deadlock-freedom (Kobayashi, 99, 00) – Liveness (Kobayashi, 01; Yoshida, et al, 02) – Security (Abadi et al; Cardelli and Gordon; Berger, Honda, Yoshida) – Resource management (Tofte; Kobayashi; Gordon and Dal Zillio; Yoshida, et al) – Race-condition detection (refs) Which are extensions to CCS/CSP and π-calulus (Milner)

Approach Contract Does roughly what client wants it to do Web service Implementation Behaviorial type Bisimulation approximation Process Mobile process calculi provided a natural candidate.

Why process calculi? CCS Petri Nets Lambda Turing Machines ResourcesParallelismCompositionalityCompletenessModel

Related work WFMC – Workflow Oasis BPEL TC – Business process over web services Agent technologies – Common use of pi-calculus Vertical standards – FIX, TWIST, fpML, SWIFT etc….

Summary The CDL is a language for describing observable behavior of multiple participants within the context of a global model Formally based on pi-calculus Useful outside of Web Services – Robustness, conformance, testing, verification Complimentary to many areas – BPEL, Agents, FIX, fpML, TWIST etc Moving to Last Call end of 2004