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The aim of this document is to provide a conceptual model and a formalism for describing choreographies in WSMO.
The conceptual model of WSMO differentiates between Chreography and Choreography Interface.
WSMO Choreography is a collaboration between some WSMO participants in order to achieve a certain goal[1]. It describes the tasks (simple or complex actions) [2] that WSMO participants need to perform in order to achieve that goal, the interactions associated to the executions of these tasks, and the relations between tasks. As [Dijkman & Dumas, 2004] observe, the reasons for defining a choreography are twofold:
WSMO Choreography Interface describes the behaviour of a particular WSMO service in its communication with its environment (other parties). It corresponds to the communication tasks and their associated interactions which the service performs and the relations between these communication tasks. Choreography Interface provides a point of entry for a service into a choreography, allowing it to check whether it can participate or not in a choreography. The reason for specifying a choreography interface is to make clear what a WSMO service can do and/or expects in terms of communication tasks and their associated interactions.
The rest of the document deals only with the WSMO Choregraphy interface; WSMO Choreography will be considered in a later stage of the development of WSMO Choreography Interface. Section 2 presents the conceptual model for WSMO Choregraphy Interface, Section 3 aims to provide a formal representation for the conceptual model, Section 4 provides a grounding for the conceptual model, Section 5 aims to provide a comparison to existing approaches to choreography modeling, and Section 6 concludes this document and and points out further directions.
The conceptual model of the WSMO choreography interface is presented in Figure 1.
| Figure 1. Choreography Interface Meta-model |
 |
A Communication Pattern is a pattern for communication with the service and constraints the order of messages that are exchanged during the communication by the service.
A Communication Pattern consists of a set of guarded actions. A guarded action is an action for which its execution is constrained by a condition.
An action represents the basic unit of behavior of a WSMO service. It can be of two types:
Proposals like [Hull et all., 2003] consider also the possibility for non specified actions to appear in between send and receive actions to allow the modeling of the services’ internal state transitions. We adopt here the idea from [Frølund & Govindarajan, 2003] to do not model the internal transitions of the service. This separation of concerns provides the basis for inter-operability amongst services while preserving loose coupling.
Conditions are boolean expression which specify, when evaluated, wheter or not their associated actions can be performed. An action is always guarded by a condition. We write <c,a> to denote the fact that action a can be performed when condition c is true.
We clasify Communication Patterns in Simple Communication Patterns and Complex Communication Patterns.
Simple Communication Patterns are:
Complex Communication Patterns can be derived by composing simple communication patterns. We adopt here the ordering constructs of [WS-CDL, 2004] and [Frølund & Govindarajan, 2003]:
In terms of synchronization, Communication patterns can be either Synchronous, i.e the sender sends a message and then stops processing while it waits for receiving a correlated message (it might influence further processing on the sender’s side), or Asynchronous, i.e the sender continues to process while waiting for receiving a correlated message. In/Out and Out/In can be either Synchronous or Asynchronous, In, Out are Asynchronous. However, this is a problem of implementation of the patterns and doesn’t directly concern our model.
A Communication Pattern is said to be instantiatiated if:
As a content language for messages, the WSML is used.
We also adopt the notion of a conversation history from [Frølund & Govindarajan, 2003] as the sequence of messages exchanged (or guarded actions executed) so far.
Formalism to be considered:
We provide grounding (i.e. show how the conceptual model can be coupled to existing technologies) to Web Services technologies.
Web service technologies provide a loosely coupled infrastructure that enables cross-enterprise integration. They consist of multiple layers that, when stacked together, form the basis for a standard mechanism for discovering, describing, and invoking the functionality provided by a standalone web service:
As the description of the choreography interface is part of a service description, the focus here will be on how a complex behavior of a WSMO service can make use of the service layer (WSDL) of the web services stack. For this, the following equivalences are defined:
Approaches to be considered:
This document presented a conceptual model for modeling WSMO Choreography Interfaces and proposed a grounding to web services technologies . Depending on the acceptance of this model, further versions of this document will contain a mapping to some existing formalism and comparison to existing approaches.
[Benatallah et all.,2003] Benatallah, B., Sheng, Q., & Dumas, M. (2003): The Self-Serv environment for web services composition. IEEE Internet Computing, 7(1), 40–48.
[Bultan et all., 2003] Bultan, T., Fu, X., Hull, R., & Su, J. (2003, May): Conversation specification: A new approach to design and analysis of e-service composition. Proc. of the Intl. Conf. on the World Wide Web (WWW) (pp.403–410). Budapest, Hungary: ACM Press.
[Dijkman & Dumas, 2004] R. Dijkman and M. Dumas: Service-oriented Design: A Multi-viewpoint Approach. CTIT Technical Report Series No. 04-09, Centre for Telematics and Information Technology, University of Twente, The Netherlands, February 2004.
[Hull et all., 2003] Richard Hull, Michael Benedikt, Vassilis Christophides, and Jianwen Su: E-services: A look behind the curtain. In Proceedings of ACM Symposium on Principles of Database Systems. ACM, 2003.
[FIPA ACL, 2002] http://www.fipa.org/specs/fipa00037/SC00037J.html
[Frølund & Govindarajan, 2003] Frølund, S., & Govindarajan, K.: cl: A language for formally defining web services interactions (Technical Report HPL-2003-208). Hewlett-Packard.
[WS-CDL, 2003] http://www.w3.org/TR/2004/WD-ws-cdl-10-20040427/
The work is funded by the European Commission under the projects DIP, Knowledge Web, SEKT, SWWS, and Esperonto; by Science Foundation Ireland under the DERI-Lion project; and by the Vienna city government under the CoOperate program.
The editors would like to thank to all the members of the WSMO working group for their advice and input into this document.
[1] Note that this goal is not not necessarily equivalent to the way WSMO-Standard defines a goal, as it involves more than one party when defining the goal.
[2] The tasks described in a choreography are only tasks that involve communication between parties involved.
$Date: Saturday 17 July 2004 - 15:19:02$