A Mediated Approach towards Web Service Choreography Michael Stollberg, Dumitru Roman, Juan Miguel Gomez DERI – Digital Enterprise Research Institute Semantic Web Services: Preparing to Meet the World of Business Applications (ISWC 2004 Workshop)

WSMO A Mediated Approach towards Web Service Choreography 2 Problem Setting promise of Web Services as “next generation EAI technology” –a functionality for integrating something –can be arbitrarily combined –have a seamless usage interface semantic technologies allow more sophisticated support for this than the current Web Service tech stack => what is needed and how shall it work ?

WSMO A Mediated Approach towards Web Service Choreography 3 Overview – what we aim at The Choreography of a Web Service defines its usage interface A Global Model defines how several Web Service interact to achieve a common business goal Usage of Mediators to resolve possibly occurring mismatches between the Choreographies of Web Services participating in a Global Model  A Semantic Web Service solution for the Web Service promise

WSMO A Mediated Approach towards Web Service Choreography 4 WSMO Web Service Description Web Service Implementation (not of interest in Web Service Description) Choreography --- Interfaces --- Capability functional description WS - Advertising of Web Service - Support for WS Discovery Interaction Interface for consuming WS - Messages - External Visible Behavior - ‘Grounding’ Realization of WS by using other Web Services - Functional decomposition - WS Composition Non-functional Properties Core + WS-specific - complete item description - quality aspects - Web Service Management WS Orchestration

WSMO A Mediated Approach towards Web Service Choreography 5 Choreography in WSMO “Interface of Web Service for client-service interaction when consuming the Web Service” Aspects: 1) Choreography: Interaction Interface of a Web Service 2) Global Model: multiple Web Services collaboration 3) Choreography Mediation: establish valid Choreographies

WSMO A Mediated Approach towards Web Service Choreography 6 Choreography Aspects External Visible Behavior –those aspects of the workflow of a Web Service where User Interaction is required –described by process / workflow constructs Communication Structure –messages sent and received –their order (messages are related to activities) Choreography Grounding –concrete communication technology for interaction –choreography related errors (e.g. input wrong, message timeout, etc.) related to WS behavior descriptions, e.g. WSCI, BPEL4WS under construction

WSMO A Mediated Approach towards Web Service Choreography 7 Choreography Formal Model Formalization needed to allow operations / mediation on WSMO Choreography Interfaces Formal Basis: Abstract State Machines (ASM) –ASM consists of Constants and Transition Rules; all Transition Rules are executed simultaneously according to conditions –Generic, expressive technology to model behavior –Overcome the ‘Frame Problem’ Usage: –A general ASM defines the formal semantics of the WSMO Choreography Description Language –Choreography Specifications are mapped into ASM representation to allow protocol level mediation operations under construction

WSMO A Mediated Approach towards Web Service Choreography 8 Global Model Aspects Definition a multi-participant contract that describes from a global viewpoint the common observable behavior of the collaborating WS participants (this is the definition of “choreography” from the W3C WS Choreography working group) Main Description Notions –collaboration participants –information interchanged –information-driven interaction rules (channel, workunit,…) this is basically what WS CDL is about under construction

WSMO A Mediated Approach towards Web Service Choreography 9 Mediation Aspects Heterogeneity … –Mismatches on structural / semantic / conceptual / level –Occur between different components that shall interoperate –Especially in distributed & open environments like the Internet Concept of Mediation (Wiederhold, 94): –Mediators as components that resolve mismatches –Declarative Approach: Semantic description of resources ‘Intelligent’ mechanisms that resolve mismatches independent of content –Mediation cannot be fully automated (integration decision) Levels of Mediation within Semantic Web Services (WSMF): – Data Level: mediate heterogeneous Data Sources – Protocol Level: mediate heterogeneous Communication Patterns – Process Level: mediate heterogeneous Business Processes

WSMO A Mediated Approach towards Web Service Choreography 10 WSMO Mediators Structure WSMO Mediator uses a Mediation Service via Source Component Source Component Target Component 1.. n 1 Mediation Services - as a Goal - directly - optionally incl. Mediation under construction: Semantic Mediation Techniques for all levels

WSMO A Mediated Approach towards Web Service Choreography 11 if we have all this … mediated collaboration of multiple Semantic Web Service C WS 2 C WS 1 C WS n WS 3 C M 1 M M 5

WSMO A Mediated Approach towards Web Service Choreography 12 Status & Outlook What we have so far … –WSMO supports this idea by its design –general approach seems to be fitting –Especially in distributed & open environments like the Internet What we are currently working on … –WSMO Choreography General Approach Formal Model –WSMO Mediators – general specification –EU-IP proposal that targets these issues Future Issues –Global Models / relation to Choreography –Mediation techniques for data, protocol, and business mediation level