A Mediated Approach towards Web Service Choreography

Michael Stollberg, Dumitru Roman, Juan Miguel Gomez DERI – Digital Enterprise Research Institute

{first.last}@deri.org

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 ?

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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 ServiceImplementation(not of interest in Web Service Description)

Choreography --- Interfaces ---

Capability

functional description

WS

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

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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

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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

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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

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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

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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

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WSMO Mediators Structure

WSMO Mediator

uses a Mediation Service via

Source Component

Source Component

TargetComponent 1 .. n

1

Mediation Services

- as a Goal - directly- optionally incl. Mediation

under construction: Semantic Mediation

Techniques for all levels

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if we have all this …

mediated collaboration of multiple Semantic Web Service

C WS 2

CWS 1

C

WS n

WS 3

C

1.2.

M 1

M 17

4.

3.

M 5

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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