toward a process model for gi service composition

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26 June 2003 U. Einspanier, M. Lutz, I. Simonis, K. Senkler, A. Sliwinski

Toward a Process Model for GI Service Composition

Udo Einspanier,Michael Lutz, Ingo Simonis, Kristian Senkler, Adam Sliwinski

Münsteraner GI-Tage

26-27 June 2003, Münster

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26 June 2003 U. Einspanier, M. Lutz, I. Simonis, K. Senkler, A. Sliwinski 2

Overview

• Motivation

• OGC and ISO RM-ODP

• State of the art in Web Service Composition XPDL BPEL4WS DAML-S

• Comparison & Conclusion

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Motivation

• Composability greatest value of (GI) web services Service Composition is a „hot topic“

• Concepts for GI service composition have several deficits, but...

... there are a number of approaches outside the GI domain

• Goal: Compare these approaches to OGC/ISO approach and point out possible connections

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ISO RM-ODP

• Specifies: concepts and framework for the description of

distributed systems characteristics that qualify a distributed system as

“open”

• Objective: development of standards that allow distributed services in a heterogeneous environment

• Division of an ODP system into 5 viewpoints

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ISO RM-ODP Viewpoints

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OpenGIS and ISO 19100

• RM-ODP only provides the „big picture“

• Specification of geospatial processing components is the objective of OGC & ISO 19100

• concepts service interface operation service chain workflow

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3 types of service chaining

• User defined (transparent) chaining

• Workflow-managed (translucent) chaining

• Aggregate service (opaque chaining)

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Limitations

• No uniform model to integrate web services into higher level architectures or business processes

• No descriptive language to define a chain and rules or execution constraints

• Only weak approaches to ensure „semantic interoperability“

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XML Based Process Definition Language (XPDL)

• XPDL is a graph-structured process definition language

• XPDL describes a process definition in terms of what is to be done, when it has to be done, under what conditions, and by whom or what

• ‘activity’ is the key concept of an XPDL process definition

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XPDL – Language Details

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What about Web Services?

• An external reference can be defined that points to an application, e. g. a web service

• Mature metamodel

• Lacks crucial concepts for building processes on web service architectures

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BPEL4WS a.k.a BPEL

• XML-based process definition language released by IBM, Microsoft and BEA

• supersedes process definition languages XLANG and WSFL

• models the behaviour of web services in a business process interaction

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BPEL Concepts

• BPEL builds on top of WSDL „stateful extension“

• BPEL supports two kinds of business processes:

Business protocols specify the mutually visible message exchange behaviour without revealing internal behaviour.

Executable business processes model actual behaviour of participant in a business interaction.

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BPEL – Language Details

• A BPEL process has three main parts:

partners (i.e. either a service the process invokes or those that invoke the process),

activities (i.e. an operation in a business process),

containers (provide means to store messages that constitute the state of the business process).

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BPEL – Information Flow

• Control flow is handled via “service links”: interaction with each partner occurs through web

service interfaces; the structure of the relationship at the interface

level is encapsulated in service links.

• Data flow is handled by containers.

• Message flow is handled by three types of activities: receive, reply, invoke

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DAML-based Web Service Ontology (DAML-S)

• Both an ontology of and language for describing services

• Goal: Enable automatic invocation, execution monitoring, discovery and composition of web services

• Service description consists of service profile what it requires/provides service model how it works service grounding how it can be accessed

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DAML-S – Language Details

Processinputpreconditionconditional outputconditional effect

Profile hasProfilehasProcess

Atomic Process

hasGrounding

Simple Process

realizedByrealizes

Composite ProcesscomputedInputcomputedOutputcomputedEffectcomputedPreconditioninvocable

expandcollapse

• atomic processescan be directlyinvoked (WSDLgrounding)

• composite processescan be decomposedinto other processes

• simple processes are used as views on atomic or composite processes for planning and reasoning

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Comparison of Concepts of OGC and ISO RM-ODP

• necessary for integration into OGC/ISO architecture

• lexical comparison

• based on core concepts XPDL: workflow process activity, transition

information, workflow process definition BPEL: process, activity DAML-S: simple, composite and atomic process

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Comparison – a first approximation

RM-ODP OGC

XPDL

workflow process activity activity operation

transition information action transformation

workflow process definition ? workflow

BPEL

process chain of actions translucent / opaque service chain

activity action –

DAML-S

simple process activity opaque service chain

composite process chain of actions service chain

atomic process activity operation

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Conclusions & Future Research

• OGC work currently lacks crucial concepts that facilitate service composition

• There are approaches outside the GI domain that could compensate these limitations (e.g. XPDL, BPEL, DAML-S)

• A Comparison of concepts used in these approaches to those used by OGC is vital, but difficult

• Comparison has to be improved go beyond entity level properties and relationships take viewpoint-specific concepts into

account

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Thank you!

Questions?

• http://www.meanings.de

• X-Border

• DALI

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