Toward a Process Model for GI Service Composition

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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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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

• 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
XPDL transition information action transformation
XPDL workflow process definition ? workflow
BPEL process chain of actions translucent / opaque service chain
BPEL activity action
DAML-S simple process activity opaque service chain
DAML-S composite process chain of actions service chain
DAML-S 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 intoaccount

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Thank you! Questions?

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