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From Centralized Orchestration to De-centralized
Orchestration of Web Services
By Asif Iqbal
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Agenda

• Introduction
• What is a Web Service?
• What is Service Oriented Computation?
• What is Orchestration of Web Services?
• Why Orchestrate web services?
• BPEL (Business Process Execution Language)?
• Centralized Orchestration of Web Services
• Decentralized orchestration of Web Services
• Example 01 (Add two Numbers)?
• Rules
• Examples 01 (Apply above rule)?
• Example 02 (divide Two Numbers)?

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Agenda

• Example 02 (Apply above rule)?
• Related Work
• Conclusion

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Introduction

• What is a Web Service?
• Web Service is defined by the W3C as "a software
  system designed to support interoperable Machine
  to Machine interaction over a network."
• What is Service Oriented Architecture?
• SOA Magazine define SOA as
• SOA is an approach to delivering software
  capabilities through building blocks called
  "services."

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Introduction

• What is Orchestration of Web Services?
•  Web service orchestration is a task of
  combining and linking existing web services to
  create new business process.
• Why Orchestrate Web Services?
• The right music played by the right
  instruments at the right time in the right
  combination thats good orchestration, said
  Leonard Bernstein, the great conductor and
  composer.
• Like a music composer, the Web Service
  composer has to select the necessary Web
  Services, to decide how they are going to
  interact with each other.

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BPEL (Business Process Execution language)

• Important technology of SOA
• BPEL is an XML-based language.
• A BPEL process consists of steps. Each step is
  called an activity.
• Enables easy and flexible orchestration of
  services into business processes
• Basic Activities the ones that do something
• Invoke (Invoking Web services)?
• Receive (Waiting for the request)?
• Assign (Manipulating data variables)?
• Structure Activities thet ones that organize
  basic activities without doing anything by
  themselves
• Sequence (set of activities that will be
  invoked in an ordered sequence)?
• Flow (a set of activities that will be invoked
  in parallel)?
• Switch (for implementing branches)?

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Orchestration of Web Services

• Two Methods for orchestration of web services
• Centralized Orchestration of Web Services
• Decentralized orchestration of Web Services

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Centralized Orchestration of WS
A composed web service interacts with set of
published web services and satisfy a given
composition requirements.
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Centralized Orchestration of WS

• All control is at one place
• Services are isolated from each other
• Every service will communicate with composed
  service
• All data is transferred between various
  services via centralized coordinator

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Centralized Orchestration of WS

• This approach (centralized composition of web
  services) gives many advantages like
• Efficient Monitoring
• Fault Handling
• Maintenance
• But also have some cons like
• The composed service is responsible for
  co-ordination of all data and control flow
  between services and hence become performance
  bottleneck.
• All data is transferred between various
  services via centralized composed service, this
  lead to unnecessary traffic on net work.
• Single point of failure

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Decentralized Orchestration of WS

• In de-centralized orchestration BPEL program
  is partitioned into independent sub programs that
  interacts with each other without any centralized
  control
• In decentralized composition a wrapper
  (derived from composed service) is placed on each
  service
• Wrapper communicates with other wrappers and
  forms P2P interconnection. Wrapper contains
  activities.

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Decentralized Orchestration of WS
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Example 01

• User
• Send two numbers
• Receive Addition Result
• Add Two Numbers
• Receive two numbers
• Apply operation
• Send back addition result

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Example 01 (Centralized Orchestration)
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Rules

• Pick interface activities of each service from
  centralized composition
• Activities will be partitioned and placed in a
  wrapper of service for which picked activities
  were interface in centralized composition
• For Assignment activities, check dependency of
  assignment activities
• These activities will be a part of wrapper
  that have activities on which assignment
  activities are dependent
• In case of conditional behavior (if)?
• Check dependency of condition
• This will be a part of wrapper that has
  activity/activities on which condition has data
  dependency.

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Rules

• In order to control Execution and to preserve
  control and data dependency of centralized
  specification, activities distributed in
  different wrappers must be linked.
• Add activities to transfer control from one
  wrapper to another
• To transfer control (Invoke)?
• To get control (receive)?

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Example 01 (Decentralized Orchestration)
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Example 02

• User
• Send request to divide two numbers
• Receive Result
• Divide
• Receive Request to divide two numbers
• Send back results of division

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Example 02 (Centralized Orchestration)
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Example 02 (Dcentralized Orchestration)
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Related Work

• 45 Focus on the Use of Program Dependence
  Graph (PDG) in partitioning of composite web
  service application for decentralized
  orchestration
• Code Partitioning Algorithm to partition a
  BPEL program represented as a PDG.
• Tool to partition composite web service
  specification and also generate WSDL for each of
  partitioned fragment.
• Yildiz and Godart work is based on
  Inter-Organizational processes

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Conclusion

• Decentralized Composition of web services
  offer benefits like
• There is no centralized coordinator which can
  be a potential bottleneck.
• Because of distributed wrappers network
  traffic is reduced and improves transfer time.
• Distributing the control improves concurrency.
• Also introduced some cons like
• Fault Handling more complex
• Increased complexity (P2P interconnection)?

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References
1 M Pistore, P Traverso and P Bertoli Automated
composition of web services by Planning in
Asynchronous Domains. 2 Annapaola Marconi,
Marco Pistore and Paolo Traverso Specifying data
fl0w Requirement for the automated composition of
web services 3 Ustun Yildiz , Claude Godart
Centralized vs Decentralized conversation based
Orchestration 4 Mangala , Satish and Vivek
Decentralized execution of composite web
services 5 G Cahfle , S Chandra, V Mann
Decentralized Orchestration of composite web
services
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