Towards a Unifying Theory for Web Services Composition

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Towards a Unifying Theory for Web Services
Composition
Ivan Lanese Computer Science Department University
of Bologna Italy
Manuel Mazzara Faculty of Computer Science Free
University of Bozen - Bolzano Italy
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Roadmap

• Service Oriented Computing
• Webp8 as a model for SOC
• How to compute process equivalence
• Conclusions

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Roadmap

• Service Oriented Computing
• Webp8 as a model for SOC
• How to compute process equivalence
• Conclusions

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Web services

• Set of technologies
• to facilitate B2B integration
• using a standard web-messaging infrastructure
• support Service Oriented Computing
• There is no web services revolution
• an evolution based on already existing Internet
  protocols

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Service Oriented Computing

• Emerging paradigm for distributed computing and
  e-business processing
• Based on web services composition
• Finds its origin in object-oriented and component
  computing
• Goal
• Enabling developers to build networks of
  integrated and collaborative applications,
  regardless of
• the platform where the application runs
• the used programming language

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Web services composition

• Allows to build complex services and business
  processes out of simpler ones
• Called web services composition or orchestration
• Web services composition transparent to the final
  consumer
• Many industrial proposals to deal with web
  services orchestration
• WSFL (IBM) graph-oriented
• XLANG (Microsoft) block-structured
• WS-BPEL (OASIS) tries to unify both

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WS-BPEL

• The most credited candidate to become a future
  standard for web services composition
• Workflow-based programming language
• Describes sophisticated business processes by
  orchestrating web services
• Allows for a mixture of block and
  graph-structured process models
• We are mostly interested in the first style
• It is expressive at the price of being complex

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Error handling

• An important aspect of web services orchestration
• Many things can go wrong in a web service
  execution
• Broken connections
• Server crashes
• Unsatisfied requirements
• Usually treated by transactions
• ACID transactions
• From database field
• If a transaction commits then its
• complete effect is granted
• If a transaction fails all its effects are
  annulled

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Long-running transactions

• ACID transactions not practical in SOC
• Web services are loosely coupled
• Transactions can last long-time (e.g., shipping
  good)
• Some actions cannot be simply undone (e.g.,
  message communication)
• Non ACID transactions are usually called
  long-running
• Based on the idea of compensations
• If a transaction fails then execute the
  compensation for it
• Should be explicitly programmed since
  application-dependent
• Send annul communication to partners
• Ask for refunding
• Mark order as no more valid

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Transactions in WS-BPEL

• WS-BPEL has three different mechanisms for error
  handling
• fault handling, compensation handling, event
  handling
• Able to deal with many possibilities
• Complex and difficult to use
• Informal and unclear documentation

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Towards formal methods

• Need of a formal framework
• To provide a clear and rigorous semantics
• To prove properties of web services composition
• We start from p-calculus
• Many languages (included WS-BPEL) claim being
  based on it (but the connection is not always
  evident)
• We add a transaction operator to easily deal with
  error handling

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Roadmap

• Service Oriented Computing
• Webp8 as a model for SOC
• How to compute process equivalence
• Conclusions

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Webp8

• Extension of asynchronous p-calculus
• To deal with asynchronous communications among
  loose coupled web services
• With an operator of transaction to deal with
  compensations
• Originally proposed as the timed language Webp
  Laneve, Zavattaro FOSSACS05
• We are interested in the untimed version

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Webp8 syntax
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The semantics

• We give a reduction semantics in Milner style
• Structural congruence reduction rules
• Weak barbed congruence as abstract semantics
• Barbs allow to observe basic process behaviours
• Weak to abstract from internal activities
• Congruence to ensure preservation under
  composition
• Two equivalent web services remain equivalent
  also when composed in two complex business
  processes

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Structural congruence

• Standard rules for parallel composition and
  restriction

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Structural congruence

• Standard rules for parallel composition and
  restriction
• Commit of a workunit

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Structural congruence

• Standard rules for parallel composition and
  restriction
• Commit of a workunit
• Flattening of workunit nesting different e.g.
  w.r.t. c-join

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Structural congruence

• Standard rules for parallel composition and
  restriction
• Commit of a workunit
• Flattening of workunit nesting different e.g.
  w.r.t. c-join
• Messages can float away

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Reduction rules

• Standard rules for communication

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Reduction rules

• Standard rules for communication
• Rule for transaction abort
• only if P contains only (and at least one) inputs

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Weak barbed congruence

• Barbs P ? x if P contains a message with subject
  x
• Weak reduction P ? Q if P has a sequence of
  reductions leading to Q
• Weak barb P ? x if P ? P? x
• Barbed bisimulation relation S such that P S Q
  implies
• If P ? x then Q ? x
• If P ? P then Q ? Q and P S Q
• Barbed congruence largest barbed bisimulation
  that is also a congruence

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Properties of barbed congruence

• Good abstraction of process behaviour
• Preserved by composition
• Problem difficult to compute
• Requires quantification over all contexts
• Solution using an auxiliary labeled semantics
  that implies barbed congruence and is easier to
  compute

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Roadmap

• Service Oriented Computing
• Webp8 as a model for SOC
• How to compute process equivalence
• Conclusions

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Labeled semantics

• Most rules as in standard late p-calculus
• Special rules to deal with transactions
• Predicate inp(P) true if P contains at least an
  input
• Function xtr(P) extracts messages and
  transactions inside P

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Deriving a suitable abstract semantics

• We use weak asynchronous bisimilarity
• Labels must be matched but
• An action µ can be matched by a sequence tt µ
  tt (weak)
• An input can be matched by a t which leaves the
  message available (asynchronous)
• Weak asynchronous bisimilarity is not a
  congruence
• Closed bisimilarity closed under inp and xtr
• Closed bisimilarity is a congruence
• Closed bisimilarity implies barbed congruence
• Useful tool to prove behavioural equivalence

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Example compensations reducibility
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Example compensations reducibility

• Compensations can always be reduced to a single
  output
• if x,y fresh
• Proved using closed bisimulation

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Example compensations reducibility

• Compensations can always be reduced to a single
  output
• if x,y fresh
• Proved using closed bisimulation
• The following relation is a closed bisimulation
• Note property not valid in a strong or timed
  setting

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Other applications
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Other applications

• Properties of web services composition can be
  proved
• Decoupling of a service body
• Webp8 has been used to formalize a simplification
  of BPEL recovery framework Lucchi, Mazzara JLAP

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Roadmap

• Service Oriented Computing
• Webp8 as a model for SOC
• How to compute process equivalence
• Conclusions

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Conclusions

• Webp8 is a suitable formal framework
• For analyzing web services orchestration
• For reasoning about transactions
• Able to treat WS-BPEL
• Labeled semantics makes it practically usable to
  prove properties of web services
• Next step exploiting it via tools

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Related work

• Webp extension of Webp8 with time
• Similar semantic properties but different aim
• Allows to use timeouts
• C-join extends join instead of p
• The nesting of transactions matters
• When transactions interact are merged
• Compensating CSP extends CSP
• Analyze the composition of compensations
• No synchronization nor mobility

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End of talk

• Thanks!