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Supporting Adaptive Web-Service Orchestration with an Agent Conversation Framework . Warren Blanchet, Eleni Stroulia, Renée Elio University of Alberta. Outline . Introduction & Motivation Workflow & Conversations WRABBIT: Workflow reconfiguration Case studies Conclusion & Future work.

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supporting adaptive web service orchestration with an agent conversation framework

Supporting Adaptive Web-Service Orchestration with an Agent Conversation Framework

Warren Blanchet,

Eleni Stroulia, Renée Elio

University of Alberta

outline
Outline
  • Introduction & Motivation
  • Workflow & Conversations
  • WRABBIT: Workflow reconfiguration
  • Case studies
  • Conclusion & Future work

ICWS 2005, Orlando

web services
Web services
  • Pool of available services
    • User doesn’t care about implementation
  • Compose services
    • Use services together to provide higher-level service

ICWS 2005, Orlando

change
Change
  • Web services accommodate change
    • …that doesn’t expose itself at the interface level
    • What about rules on messages exchanged?
  • Compositions:
    • No peers: too centralized
    • Peers: hard to change messages exchanged due to effect on others

ICWS 2005, Orlando

outline1
Outline
  • Introduction & Motivation
  • Workflow & Conversations
  • WRABBIT: Workflow reconfiguration
  • Case studies
  • Conclusion & Future work

ICWS 2005, Orlando

workflow conversations
Workflow & Conversations
  • Workflows are processes that produce work products
    • Distribution of abilities requires cooperation
    • Cooperation requires communication
    • Communication requires shared understanding

ICWS 2005, Orlando

conversation model
Conversation model
  • Agent-communication community
    • Agents have a shared conversation model
      • Defines the permissible messages at any point in a conversation
      • Deviation from this model is a conversation error, which will cause the conversation to fail
        • This is signaled by a ‘not-understood’ message
      • Conversation failure indicates the shared model is inconsistent: the parts shared do not match

ICWS 2005, Orlando

web services workflow
Web-services Workflow
  • Accomplishing work: web-services
  • Controlling flow between services
    • Equivalent to ‘composition’
    • BPEL4WS tailored for separation of work and flow for a given workflow participant
    • WS-CDL (forthcoming) means to verify correct information flow between participants

ICWS 2005, Orlando

aside on bpel based interaction
Aside on BPEL-based interaction
  • Suppose a composed service is implemented in BPEL, and has a non-trivial communication protocol
  • How to ensure the service client conforms?
    • Inspect by hand
    • Use a monitoring proxy with an abstract BPEL specification of the client’s communication
    • Merge the client’s abstract BPEL specification of the client’s communication into the client’s executable BPEL specification
    • Use an adapter pattern style executable BPEL client specification
    • Integrate the client’s abstract BPEL specification into the client’s workflow dynamically using constraint-based matching

ICWS 2005, Orlando

observations
Observations
  • Each participant in a workflow can be modeled in BPEL4WS
  • Each BPEL file specifies the messages exchanged with its partners
  • The shared workflow model is composed of these per-participant pieces
    • The shared conversation model is a subset of this workflow model

ICWS 2005, Orlando

slide11
Goal
  • Given:
    • Per-participant workflow model segments (which we call workflow scripts)
    • Conversation failure detection & signaling
  • Produce:
    • A system which can repair inconsistent workflow models (as signaled by conversation failures) automatically

ICWS 2005, Orlando

outline2
Outline
  • Introduction & Motivation
  • Workflow & Conversations
  • WRABBIT: Workflow reconfiguration
  • Case studies
  • Conclusion & Future work

ICWS 2005, Orlando

introduction
Introduction
  • WRABBIT:
    • Workflow Reconfiguration with Agent- and BPEL-Based Intercommunication Technology
  • Website:
    • www.cs.ualberta.ca/~stroulia/WRABBIT

ICWS 2005, Orlando

requirements
Requirements
  • WRABBIT agents need:
    • declarative specifications for workflow scripts (BPEL4WS + extra custom stuff)
    • to execute these scripts
    • to recognize conversation failures
    • to determine where to obtain a correct workflow script (if it is at fault)
    • to retrieve the correct workflow script, if necessary
    • to incorporate the new workflow script into its operation

ICWS 2005, Orlando

wrabbit architecture
WRABBIT Architecture

ICWS 2005, Orlando

wrabbit details messaging
WRABBIT Details: Messaging
  • WRABBIT agents use an ACL for a message transport layer
    • This allows agents to send out-of-band information such as conversation failure signals (not-understood)
    • A straightforward mapping from WSDL operation types to mini-protocols
      • one-way: (inform)
      • request-response: (inform)(request)

ICWS 2005, Orlando

wrabbit details workflow script execution
WRABBIT Details: Workflow Script Execution
  • WRABBIT agents may be configured:
    • to execute particular scripts in response to incoming messages
    • to execute a script that satisfies some criteria
  • Workflow scripts inherit BPEL features
    • importantly can be executable or abstract

ICWS 2005, Orlando

wrabbit details workflow script composition
WRABBIT Details: Workflow Script Composition
  • Why leave scripts abstract?
    • Loose coupling
    • Easy updating
  • WRABBIT agents compose abstract scripts automatically through means-end analysis
  • Others have done this more thoroughly, this work uses a very simplistic method, and assumes a better mechanism exists

ICWS 2005, Orlando

wrabbit details conversation failure detection
WRABBIT Details: Conversation Failure Detection
  • WRABBIT agents use the BPEL-derived workflow scripts to detect conversation failure
  • Failure categories:
    • Invalid content: message payload unfamiliar
    • Invalid sender: the model does not allow this sender for this message
    • Message unexpected: the message is not valid in the current context

ICWS 2005, Orlando

wrabbit details conversation failure recovery
WRABBIT Details: Conversation Failure Recovery
  • WRABBIT agents first determine who is the authority for a given failure
    • May depend on failure type, workflow script, etc.
    • WRABBIT agents rely on a shared policy to determine the authority
  • Then, agents exchange the files that define the authoritative workflow script

ICWS 2005, Orlando

outline3
Outline
  • Introduction & Motivation
  • Workflow & Conversations
  • WRABBIT: Workflow reconfiguration
  • Case studies
  • Conclusion & Future work

ICWS 2005, Orlando

environment
Environment
  • Academic department
  • Agents:
    • Instructor
    • Department administration employee
  • Story:
    • the Department agent dispenses student transcripts
    • the Instructor agent uses this service

ICWS 2005, Orlando

missing precondition study
Missing Precondition Study
  • Change:
    • To get a student transcript, you now have to provide an authorization token
      • A separate service of the Department agent provides these tokens
    • The token is provided in a new message at the beginning of the agents’ interaction

ICWS 2005, Orlando

missing precondition study 2
Missing Precondition Study (2)
  • The Department agent is updated
  • The Instructor agent has not
  • The Instructor agent begins the conversation with the first message (from its perspective) which is the second message from the Department agent’s view
    • Not-understood!

ICWS 2005, Orlando

missing precondition study 3
Missing Precondition Study (3)
  • The Department agent is the authority, so it does nothing
  • The Instructor gets the new script from the Department agent
  • The Instructor then retries
    • It discovers it needs to satisfy the new abstractness
    • It does so with the authorization workflow, and successfully obtains its transcript

ICWS 2005, Orlando

outline4
Outline
  • Introduction & Motivation
  • Workflow & Conversations
  • WRABBIT: Workflow reconfiguration
  • Case studies
  • Conclusion & Future work

ICWS 2005, Orlando

summary
Summary
  • WRABBIT
    • is an agent-based (peer-to-peer, distributed) framework
    • executes BPEL4WS web-service compositions
    • identifies and dynamically recovers from conversation failures
    • has been tested with case studies

ICWS 2005, Orlando

contributions
Contributions
  • WRABBIT enables flexibility
    • Service owners can change their services without an involved migration process
    • Service ownership can be distributed without anarchy

ICWS 2005, Orlando

future work
Future work
  • Better support for new script preconditions
  • Rollback/compensation
  • Negotiation policies

ICWS 2005, Orlando

questions
Questions?
  • blanchet@cs.ualberta.ca
  • www.cs.ualberta.ca/~stroulia/WRABBIT

ICWS 2005, Orlando