Supporting Adaptive Web-Service Orchestration with an Agent Conversation Framework Warren Blanchet, Eleni Stroulia, Renée Elio University of Alberta.

Supporting Adaptive Web-Service Orchestration with

an Agent Conversation Framework

Warren Blanchet,Eleni Stroulia, Renée Elio

University of Alberta

ICWS 2005, Orlando

Outline

Introduction & MotivationWorkflow & ConversationsWRABBIT: Workflow

reconfigurationCase studiesConclusion & Future work

ICWS 2005, Orlando

Web services

Pool of available servicesUser doesn’t care about

implementationCompose services

Use services together to provide higher-level service

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Change

Web services accommodate change…that doesn’t expose itself at the

interface levelWhat about rules on messages

exchanged?Compositions:

No peers: too centralizedPeers: hard to change messages

exchanged due to effect on others

ICWS 2005, Orlando

Outline

Introduction & MotivationWorkflow & ConversationsWRABBIT: Workflow

reconfigurationCase studiesConclusion & Future work

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Workflow & Conversations

Workflows are processes that produce work productsDistribution of abilities requires

cooperationCooperation requires

communicationCommunication requires shared

understanding

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Conversation model

Agent-communication communityAgents 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

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Web-services Workflow

Accomplishing work: web-servicesControlling 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

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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 handUse a monitoring proxy with an abstract BPEL

specification of the client’s communicationMerge 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

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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 piecesThe shared conversation model is a

subset of this workflow model

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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

Outline

Introduction & MotivationWorkflow & ConversationsWRABBIT: Workflow

reconfigurationCase studiesConclusion & Future work

ICWS 2005, Orlando

Introduction

WRABBIT:Workflow Reconfiguration with

Agent- and BPEL-Based Intercommunication Technology

Website:www.cs.ualberta.ca/~stroulia/

WRABBIT

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Requirements

WRABBIT agents need:declarative specifications for workflow

scripts (BPEL4WS + extra custom stuff)to execute these scriptsto recognize conversation failuresto determine where to obtain a correct

workflow script (if it is at fault)to retrieve the correct workflow script, if

necessaryto incorporate the new workflow script

into its operation

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WRABBIT Architecture

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WRABBIT Details: Messaging

WRABBIT agents use an ACL for a message transport layerThis allows agents to send out-of-band

information such as conversation failure signals (not-understood)

A straightforward mapping from WSDL operation types to mini-protocolsone-way: (inform)request-response: (inform)(request)

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WRABBIT Details: Workflow Script Execution

WRABBIT agents may be configured:to execute particular scripts in

response to incoming messagesto execute a script that satisfies some

criteriaWorkflow scripts inherit BPEL

featuresimportantly can be executable or

abstract

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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

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WRABBIT Details: Conversation Failure Detection

WRABBIT agents use the BPEL-derived workflow scripts to detect conversation failure

Failure categories:Invalid content: message payload

unfamiliarInvalid sender: the model does not allow

this sender for this messageMessage unexpected: the message is not

valid in the current context

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WRABBIT Details: Conversation Failure Recovery

WRABBIT agents first determine who is the authority for a given failureMay depend on failure type, workflow

script, etc.WRABBIT agents rely on a shared policy

to determine the authorityThen, agents exchange the files that

define the authoritative workflow script

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Outline

Introduction & MotivationWorkflow & ConversationsWRABBIT: Workflow

reconfigurationCase studiesConclusion & Future work

ICWS 2005, Orlando

Environment

Academic departmentAgents:

InstructorDepartment administration employee

Story:the Department agent dispenses

student transcriptsthe Instructor agent uses this service

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Missing Precondition Study

Change:To get a student transcript, you

now have to provide an authorization tokenA separate service of the Department

agent provides these tokens

The token is provided in a new message at the beginning of the agents’ interaction

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Missing Precondition Study (2)

The Department agent is updatedThe Instructor agent has notThe Instructor agent begins the

conversation with the first message (from its perspective) which is the second message from the Department agent’s viewNot-understood!

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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 retriesIt discovers it needs to satisfy the new

abstractnessIt does so with the authorization workflow,

and successfully obtains its transcript

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Missing Precondition Study Trace

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Outline

Introduction & MotivationWorkflow & ConversationsWRABBIT: Workflow

reconfigurationCase studiesConclusion & Future work

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Summary

WRABBITis an agent-based (peer-to-peer,

distributed) frameworkexecutes BPEL4WS web-service

compositionsidentifies and dynamically recovers

from conversation failureshas been tested with case studies

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Contributions

WRABBIT enables flexibilityService owners can change their

services without an involved migration process

Service ownership can be distributed without anarchy

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Future work

Better support for new script preconditions

Rollback/compensationNegotiation policies

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Questions?

[email protected]

www.cs.ualberta.ca/~stroulia/WRABBIT