Realizabilityof Interaction Models

Gero Decker

ZEUS Workshop, Stuttgart

3 Mar 2009

Realizability of Interaction Models | Gero Decker | 24. April 2006

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Agenda

1. Why use the interaction modeling style (rather than the interconnection modeling style)?

2. When are choreographies realizable, i.e. there exist participants that collectively realize the specified behavior?

3. What impact do branching structures have on realizability checking?

Realizability of Interaction Models | Gero Decker | 24. April 2006

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Process Choreographies

■ Inter-organizational integration requires interaction contracts

□ Defined document structures / message types

□ Defined interaction behavior: constraints and obligations

“A choreography defines the sequence and conditions under which multiple cooperating independent agents exchange messages in order to perform a task to achieve a goal state.” [W3C Glossary]

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Realizability of Interaction Models | Gero Decker | 24. April 2006

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Choreography Modeling

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Realizability of Interaction Models | Gero Decker | 24. April 2006

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Two Modeling Styles

■ Local ordering constraints

■ Distinction between control flow and message flow

■ MSC, BPMN

■ BPEL4Chor

■ Global ordering constraints

■ Interactions as atomic building blocks

■ UML Comm. Diagrams

■ BPSS, WS-CDL

■ Let’s Dance, iBPMN

Interconnection Models Interaction Models

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Interconnection Models

incompatibleloops

incompatiblebranching structures

incompatible sequence flow

abstraction level =messages

Realizability of Interaction Models | Gero Decker | 24. April 2006

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Interaction Models

atomicinteractions

decisionownership

global controlflow dependencies

complexinteractionsglobal loops

Realizability of Interaction Models | Gero Decker | 24. April 2006

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Interaction Petri Nets

Realizability of Interaction Models | Gero Decker | 24. April 2006

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Interaction Petri Nets

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Realizability of Interaction Models | Gero Decker | 24. April 2006

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Realizability

A B

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Realizability of Interaction Models | Gero Decker | 24. April 2006

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Realizability – Two Dimensions

■ Communication model

□ Synchronous (blocking send vs. failing)

□ Asynchronous (FIFO queue vs. standard buffer)

■ “Similarity” of behavior

□ Complete behavior

□ Subset of behavior (proper termination vs. reachability constraints)

□ Equivalence notion (trace vs. bi-simulation vs. …)

Realizability of Interaction Models | Gero Decker | 24. April 2006

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Realizability – Existing Work

■ Fu, Bultan & Su: Realizability (2004)

□ Projection: remove transitions + minimal deterministic automaton

□ Asynchronous, complete behavior (trace equivalence)

■ Decker & Weske: Local Enforceability (2007)

□ Projection: remove transitions + keep branching structure

□ Synchronous, subset of behavior (pattern-based + “blocked” transitions)

■ Decker, Barros, Kraft & Lohmann: Desynchronizability (2008)

□ Requires realizability for the synchronous case

□ Weak termination for the asynchronous case

Realizability of Interaction Models | Gero Decker | 24. April 2006

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Fu, Bultan & Su: Realizability (2004)

Traces = { [(A,τ,A),(A,x,B),(B,y,C)], [(A,τ,A),(A,x,B)]}

Traces = { [(A,τ,A),(A,x,B),(B,y,C)], [(A,τ,A),(A,x,B)]}

(A,τ,A)

(A,τ,A)

(A,x,B)

(A,x,B)

(B,y,C)

(A,τ,A) (A,x,B) (B,y,C)

(A,x,B)(A,τ,A) (B,y,A)

(A,x,B) (B,y,C)

Realizability of Interaction Models | Gero Decker | 24. April 2006

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Now: Focus on Branching Structures

(A,τ,A)

(A,τ,A)

(A,x,B)

(A,x,B)

(B,y,C)

(B,y,C)(A,x,B) (B,y,C)

(A,τ,A)

(A,τ,A)

(A,x,B)

(A,x,B)

(A,τ,A)

(A,τ,A)

(A,x,B)

(A,x,B)

(B,y,C)

(B,y,C)

(A,x,B)

(A,x,B)

(B,y,C)

Realizability of Interaction Models | Gero Decker | 24. April 2006

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Role Projection: Preserving Branching Structures

(A,τ,A)

(A,τ,A)

(A,x,B)

(A,x,B)

(B,y,C)

s1

s2 s3 s4

s5s6

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s3 s6(A,x,B) s4 (B,y,C)

Realizability of Interaction Models | Gero Decker | 24. April 2006

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Role Projection: Preserving Branching Structures

(A,τ,A)

(A,τ,A)

(A,x,B)

(A,x,B)

s1

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(A,y,B)

(A,z,B)

s3

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s3 s6(A,x,B)

s4

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(A,y,B)

(A,z,B)

Realizability of Interaction Models | Gero Decker | 24. April 2006

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Role Projection: Preserving Branching Structures

(A,τ,A)

(A,τ,A)

(A,x,B)

(A,x,B)

s1

s2

s7

s8

s5

(B,z,A)

s3

(B,τ,B)

(B,τ,B)

s4

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(B,τ,B)

(B,τ,B)

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s11

(B,z,A)

(B,y,A)

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s1

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s4 s6

(A,x,B)s2

s8

(A,τ,A)

(A,τ,A) s9

s10 s11

(A,x,B)

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(B,z,A)

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s5(B,y,A)

(B,y,A)

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(B,y,A)

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(B,z,A)

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(B,z,A)(B,y,A)

n1

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Realizability of Interaction Models | Gero Decker | 24. April 2006

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Open Issue: Proofs

■ Define Full Realizability: There exist role behavior models the composition of which are bi-similar to the choreography

■ Theorem: A choreography is fully realizable if and only if the composition of the role projections is bi-similar to the choreography

■ Proof (idea): Show that the role behavior models must look exactly like they look if their composition is to be bi-similar

Realizability of Interaction Models | Gero Decker | 24. April 2006

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Open Issue: Subset of Behavior

■ If a choreography is not fully realizable then

□ Remove transitions from the composition that are not allowed by to the choreography

□ Imply which transitions in the role behavior models must be removed to achieve this removal in the composition

□ Try to find (reduced) role behavior models the composition of which shows a valid subset of the behavior of the choreography

Realizability of Interaction Models | Gero Decker | 24. April 2006

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Conclusion

■ Interaction modeling imposes challenge of realizability

■ Different flavors of realizability

□ Communication model / similarity of behavior

■ Role projection as “best guess” for local behavior

■ Branching structures important if local choices to be considered

■ Role projection must reflect branching structures

■ Unifying theory still missing Niels?

Realizability of Interaction Models | Gero Decker | 24. April 2006

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Questions?

■ Mail gero.decker@hpi.uni-potsdam.de

■ Tool http://oryx-editor.org

■ Blog http://bpmn.info