Desynchronizable Choreographies

Non-desynchronizable Service Choreographies

Gero Decker1, Alistair Barros2,

Frank Michael Kraft3, Niels Lohmann4

1Hasso-Plattner-Institute, Potsdam, Germany2SAP Research, Brisbane, Australia3SAP AG, Walldorf, Germany4University of Rostock, Germany

Non-desynchronizable Service Choreographies | Gero Decker | 24. April 2006

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Agenda

■ Modeling service choreographies

■ Synchronous / asynchronous communication

■ Desynchronizability

■ Resolution strategies

■ Conclusion


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Service Choreographies

■ Inter-organizational integration requires interaction contracts

□ Defined set of interactions

□ Defined document structures / message types

□ Defined interaction behavior: constraints and obligations

■ Choreographies as behavioral specification for services

□ Starting point for adapting existing services

□ Starting point for implementing new services


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Send auction creation req.

Send details

Auction crea-tion confirm.

Sel

ler

Request details

Send creation confirmation

Auc

tioni

ng S

ervi

ce

Auction creation request

Trusted customer

no

yes

Send payment

Payment

Modeling ChoreographiesInterconnection Modeling


5

Send auction creation req.

Send details

Auction crea-tion confirm.

Sel

ler

Request details

Send creation confirmation

Auc

tioni

ng S

ervi

ce


Trusted customer

no

yes

Send payment

Payment

Modeling ChoreographiesInterconnection Modeling

incompatibleloops

definition of processinstantiation

incompatiblebranching structures

incompatible sequence flow

abstraction level =messages


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Seller

Auctioning Service

Auctioncreation confirm.


Trusted customer?

no

yes

Payment

Exchange details

Modeling ChoreographiesInteraction Modeling

atomicinteractions

decisionownership

global controlflow dependencies

complexinteractionsglobal loops

process instantiationunspecified


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Modeling Choreographies

■ BPMN, BPEL4Chor

■ Send and receive as separate steps (asynchronous)

■ Let’s Dance, WS-CDL

■ BPMN 2.0

■ Send and receive in one step (synchronous)

Interconnection ModelsInteraction Models

Place order

Send ack

Send paymentB

uyer

Sel

ler

Buyer

Seller

Order Order ack Payment


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Buyer

Seller

Purchase order

PO ack

PO cancel

PO cancel

PO ok

PO ok

Cancelled

Accepted


9


Buyer

Seller

Purchase order

PO ack

PO cancel

PO cancel

PO ok

PO ok

Cancelled

Accepted

Does this choreography make sense

in an asynchronous world?


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Modeling Choreographies Synchronous vs. Asynchronous

■ Assumes asynchronous communication

■ Assumes synchronous communication

Desynchronized NetInteraction Petri net

so ra sp

ro sa rp

B S

order

S B

order ack

B S

payment


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Studying the Desynchronized Net

S B

PO ack

S B

PO ok

B S

cancel

B S

PO req

S B

cancel

B S

PO ok

cancelPO ackPO req cancel PO ok PO ok

Deadlock


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Observable behavior model (role Z)

Interaction Model

Desynchronized Net

Observable behavior model (role A)

Desynchronizability

…

Role projection Role projection

Interconnection

■ Has final markings

■ Must weakly terminate

■ Must be realizable

■ Final markings are generated

■ Check for weak termination

B A

PO ack

B A

PO ok

A B

cancel

A B

PO req

B A

cancel

A B

PO ok

cancelPO ackPO req cancel PO ok PO ok


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DesynchronizabilityLocation of conflict transitions

■ The interaction model

□ weakly terminates and

□ is realizable

■ There exist choreography instances that end in final markings

■ Those transitions can be identified that lead to a deadlock

□ “Conflict transitions”


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ToolingDesynchronizability in IPNs

http://oryx-editor.org


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ToolingDesynchronizability in IPNs



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ToolingDesynchronizability in BPMN 2.0



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ToolingDesynchronizability in BPMN 2.0



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Resolution Strategies in Practice (1/2)

■ Not a solution: Removing the possibility of cancellation

■ Solution 1: Precedence

□ Agreement on precedence relationships at design-time

□ Assumption: the services will notice that conflicting messages are present

□ Predefined behavior upon conflicting messages

□ a. Singular Interaction Partner Precedence

□ b. Type-based Precedence between Multiple Interactions

□ c. Situation-based Precedence between Multiple Interactions


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Resolution Strategies in Practice (2/2)

■ Solution 2: Precedence with Decision

□ Precedence relationships define who has the right to decide on the outcome upon conflicting messages

■ Solution 3: Allowing Individual Decisions

□ Upon conflicting messages each service decides how to handle this and tells the other services about the decision

□ Main problem: contradicting decisions

■ Solution 4: Negotiation of Outcome

□ Upon conflicting messages the conversation is suspended and a negotiation phase starts


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Summary & Outlook

■ Interaction modeling eases choreography modeling (but hides the challenges of asynchronous communication)

■ Desynchronizability checking detects and locates errors

■ Resolution strategies have business impact must be chosen on a business level!

■ Future work

□ Formalization of resolution strategies

□ Declarative integration of resolution strategies


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Questions?

■ Mail [email protected]

■ Tool http://oryx-editor.org

■ Blog http://bpmn.info

Desynchronizable Choreographies

Technology

Transcript of Desynchronizable Choreographies