Service Choreography and Orchestration with Conversations

Tevfik BultanDepartment of Computer Science

University of California, Santa Barbarabultan@cs.ucsb.edu

http://www.cs.ucsb.edu/~bultan

Acknowledgements

• Joint work with

– Xiang Fu, Hofstra University

– Jianwen Su, University of California, Santa Barbara

– Aysu Betin Can, Middle East Technical University

• [Bultan, Fu, Hull, Su, WWW’03] Conversation specification • [Fu, Bultan, Su, CIAA’03, TCS’04] Conversation protocols, realizability• [Fu, Bultan, Su WWW’04, TSE’05] Analyzing interacting BPEL

processes, realizability• [Fu, Bultan, Su CAV’04] Web Service Analysis Tool (WSAT)• [Betin Can, Bultan, Fu WWW’05] Peer controller pattern for modular

interaction analysis

Web Services

• The World Wide Web Consortium (W3C) defines a Web service as – "a software system designed to support interoperable machine-to-

machine interaction over a network”

• The basic architecture

Service Requester

Service Provider

Service Broker

RegisterSearch

Request

Response

Web Services Standards Stack

Data

Type

Service

Registry

Protocol

Universal Description, Discovery & Integration (UDDI)

Web Services Description Language (WSDL)

Simple Object Access Protocol (SOAP)

XML Schema (XSD)

Extensible Markup Language (XML)

Service Requester

Service Provider

Service Broker

RegisterSearch

Request

Response

SOAP

WSDLWSDL

UDDI

Web Services Characteristics/Goals

• Interoperability– Platform independent (.NET, J2EE)– Service interactions across organizational boundaries

• Loose coupling– Standardized data transmission via XML– Interaction based on standardized interfaces such as WSDL

• Communication via messages– Synchronous and asynchronous messaging

Basic Usage of Web Services

• What we have so far supports basic client/server style interactions

• Example: Amazon E-Commerce Web Service (AWS-ECS) • AWS-ECS WSDL specification lists 40 operations that provide differing

ways of browsing Amazon’s product database such as– ItemSearch, CartCreate, CartAdd, CartModify, CartGet, CartClear

• Based on the AWS-ECS WSDL specification one can implement clients that interact with AWS-ECS

Service Requester

Service Provider

Request

Response

SOAP

WSDL

Client Server

Composing Services

• Can this framework support more than basic client/server style interactions?

• Can we compose a set of services to construct a new service?

• For example:– If we are building a bookstore service, we may want to use both

Amazon’s service and Barnes & Noble’s service in order to get better prices

• Another (well-known) example:– A travel agency service that uses other services (such as flight

reservation, hotel reservation, and car rental services) to help customers book their trips

Composing Services

Two dimensions:

1. Define an executable process that interacts with existing services and executes them in a particular order and combines the results to achieve a new goal• Orchestration: From atomic services to stateful services

2. Specify how the individual services should interact with each other. Find or construct individual services that follow this interaction specification• Choreography: Global specification of interactions among

services

Orchestration vs. Choreography

• Orchestration: Central control of the behavior of a distributed system• Like a conductor conducting an orchestra• Conductor is in charge during the performance

• Orchestration specifies an executable process, identifying when and how that process should interact with other services– Orchestration is used to specify the control flow of a composite web

service (as opposed to an atomic web service that does not interact with any other service)

Orchestration vs. Choreography

• Choreography: Specification of the behavior of a distributed system without centralized control• Choreographer specifies the behavior of the dancing team• Choreographer is not present during the execution

• A choreography specifies how the services should interact– It specifies the legal sequences of messages exchanged among

individual services (peers)– It is not necessarily executable

• A choreography can be realized by writing an orchestration for each peer involved in the choreography– Choreography as global behavior specification– Orchestration as local behavior specification that realizes the global

specification

Orchestration with WS-BPEL

• Web Services Business Process Execution Language (WS-BPEL) is an orchestration language

• A WS-BPEL specification describes the execution logic using basic and structured activities– Basic activities: RECEIVE, REPLY, INVOKE, ASSIGN, THROW, TERMINATE, WAIT,

EMPTY RECEIVE, REPLY, INVOKE

– Structured activities: SEQUENCE, SWITCH, WHILE, PICK, FLOW, SCOPE, COMPENSATE

• WS-BPEL supports messaging (RECEIVE, REPLY, INVOKE) and multi-threading (FLOW)

Choreography with WS-CDL

• Web Services Choreography Description Language (WS-CDL)

• WS-CDL specifications describe ``peer-to-peer collaborations of Web Services participants by defining, from a global viewpoint, their common and complementary observable behavior; where ordered message exchanges result in accomplishing a common business goal.''

• A WS-CDL specification describes the interaction ordering among a set of peers using basic and structured activities– Basic activities: INTERACTION, PERFORM, ASSIGN, SILENT ACTION, NO ACTION

– Structured activities: SEQUENCE, PARALLEL, CHOICE, PICK, FLOW, SCOPE, COMPENSATE

Web Services Standards Stack

Data

Type

Service Orchestration

Protocol

Web Services Business Process Execution Language (WS-BPEL)

Web Services Description Language (WSDL)

Simple Object Access Protocol (SOAP)

XML Schema (XSD)

Extensible Markup Language (XML)

AtomicService

AtomicService

OrchestratedService

SOAP

WSDL

WSDL

Choreography Web Services Choreography Description Language (WS-CDL)

WS-BPEL

OrchestratedService

WS-BPEL

SOAP

SOAP

SOAP

SOAPWS-CDL

Asynchronous Messages

• Sender does not have to wait for the receiver– Message is inserted to a message queue– Messaging platform guarantees the delivery of the message

• Why support asynchronous messaging?– Otherwise the sender has to block and wait for the receiver – Sender may not need any data to be returned– If the sender needs some data to be returned, it should wait when it

needs to use that data– Asynchronous messaging can alleviate the latency of message

transmission through the Internet– Asynchronous messaging can prevent sender from blocking if the

receiver service is temporarily unavailable• Rather then creating a thread to handle the send, use

asynchronous messaging

Outline

• Motivation: Web Services• Conversations• Realizability• Synchronizability • Web Service Analysis Tool• An Application (Reality Check)• Conclusions

Going to Lunch at UCSB

• Before Xiang left UCSB, Xiang, Jianwen and I were using the following protocol for going to lunch:

– Sometime around noon one of us would call another one by phone and tell him where and when we would meet for lunch.

– The receiver of this first call would call the remaining peer and pass the information.

• Let’s call this protocol the First Caller Decides (FCD) protocol.

• At the time we did not have answering machines or voicemail!

FCD Protocol Scenarios

• Possible scenario

1. Tevfik calls Jianwen with the decision of where and when to eat

2. Jianwen calls Xiang and passes the information• Another scenario

1. Jianwen calls Tevfik with the decision of where and when to eat

2. Tevfik calls Xiang and passes the information• Yet another scenario

1. Tevfik calls Xiang with the decision of where and when to eat• Maybe Jianwen also calls Xiang at the same time with a

different decision. But the phone is busy.• Jianwen keeps calling. But Xiang is not going to answer

because according to the protocol the next thing Xiang has to do is call Jianwen.

2. Xiang calls Jianwen and passes the information

FCD Protocol: Tevfik’s Behavior

Tevfik calls Jianwen with the lunch decision

Let’s look at all possible behaviors of Tevfik based on the FCD protocol

Tevfik is hungry

Tevfik calls Xiang with the lunch decision

Tevfik receives a call from Jianwen passing him the

lunch decision

Tevfik receives a call from Xiang passing him the

lunch decisionTevfik receives a call from Xiang telling him the lunch decision that Tevfik has to pass to

Jianwen

FCD Protocol: Tevfik’s Behavior

!T->J(D)

!T->X(D)

?J->T(P)

?X->T(P)

?X->T(D)?J->T(D)

!T->J(P)!T->X(P)

T->J(D) Tevfik calls Jianwen with the lunch decision

Message Labels:

! send

? receiveJ->X(P)

Jianwen calls Xiang to pass the decision

!T->J(D)

?X->T(D)

!T->J(D)

?J->T(D)

!T->X(P)

Tevfik

!T->J(P)

?J->T(P)

?X->T(P)

!X->J(D)

?T->X(D)

!X->T(D)

?J->X(D)

!X->T(P)

Xiang

!X->J(P)

?J->X(P)

?T->X(P)

!J->T(D)

?X->J(D)

!J->X(D)

?T->J(D)

!J->X(P)

Jianwen

!J->T(P)

?T->J(P)

?X->J(P)

State machines for the FCD Protocol

• Three state machines characterizing the behaviors of Tevfik, Xiang and Jianwen according to the FCD protocol

FCD Protocol Has Voicemail Problems

• When the university installed a voicemail system FCD protocol started causing problems– We were showing up at different restaurants at different times!

• Example scenario: – Tevfik calls Xiang with the lunch decision – Jianwen also calls Xiang with the lunch decision

• The phone is busy (Xiang is talking to Tevfik) so Jianwen leaves a message

– Xiang calls Jianwen passing the lunch decision• Jianwen does not answer (he already left for lunch) so Xiang

leaves a message– Jianwen shows up at a different restaurant!

• Message sequence is: T->X(D) J->X(D) X->J(P)– The messages J->X(D) and X->J(P) are never consumed

• This scenario is not possible without voicemail!

A Different Lunch Protocol

• To fix this problem, Jianwen suggested that we change our lunch protocol as follows:

– As the most senior researcher among us Jianwen would make the first call to either Xiang or Tevfik and tell when and where we would meet for lunch.

– Then, the receiver of this call would pass the information to the other peer.

• Let’s call this protocol the Jianwen Decides (JD) protocol

?X->T(P)?J->T(D)

!T->X(P)

Tevfik XiangJianwen

?T->X(P)?J->X(D)

!X->T(P)

!J->T(D) !J->X(D)

State machines for the JD Protocol

• JD protocol works fine with voicemail!

Conversations

• The FCD and JD protocols specify a set of conversations – A conversation is the sequence of messages generated during an

execution of the protocol

• We can specify the set of conversations without showing how the peers implement them– we call such a specification a conversation protocol

T->J(D)

T->X(D)

X->J(P)

X->T(D) X->J(D)J->T(D)

J->X(D)

J->X(P)T->J(P) J->T(P)

T->X(P)

X->T(P)

FCD Protocol

J->T(D) J->X(D)

T->X(P) X->T(P)

JD Protocol

FCD and JD Conversation Protocols

Conversation set: { T->X(D) X->J(P), T->J(D) J->X(P), X->T(D) T->J(P), X->J(D) J->T(P), J->T(D) T->X(P),

J->X(D)X->T(P) }

Conversation set: { J->T(D) T->X(P),

J->X(D) X->T(P)}

Observations & Questions

• The implementation of the FCD protocol behaves differently with synchronous and asynchronous communication whereas the implementation of the JD protocol behaves the same. – Can we find a way to identify such implementations?

• The implementation of the FCD protocol does not obey the FCD protocol if asynchronous communication is used whereas the implementation of the JD protocol obeys the JD protocol even if asynchronous communication used.– Given a conversation protocol can we figure out if there is an

implementation which generates the same conversation set?

Conversations, Choreography, Orchestration

• A conversation protocol is a choreography specification– A conversation set corresponds to a choreography– A conversation set can be specified using a choreography

language such as WS-CDL– One can translate WS-CDL specifications to conversation protocols

• Peer state machines are orchestrations– A peer state machine can be specified using an orchestration

language such as WS-BPEL– One can translate WS-BPEL specifications to peer state machines

A Model for Composite Web Services

T->X(P)

X->T(P)

J->X(D)J->T(D)

Peer T

Peer J

Peer X

• A composite web service consists of– a finite set of peers

• Lunch example: T, X, J– and a finite set of messages

• Lunch example (JD protocol): J->T(D), T->X(P), J->X(D), X->T(P)

Communication Model

• We assume that the messages among the peers are exchanged using reliable and asynchronous messaging– FIFO and unbounded message queues

• This model is similar to existing messaging platforms such as – JMS (Java Message Service)– Java API for XML messaging (JAXM)– MSMQ (Microsoft Message Queuing Service)

J->T(D)Peer J Peer TJ->T(D)

Conversations

• Record the messages in the order they are sent

Generated conversation:

• A conversation is a sequence of messages generated during an execution

T->X(P)

J->T(D)

Peer T

Peer J

Peer X

T->X(P)J->T(D)

Properties of Conversations

• The notion of conversation enables us to reason about temporal properties of the composite web services

• LTL framework extends naturally to conversations– LTL temporal operators

X (neXt), U (Until), G (Globally), F (Future)– Atomic properties

Predicates on message classes (or contents)

Example: G ( payment F receipt )

• Model checking problem: Given an LTL property, does the conversation set satisfy the property?

Bottom-Up vs. Top-Down

Bottom-up approach• Specify the behavior of each peer

– For example using an orchestration language such as WS-BPEL• The global communication behavior (conversation set) is implicitly

defined based on the composed behavior of the peers• Global communication behavior is hard to understand and analyze

Top-down approach• Specify the global communication behavior (conversation set) explicitly

as a protocol– For example using a choreography language such as WS-CDL

• Ensure that the conversations generated by the peers obey the protocol

ConversationProtocol GF(T->X(P) X->T(P))

? LTL property

InputQueue

...Virtual Watcher?

LTL property

Peer T Peer XPeer J

J->T(D) J->X(D)

T->X(P) X->T(P)

GF(T->X(P) X->T(P))

!J->T(D)

!J->X(D)

?X->T(P)

?J->T(D)

!T->X(P)

?T->X(P)?J->X(D)

!X->T(P)

Top-Down vs. Bottom-Up

Conversation Protocols

• Conversation Protocol: – An automaton that accepts the desired conversation set

• A conversation protocol is a contract agreed by all peers– Each peer must act according to the protocol

• For reactive protocols with infinite message sequences use:– Büchi automata which accept infinite strings

• For specifying message contents, use:– Guarded automata– Guards are constraints on the message contents

Synthesize Peer Implementations

• Conversation protocol specifies the global communication behavior– How do we implement the peers?

• How do we obtain the contracts that peers have to obey from the global contract specified by the conversation protocol?

• Project the global protocol to each peer– By dropping unrelated messages for each peer

Question

If this equality holds the conversation protocol is realizable

• The JD protocol is realizable • The FCD protocol is not realizable

Are there conditions which ensure the equivalence?

Conversations generated by the projected services

Conversations specified by the conversation protocol

?

Outline

• Motivation: Web Services• Formalizing Conversations • Realizability• Synchronizability• Web Service Analysis Tool• An Application (Reality Check)• Conclusions

Realizability Problem

• Not all conversation protocols are realizable!

AB: m1

CD: m2

Conversation protocol

Conversation “m2 m1m2 m1” will also be generated by all peer implementations which follow the protocol

!m1 ?m1 !m2 ?m2

Peer A Peer B Peer C Peer D

Projection of the conversation protocol to the peers

Another Unrealizable Protocol

m3

m1

m2

m2 m1 m3

m1

m2

m3AB: m1BA: m2

AC: m3

BA: m2

AB: m1

A

B

C

m1m2

m3

Watcher

A B

C

Generated conversation:

B A, C

Realizability Conditions

Three sufficient conditions for realizability (no message content) • Lossless join

– Conversation set should be equivalent to the join of its projections to each peer

• Synchronous compatible– When the projections are composed synchronously, there should

not be a state where a peer is ready to send a message while the corresponding receiver is not ready to receive

• Autonomous– At any state, each peer should be able to do only one of the

following: send, receive or terminate

(a peer can still choose among multiple messages)

Realizability Conditions

AB: m1

CD: m2

AB: m1BA: m2

AC: m3

BA: m2

AB: m1

• Following protocols fail one of the three conditions but satisfy the other two

Not lossless join

Not autonomous

AB: m1

CA: m2

Not synchronous compatible

Outline

• Motivation: Web Services• Formalizing Conversations • Realizability• Synchronizability• Web Service Analysis Tool• An Application (Reality Check)• Conclusions

Bottom-Up Approach

• We know that analyzing conversations of composite web services is difficult due to asynchronous communication– Model checking for conversation properties is undecidable even for

finite state peers

• The question is:– Can we identify the composite web services where asynchronous

communication does not create a problem?• We call such compositions synchronizable

• The implementation of the JD protocol is synchronizable • The implementation of the FCD protocol is not synchronizable

Three Examples, Example 1

requester server

!r2

?a1 ?a2

!e

!r1

• Conversation set is regular: (Conversation set is regular: (rr11aa11 | | rr22aa22)* )* ee

• During all executions the message queues are bounded

r1, r2

a1, a2

e ?r1

!a1 !a2

?r2

?e

Example 2

• Conversation set is not regularConversation set is not regular• Queues are not bounded

requester server

!r2

?a1 ?a2

!e

!r1

r1, r2

a1, a2

e ?r1

!a1 !a2

?r2

?e

Example 3

• Conversation set is regular: (Conversation set is regular: (rr11 | | rr22 | | rara)* )* ee

• Queues are not bounded

requester server

!r2

?a !r

!e!r1

r1, r2

a1, a2

e

?r1 ?r2

?e

?r !a

State Spaces of the Three Examples

0

200

400

600

800

1000

1200

1400

1600

1 3 5 7 9 11 13

Example 1

Example 2

Example 3

queue length

# o

f st

ates

in

th

ou

san

ds

• Verification of Examples 2 and 3 are difficult even if we bound the queue length

• How can we distinguish Examples 1 and 3 (with regular conversation sets) from 2?– Synchronizability Analysis

Synchronizability Analysis

• A composite web service is synchronizable if its conversation set does not change – when asynchronous communication is replaced with synchronous

communication

• If a composite web service is synchronizable we can check the properties about its conversations using synchronous communication semantics – For finite state peers this is a finite state model checking problem

Synchronizability Analysis

Sufficient conditions for synchronizability:• A composite web service is synchronizable, if it satisfies the

synchronous compatible and autonomous conditions

• Connection between realizability and synchronizability:– A conversation protocol is realizable if its projections to peers are

synchronizable and the protocol itself satisfies the lossless join condition

Outline

• Motivation: Web Services• Formalizing Conversations • Realizability• Synchronizability• Web Service Analysis Tool• An Application (Reality Check)• Conclusions

BPEL to

GFSAGuardedautomata

GFSA to Promela (bounded queue)

BPEL

WebServices

Promela

SynchronizabilityAnalysis

GFSA to Promela(synchronous

communication)

IntermediateRepresentation

ConversationProtocol

Front End

Realizability Analysis

Guardedautomaton

skip

GFSAparser

success

fail

GFSA to Promela(single process,

no communication)

success

fail

Analysis Back End

(bottom-up)

(top-down)

Verification Languages

Web Service Analysis Tool (WSAT)

Are These Conditions Too Restrictive?

Problem Set Size Pass?Source Name #msg #states #trans.

ISSTA’04 SAS 9 12 15 yes

IBM

Conv.

Support

Project

CvSetup 4 4 4 yesMetaConv 4 4 6 no

Chat 2 4 5 yesBuy 5 5 6 yes

Haggle 8 5 8 noAMAB 8 10 15 yes

BPEL

spec

shipping 2 3 3 yes

Loan 6 6 6 yes

Auction 9 9 10 yesCollaxa.

com

StarLoan 6 7 7 yesCauction 5 7 6 yes

Outline

• Motivation: Web Services• Formalizing Conversations • Realizability• Synchronizability• Web Service Analysis Tool• An Application (Reality Check)• Conclusions

Checking Service Implementations

• People write web service implementations using programming languages such as Java, C#, etc.– Then automatically generate

specifications (such as WSDL)

• Synchronizability analysis works on state machine models

• How do we generate the state machines from a given Java implementation?

SynchronizabilityAnalysis

Checking Service

ImplementationsWritten In Java

Design for Verification Approach

1. Use of design patterns that facilitate automated verification

2. Use stateful, behavioral interfaces which isolate the behavior and enable modular verification

3. Use an assume-guarantee style modular verification strategy that separates verification of the behavior from the verification of the conformance to the interface specifications

4. Use a generic model checking technique for interface verification

5. Use domain specific and specialized verification techniques for behavior verification

Peer Controller Pattern• Eases the development of web services

• Uses Java API for XML messaging (JAXM)– Asynchronous communication among peers

• Supported by a modular verification technique– Behavior Verification: Checks properties of conversations of a web

service composed of multiple peers • assuming that peers behave according to their interfaces

– Interface Verification: Checks if each peer behaves according to its interface

ApplicationThread

CommunicationInterface

StateMachine

Communicator

CommunicationController

PeerServlet

ThreadContainersessionId

Peer Controller Pattern

used at runtimeused at interface verificationused both times

Red Bordered classes are the ones the user has to implement

CompositeService Peer

StateMachine

Promela

JavaPath Finder

Spin

PeerCode

InterfaceVerification

Verification Framework

ThreadThreadPeer

StateMachines

WSAT

SynchronizabilityAnalysis Conversation

Verification

PromelaTranslation

InterfaceMachine

Pee

r 1

Pee

r 2

Pee

r n

Pee

r 1

InterfaceMachine

Pee

r 2

InterfaceMachine

Pee

r n

Peer Modular Interface Verification

Composite Service

Conversation Behavior

Modular Conversation Verification

Modular Design / Modular Verificationin

terf

ace

inte

rfac

e

inte

rfac

e

Behavior Verification

• Uses WSAT for synchronizability analysis• Uses Spin model checker for conversation verification

– Automated translation to Promela using WSAT

• Spin is a finite state model checker– We have to bound the channel sizes, session numbers, message

types

• Synchronizability analysis – Enables us to verify web services efficiently by replacing

communication channels with channels of size 0 (i.e., synchronous communication)

– The verification results hold for unbounded channels

Interface Verification

• If the call sequence to the Communicator class is accepted by the state machine specifying the interface, then the peer implementation conforms to the behavior in the contract

• Uses JPF model checker

• Isolated check of individual peer implementations – CommunicationController is replaced with CommunicatorInterface– Drivers simulating other peers are automatically generated

• State Space reduction– Usage of stubs – Each session is independent

• just need to check each peer for one session

Examples

• We used this approach to implement several simple web services– Travel agency– Loan approver– Product ordering

• Performance of both interface and behavior verification were reasonable

Interface Verification

Interface Verification with JPF for Loan Approver

Threads T (sec) M (MB)

Customer 8.86 3.84

Loan Approver

9.65 4.7

Risk Assesor

8.15 3.64

Behavior Verification

• Sample Property: Whenever a request with a small amount is sent, eventually an approval message accepting the loan request will be sent.

• Loan Approval system has 154 reachable states – Queue lengths never exceed 1

• Behavior verification used less than 1 sec and 1.49 MB

• SPIN requires restricted domains– Have to bound the channel sizes bounded message queues

• In general there is no guarantee these results will hold for other queue sizes

– Using synchronizability analysis we use queues of size 0 and still guarantee that the verification results hold for unbounded queues!

Related Work

• Specification approaches that are similar to conversation protocols

– [Parunak ICMAS 96] Visualizing agent conversations: Using enhanced Dooley graphs for agent design and analysis.

– [Hanson, Nandi, Kumaran EDOCC’02] Conversation support for business process integration

Related Work

• Message Sequence Charts (MSC)

– [Alur, Etassami, Yannakakis ICSE’00, ICALP’01] Realizability of MSCs and MSC Graphs

– [Uchitel, Kramer, Magee ACM TOSEM 04] Implied Scenarios in MSCs

Related Work

• Verification of web services– Petri Nets

• [Narayanan, McIlraith WWW’02] Simulation, verification, composition of web services using a Petri net model

– Process Algebras• [Foster, Uchitel, Magee, Kramer ASE’03] Using MSC to model

BPEL web services which are translated to labeled transition systems and verified using model checking

– Model Checking Tools• [Nakajima ICWE’04] Model checking Web Service Flow

Language specifications using SPIN– …

• See the survey on BPEL verification– [Van Breugel, Koshkina 06] Models and Verification of BPEL

http://www.cse.yorku.ca/~franck/research/drafts/

Related Work

• Modeling Choreography & Orchestration

– Process algebras, synchronous communication• [Busi, Gorrieri, Guidi, Lucchi, Zavattaro ICSOC’05]• [Qiu, Zhao, Chao, Yang WWW’07]

– Activity based (rather than message based) approaches• [Berardi, Calvanese, DeGiacomo, Hull, Mecella VLDB’05]

Current and Future Work

• Dealing with message content and data manipulations– Symbolic analysis and/or automated abstraction

• [Fu, Bultan, Su ICWS’04, JWSR] presents some symbolic analysis algorithms but not implemented

• [Bultan, Fu, SOCA 2007] Modeling conversations with Collaboration diagrams

• [Yu, Wang, Gupta, Bultan FSE’08] Modular verification of interacting BPEL processes

• Analyzing realizability of Singularity channel contracts

• Generating extra messages to achieve choreography conformance

• Analyzing choreographies with dynamically created channels

Conclusions

Applying the results I presented in practice can happen in two ways:

• Developing tools for languages that support these concepts – Such as WS-CDL, WS-BPEL– This is the approach we used in building WSAT

• Using design patterns that enable extraction of analyzable models– Such as the peer controller pattern– Using the peer controller pattern, we can isolate the interaction

behavior, leading to efficient analysis of the interaction behavior– However, interface verification is very hard

Conclusions

• Choreography specification and analysis is an interesting problem

• If people really start building systems based on choreography specifications then the problems I discussed will need to be addressed

– However, it is not clear to me if the Web services framework will achieve wide adoption

– It is possible that WSDL and SOAP will be the only commonly used ones (i.e., the simple client/server model)

• Still, choreography specification and analysis problem is likely to resurface in distributed systems in some other context– For example, see Singularity channel contracts

THE END