1 Synchronizability of Conversations Among Web Services Xiang Fu, Tevfik Bultan, Jianwen Su Abstract We present a framework for analyzing interactions among web services that communicate with asynchronous messages. We model the interactions among the peers participating to a composite web service as conversations, the global sequences of messages exchanged among the peers. This naturally leads to the following model checking problem: given an LTL property and a composite web service, do the conversations generated by the composite web service satisfy the property? We show that asynchronous messaging leads to state space explosion for bounded message queues and undecidability of the model checking problem for unbounded message queues. We propose a technique called synchronizability analysis to tackle this problem. If a composite web service is synchronizable, its conversation set remains the same when asynchronous communication is replaced with synchronous communication. We give a set of sufficient conditions that guarantee synchronizability and that can be checked statically. Based on our synchronizability results, we show that a large class of composite web services with unbounded message queues can be verified completely using a finite state model checker such as Spin. We also show that synchronizability analysis can be used to check realizability of top-down conversation specifications and we contrast the conversation model with the message sequence charts. We integrated synchronizability analysis to a tool we developed for analyzing composite web services. Index Terms web services, asynchronous communication, conversations, model checking, verification, synchronizability, re- alizability I. I NTRODUCTION Browser-based web accessible software systems have been extremely successful in electronic commerce, es- pecially for business-to-consumer applications. However, the diffi culty of integrating business processes across heterogeneous platforms has been a major hurdle in extending this success to business-to-business applications. Web services address this problem by providing a framework for integration and interoperability of web accessible software applications regardless of implementation platforms and across boundaries of business entities [3], [13], [27], [33]. Since communicating web services can be deployed on different locations using different implementation plat- forms, agreeing on a set of standards for data transmission and service descriptions is clearly very important. Fig. 1 displays a collection of most frequently used standards for web services where Extensible Markup Language (XML) [12] forms the foundation. Web services interact with each other by exchanging messages in the XML format. XML Schema [44] provides the type system for XML messages and Simple Object Access Protocol (SOAP) [38] is a standard communication protocol for transmitting XML messages. Each web service has to publish its invocation interface, e.g., network address, ports, functions provided, and the expected XML message format to invoke the service, using the Web Services Description Language (WSDL) [43]. Although a WSDL specifi cation defi nes the public interface of a web service, it does not provide any information about its behavior. Behavioral descriptions of web services are defi ned using higher level standards such as Business Process Execution Language for Web Services (BPEL) [7]. BPEL specifi cations can be used to defi ne behavioral interfaces of web services. The Web Service Choreography Description Language (WS-CDL) [42] is used to specify the interactions among multiple web services. The specifi cations of web services are registered in a Universal Description, Discovery and Integration (UDDI) [40] registry, which allows registered web services to be discovered by other services. Web service development based on these standards is supported by different implementation platforms such as .Net [29] This work is supported in part by NSF grants CCR-0341365, IIS-0101134, and IIS-9817432. X. Fu is with the Georgia Southwestern State University. T. Bultan and J. Su are with the University of California, Santa Barbara. Email: xfu@canes.gsw.edu, {bultan,su}@cs.ucsb.edu