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Web Services Coordination
In this section, we provide an overview of Web services coordination and its applications for business transactions and activities.
Web services interactions are becoming more and more complex in structure and relationships. More complex means we need longer time to execute them, because of business latencies and user interactions. The Web Services Coordination specification (WS-Coordination) (Feingold & Jeyaraman, 2009) describes an extensible framework for plugging in protocols that coordinate the actions of Web services applications. Such coordination protocols can be used to support a variety of business applications, including those that require strict consistency and those that require agreement of a proper subset of the participants. The framework enables a Web service to create a context needed to propagate an activity to other Web services and to register for a particular coordination protocol.
There are two types of business transactions. One follows the traditional atomic transaction semantics, and the other is referred to as business activities, which implies that the atomicity property may be relaxed. The former is suitable for short transactions that require strong atomicity, such as a fund transfer transaction. The latter is more suitable long running transactions, such as those used in supply chain management. Based on WS-Coordination, two specifications, namely Web Service Atomic Transaction (WS-AtomicTransaction) (Little & Wilkinson, 2009) and Web Service Business Activity (WS-BusinessActivity) (Freund & Little, 2009), were standardized by OASIS to address the coordination needs for common types of business transactions.
Key Terms in this Chapter
Web Services: The Web services technology refers to the set of standards that enable automated machine-to-machine interactions over the Web. The core standards include XML, HTTP, SOAP, WSDL and UDDI.
Distributed System: A distributed system is a computer network system, shown to end users as a single machine but actually work with a set of independent computers connected.
Atomic Transaction: An atomic transaction in the context of Web services refers to a distributed transaction to be executed atomically. It should exhibit the atomicity, consistency, isolation and durability properties, just like a local transaction.
Business Activity: A business activity is usually long running and requires flexible outcomes. A Web service business activity must conform to the WS-BusinessActivity specification and adopts one of two two coordination types, Atomic-Outcome and Mixed-Outcome, and one of two coordination protocols, Business-Agreement-with-Participant-Completion and Business-Agreement-with-Coordinator-Completion. Either protocol can be used with either coordination type.
Byzantine Fault: It is used to model arbitrary fault. A Byzantine faulty process might send conflicting information to other processes to prevent them from reaching an agreement.
Replica Consistency: The states of the replicas of an application should remain to be identical at the end of the processing of each request. Replica consistency is necessary to mask a fault in some replicas.
Business Process Execution Language (BPEL): It is a computer language used to describe the actions and execution order of these actions within a business process. It is designed specifically for the Web services paradigm. A BPEL for Web services was standardized by OASIS.
Byzantine Fault Tolerance: A replication-based technique used to ensure high availability of an application subject to Byzantine fault.