Abstract
In this article, we present an overview of our recent works on enhancing the trustworthiness of Web services coordination for business activities and transactions. The approach is based on what we call application-aware Byzantine fault tolerance. We argue that it is impractical to apply general-purpose Byzantine fault tolerance algorithms for such systems in a straightforward manner. Instead, by exploiting the application semantics, much lighter weight solutions can be designed to enhance intrusion tolerance, and hence the trustworthiness of systems that require Web services coordination.
TopBackground
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.
Key Terms in this Chapter
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.
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.
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.
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.
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.
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.
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 Tolerance: A replication-based technique used to ensure high availability of an application subject to Byzantine fault.