Web Services Orchestration and Choreography

Web Services Orchestration and Choreography I n a recent Computer Sciences Corporation survey , 1 senior information technology executives ranked " connecting to customers, suppliers, or partners electronically " as the top global IT management issue. Web services offer standards based mechanisms for addressing this issue. However, existing methods for creating business processes are not designed to work with cross-organizational components. Nor are these methods flexible enough to handle the technical interfaces that Web services introduce. The terms orchestration and choreography describe two aspects of creating business processes from composite Web services. The two terms overlap somewhat, but Figure 1 illustrates their relationship to each other at a high level. Orchestration refers to an executable business process that can interact with both internal and external Web services. The interactions occur at the message level. They include business logic and task execution order, and they can span applications and organizations to define a long-lived, transactional, multi-step process model. Orchestration always represents control from one party's perspective. This differs from choreography, which is more collaborative and allows each involved party to describe its part in the interaction. Choreography tracks the message sequences among multiple parties and sources—typically the public message exchanges that occur between Web ser-vices—rather than a specific business process that a single party executes. Proposed orchestration and choreography standards must meet several technical requirements for designing business processes that involve Web services. These requirements address both the language for describing the process workflow and the supporting infrastructure for running it. First, asynchronous service invocation is vital to achieving the reliability and scalability that today's IT environments require. The capability to invoke services concurrently can also enhance process performance. Implementing asynchronous Web services requires a mechanism to correlate requests with each other. Software architects commonly use correlation identifiers for this purpose. The process architecture must also provide a way to manage exceptions and transactional integrity. In addition to handling errors and time-out constraints, orchestrated Web services must ensure resource availability for long-running distributed transactions. Traditional ACID (atomicity, consistency, isolation, and durability) transactions are typically not sufficient for long-running, distributed transactions because they cannot lock resources in a transaction that runs over a long time. The notion of compensating transactions offers a way to undo an action if a process or user cancels it. With compensating transactions , each method exposes an undo operation that a transaction coordinator can invoke if necessary. …