Distributed Orchestration Versus Choreography: The FOCAS Approach

Web service orchestration is popular because the application logic is defined from a central and unique point of view, but it suffers from scalability issues. In choreography, the application is expressed as a direct communication between services without any central actor, making it scalable but also difficult to specify and implement. In this paper we present FOCAS, in which the application is described as a classic service orchestration extended by annotations expressing where activities, either atomic or composite, are to be executed. FOCAS analyzes the orchestration model and its distribution annotations and transforms the orchestration into a number of sub-orchestrations to be deployed on a set of distributed choreography servers, and then, deploys and executes the application. This approach seemingly fills the gap between “pure” orchestration (a single control server), and “pure” choreography (a server per service). The paper shows how FOCAS transforms a simple orchestration into a distributed one, fitting the distribution needs of the company, and also shows how choreography servers can be implemented using traditional orchestration engines.