SCA is not (yet) part of J2EE; it is a custom extension which is currently only available in WPS. SCA is a way of modeling a service interface, the separation between the two parts of an SOA application. The service interface specifies what the provider (SOA-SP) has to implement and what the coordinator (SOA-SC) can invoke. By modeling the SOA-SPs as each being an implementation of a service interface, and by modeling the SOA-SCs as each being a client that invokes a service interface, a designer can more easily develop a high level design of his SOA composite app, focusing on the services involved and deferring the implementation decisions until later.
An SCA service interface is a fell-fledged interface, ultimately realized as a WSDL document or as a Java interface.
The service provider implementations in an SCA are called service components. A service component implementation can be a:
- Java object (a POJO or SLSB (How to Implement a Service in J2EE))
- BPEL business process
- human task
- business state machine
- set of business rules
Because they are all service components, initially you just model it as a service component to be used by service clients; later, you decide how to implement it. Perhaps it starts out as a human task, but later you automate it as a Java object or business process. Through it all, the service component interface stays the same.
The parameters passed into a service component operation and the result value passed out are modeled as service data objects (SDOs). SCA and SDOs enable a service and its data to be modeled in an implementation-independent way, one which may later be implemented using a variety of technologies, the main choices currently being either a Web services provider or a Java class.
Here are several resources where you can learn more about SCA:
- Building SOA solutions with the Service Component Architecture -- Part 1
- Using Web services with Service Component Architecture for interoperability (podcast)
- WebSphere Process Server: IBM's new foundation for SOA
- IBM Education Assistant: IBM WebSphere Process Server and WebSphere Integration Developer, V6.0: Service Component Architecture
- The advantages of SOA (podcast)