The need to aggregate or combine small services into larger services is core to a service oriented architecture. In many cases, a single service will act as a front-end to many small services. There are a variety of methods for combining services, including:

• Pipe and Filters – Direct the output of one service into the input of another service. (Topology = point-to-point)
• Orchestrations – Utilize a high-level scripting language to control the sequence and flow of service execution. (Topology = hub & spoke)

Each service composition mechanism will utilize a variety of message exchange patterns and provide its own message correlation facility.