Reference document

Orchestration Services
The Orchestration Services provide the means to coordinate the execution of multiple technical services in such a way that the coordinated whole of technical services appears as a single, aggregate technical service responding to a single individual request. Such an aggregation could be said to implement a business process that is characterized by the fact that it runs within own organization boundaries, with the own organization having full control over the execution of the process. Orchestration describes one particular component activity of the composition that oversees and directs the other component activities. An orchestration has one and only one direction activity. In a service-oriented software solution, the component services of an orchestration are software services performed by software programs. A service composition described by an orchestration is again a service itself and can be re-used in further compositions. An orchestration coordinates the other sub-services in a step-wise manner, i.e. it specifies the partial order of all the steps that each of the sub-services has to do in order to produce the desired outputs in a co-operative, joined way. An orchestration can be (and usually is) stateful as the coordination of sub-services immediately requires to keep track of states. The other sub-services which are controlled by the orchestration do usually not interact with each other directly and do not have to be stateful.
Composition Services