To model system implementations, we define the language of Simple Processes. In this language, systems are defined in the classical style of giving a separate program for each process. Process programs use send and receive actions that need to match during execution in order to achieve a communication. We discuss how implementations of choreographies from the previous chapter can be written in terms of this language. We also formulate in our setting the key properties of parallelism, communication safety, and starvation-freedom, respectively: the capability of executing independent communications in any order; the property that processes never attempt to interact by performing incompatible actions; and the property that every running process eventually gets to act.

We study the properties of Recursive Choreographies and its related notion of EPP in depth. Endpoint projection is proven to guarantee choreography compliance and communication safety in this setting. Starvation-freedom does not hold anymore in general, due to the possibilities opened by general recursion in Recursive Choreographies, but it holds for the tail-recursive fragment of the language. Under some reasonable assumptions, the weaker property of deadlock-freedom holds for any implementation returned by EPP. That is, the EPP of a choreography never gets stuck.