Composition of synchronous communicating systems

Communication is an essential element of modern software, yet programming and analysing communicating systems are difficult tasks. A reason for this difficulty is the lack of compositional mechanisms that preserve relevant communication properties. This problem has been recently addressed for the well-known model of communicating systems, that is sets of components consisting of finite-state machines capable of exchanging messages. Two communicating systems can be composed by selecting one component per system, and transforming both of them into coupled gateways connecting the two systems. More precisely, a gateway forwards a message received from within its system to the other gateway, which then delivers the message to the other system. Suitable compatibility conditions between gateways have been proved sufficient for this composition mechanism to preserve properties such as deadlock freedom for asynchronous as well as symmetric synchronous communications (where sender and receiver play the same part in determining which message to exchange). The present paper gives a comprehensive treatment of the case of synchronous communications. We consider both symmetric synchronous communications and asymmetric synchronous communications (where senders decide independently which message should be exchanged). The composition mechanism preserves different properties under different conditions depending on the considered type of synchronous communication. We show here that preservation of lock freedom requires an additional condition on gateways for asymmetric communication. Such condition is also needed for preservation of deadlock freedom, lock freedom or strong lock freedom for symmetric communications. This is not needed, instead, for preservation of either deadlock freedom or strong lock freedom with asymmetric interactions.