Distributed Coordination with MESSENGERS

Messengers is a paradigm for the programming of distributed systems. It is based on the principles of autonomous messages, called Messengers, which carry their own behavior in the form of a program. This enables them to navigate freely in the underlying computational network, communicate with one another, and invoke compiled node-resident C functions in the nodes they visit. Hence a distributed application is viewed as a collection of C functions whose invocation and interoperation is orchestrated by Messengers. This provides for a clear separation between computations, as expressed by the individual node functions, and coordination, which is the order of function in-vocations and the transport of information among them as prescribed by Messengers. This separation allows each layer to be designed and implemented separately. It also supports the reuse of the coordination structures and the interactive and incremental development and use of distributed applications. The notion of separating concurrent programming into computation and coordination was introduced in CG89]. Coordination includes the synchronization, communication, and creation/destruction of computational activities required to orchestrate individual computations into a coherent system. One approach to coordination utilizes channel-based communication between processes. Processes communicate directly with each other by reading from and writing to ports. Ports of diierent processes are connected to each other via channels. This approach leads to a clean separation of computation and coordination functions. A process performs its own computation by computing values to be written to its output ports from values read from its input ports. The coordination requirements of a process, such as connecting its ports 1