Communications model

We have investigated a number of different communications models for use in our prototype system. To promote interoperability of components, we anticipate that more than one of these would be supported. The following models have been examined in detail, by building simple implementations and comparing them; we are still evaluating CORBA (the Common Object Request Broker Architecture), and the emerging MPI (Message Passing Interface) standard.

• RPC or Remote Procedure Call, offers a strict client-server model, i.e. a server publishes the availability of a service and remote clients can then make procedure calls to the server. RPC follows a well defined standard, and has the advantage that it is machine independent and provides a consistent programming interface which, at its highest level, resembles the familiar procedure call model. The RPC model is very strict (the client supplies some data, the server processes it and sends a reply, the transaction terminates) and although it can be relaxed, it is not designed to work with processes which are to behave as both clients and servers.
• Unix sockets provide a mechanism for two processes to communicate freely in a bi-directional manner. It requires that a process first set up a known connection point, then act like a server, accepting connections from client processes. Once communications are established, two processes can communicate freely; the connection may be transitory or it may persist for a number of exchanges. Sockets provide independence from the details of particular networking technologies.
• TLI or Transport Level Interface, provides a similar model to sockets, but it complies with OSI standards. The programming interface is more sophisticated than sockets and can lead to better engineered solutions. Although TLI is set to replace sockets on UNIX machines, the socket interface is only just becoming widely available on PCs.
• PVM or Parallel Virtual Machine aims to harness a collection of heterogeneous machine into a large distributed-memory computer. It provides a message handling system, as well as message parsing, process spawning and configuration library calls. It has emerged as a standard within the parallel processing community.
• ToolTalk aims to provide an interface to allow tools (such as editors, compilers, mailers etc) to be aware of what the user is doing in the environment. Tooltalk provides an announcement-like mechanism, as well as point to point communications.

Initially we will build our message passing model on sockets for our prototype; by careful design, the model will migrate to some of the other standard interfaces. The socket model provides a lightweight set of communication primitives, hence is an easy interface to implement, and its simplicity means that we can investigate and implement other communication strategies on top of this interface. Implementations of the socket library exist for both Macs and PCs, and a number of internet tools are based on this interface.