Recursively Structured Distributed Computing Systems

Two design rules which aid the construction of distributed computing systems and the provision of fault tolerance are described, namely that: (i) a distributed computing system should be functionally equivalent to the individual computing systems of which it is composed, and (ii) fault tolerant systems should be constructed from generalised fault tolerant components. The reasoning behind these two "recursive structuring principles", and the consequences of attempting to adhere to them, are discussed. Where appropriate this discussion is illustrated by reference to a distributed system based on UNIX * that is now operational at Newcastle and several other locations. This system has been implemented by adding a software subsystem, known as the Newcastle Connection, to each of a set of UNIX systems. By this means we have constructed a distributed system which is functionally equivalent at both the user and the program level to a conventional uniprocessor UNIX system.