Configuring Object-Based Distributed Programs

The popularity of the object oriented programming paradigm has stimulated research into its use for parallel and distributed programming. The major issues which impact such use are concurrency control, object interfaces, binding and inheritance. This paper discusses the relative merits of current solutions to these issues and describes an approach based on the use of active objects with essentially explicit interfaces and bindings, and composition as a pragmatic alternative to inheritance. The key feature of our approach is the use of a configuration language to define program structure as a set of objects and their bindings. The configuration language includes facilities for hierarchic definition of composite objects, for parameterisation of objects, for replication of both object instances and interface interaction points, for conditional configurations with evaluation of guards at object instantiation, and even for recursive definition of objects. This separate and explicit description of program structure complements the object oriented concepts yet is missing from most other approaches. This approach, termed Configuration Oriented Programming, is illustrated by examples from the REX environment for the development of distributable software. This environment is being developed by the REX collaborative ESPRIT II project [REX 89].