Distributed system design, configuration and reconfiguration

The paper addresses some issues related to the design of large distributed software systems ( DSS), such as office automation systems, distributed embedded applications and programming environments. The approach assumes a unique language where many features, traditionally offered by the operating system, are provided in a uniform and consistent way, namely Ada. Such distributed applications put special requirements on the programming language, in par ticular regarding configuration and reconfiguration. We distin- guish three classes of issues : i) Compile time configuration: The purpose here is to make explicit in the program itself the structure of th.e system, usually multi-layered : In section 2 a suitable structuring of the Program Library is introduced; ii) run time configuration: The idea (sec. 3) is to associate to each activation of a DSS a run~time structure keeping track of the elaboration of the units, so that distributed components that share a component share also its elaboration,regardless of their mutual activation times and without a centralised activator; iii) run time reconfigurati on: The idea presented in section 4 is based on the declaration of a "dynamic package", statically declared (as usual), whose body is computed at run~time as the value of an expression of a predefined (sub)type BODY_TYPE. The approach we have taken is that of having minimal extension~, taking into account the already great power of a language like Ada in order to have a first experimentation of the effectiveness of the approach.