Modelling remote concurrency with Ada. Case study of symmetric non-deterministic rendezvous

When developing concurrent software, a proper engineering practice is to choose a good level of abstraction for expressing concurrency control. Ideally, this level should provide platform-independent abstractions but, as the platform concurrency behaviour cannot be ignored, this abstraction level must also be able to cope with it and exhibit the influence of different possible behaviours. We state that the Ada language provides such a convenient abstraction level for concurrency description and evaluation, including distributed concurrency. For demonstrating it, we present two new cooperative algorithms based on remote procedure calls which, although simply stated, contain actual concurrency complexity and difficulties. They allow a distributed symmetric non-deterministic rendezvous. One relies on a common server and the second is fully distributed. Both realize a symmetric rendezvous using an asymmetric RPC modelled by Ada rendezvous. These case studies show that Ada concurrency features provide the adequate abstraction level both for describing and evaluating concurrency and for carrying out design decisions.