Locality, causality and continuations

Concurrency and distribution are topics exacerbated by the omnipresence of Internet. Although many languages address these topics, few offer a real opportunity to control and coordinate widely spread dynamic computations. This paper presents such a language and its prominent features. Besides explaining the software architecture of the implementation (based on objects and generic functions), it also presents an original coherency protocol for shared mutable variables. We first recall, in Section 1, the main features of our Scheme-based, concurrent, distributed and computation-oriented language already presented in more details and examples in [QD93]. Section 2 explains how to systematically implement a concurrent and distributed interpreter for that language, using a set of program transformations combining Abstract Continuation Passing Style (ACPS) and Object-Oriented Lifting. The originality of this implementation is that it chiefly uses objects and generic functions in a style that allows to concentrate the problems related to concurrency and migration of computations into the sole discriminating behavior of generic functions. ACPS is not only used to reify continuations but also to enforce locality of computations in presence of distal objects. In Section 3, we propose a new (to our knowledge) protocol to manage shared mutable variables. This protocol enhances [MSRN92], does not require atomic broadcast, tolerates short communication breakdowns and uses bounded circular clocks. This result comes from the use of a distributed GC (which allows us to maintain an approximation of Global Virtual Time) and from the exploitation of causality as stated by continuations. To give a continuation a value (and a store) clearly expresses that the computations that are present in the continuation causally depend on the invoker of the continuation. Finally the computation-orientation of our language and mainly the ability to control groups of threads, concurrently running on multiple sites for the completion of the evaluation of a single expression, is shortly sketched in Section 4. As usual, related works and conclusions end this paper.