PO: an object model to express parallelism
Antonio Corradi, Letizia Leonardi · 1988
The object paradigm has many pleasant properties such as uniformity, factorization and dynamicity.As opposed to the actor paradigm [1], the object paradigm is greatly lacking with regard to concurrency.The need to extend the semantics of objects to accommodate parallelism [10] has stimulated the proposals of several systems.In particular, investigation of the assumptions that connect objects with parallelism is fruitful.Two different solutions may lead to parallelism: i) objects are mostly passive and processes "execute through" them; ii) parallelism is closely tied to the concept of object.The first approach, adopted in Smalltalk-80, Loops and Flavors, imposes the presence of entities, the processes, that embody active execution threads.They are not 'normal' objects: therefore parallelism disrupts uniformity.Moreover, objects must be programmed taking into account the presence of processes.Parallelism obtained by following the second approach pays more attention to the 'objectness' of the system.This approach is employed by several systems such as EDEN PO [3], MELD [5].Objects, in addition to being the constructs to express information hiding and data abstraction, play the role of unit of concurrency.A combined solution may be advisable.An environment should mix both kinds of objects, passive and active, without imposing limits on reciprocal requests.This mixed solution is used by ConcurrentSmalltalk and PO.