Clusters: a Pragmatic Approach towards Supporting a Fine Grained Active Object Model in Distributed Systems

A powerful programming environment for developing distributed applications relies on its ability to abstract away the details of the underlying architecture and present a simpliied view to the developer. Along the path to transparency and uniformity lies object orientation, a uniform ne-grained active object model and powerful semantic analysis to automate the partitioning, distribution and run-time control of the user's application. This paper presents a model of object composition known as clustering. The construction model, the mapping of clusters to a run-time speciically designed to support them, and related issues are described. 1 Background DIAMONDS is an environment for the development of distributed applications (under development at Syracuse) in which the dynamic association of resources to applications reeects algorithmic parallelism, computational needs, and the current state of the systemm1]. In this system, networks of heterogeneous processors serve as a pool of computational resources for a variety of applications. Software engineering concerns drive much of the approach, focusing on providing a distributed system which is easy to program, with a clear, clean programming model. The DIAMONDS environment will include: 1) a uniform ne-grained active object model that supports and enhances good distributed object oriented analysis, design, and programming practices; 2) a means of composing ne-grained entities (language objects) into coarser grained run-time objects (the clustering process); and 3) tight coupling between the development environment and the distributed run-time support services. The programming model consists of interacting ne-grained active objects, where every object in the system is capable of processing code. Synchronization is provided by activation conditions called guards. Construction is template or class based. Objects may contain references to other objects and may export these references by sending them in a message. Communication is by message passing with three modes being supported: 1) one-way messages, 2) asynchronous two-way messages (future objectss2]), and 3) synchronous two-way messages (RPC-style). This ne-grained active object model is an ideal programming model which would have very eecient run-time attributes if a massively parallel MIMD architecture were available with an innnitely fast interprocessor communication network. On more typical hardware, the run-time system would prefer to manage coarser-grained entities. There appear to be two options for supporting ne-grained objects: