Until recently, at least one thing was clear about parallel programming: tightly coupled (shared memory) machines were programmed in a language based on shared variables and loosely coupled (distributed) systems were programmed using message passing. The explosive growth of research on distributed systems and their languages, however, has led to several new methodologies that blur this simple distinction. Operating system primitives (e.g., problem-oriented shared memory, Shared Virtual Memory, the Agora shared memory) and languages (e.g., Concurrent Prolog, Linda, Emerald) for programming distributed systems have been proposed that support the shared variable paradigm without the presence of physical shared memory. In this paper we will look at the reasons for this evolution, the resemblances and differences among these new proposals, and the key issues in their design and implementation. It turns out that many implementations are based on replication of data. We take this idea one step further, and discuss how automatic replication (initiated by the run time system) can be used as a basis for a new model, called the shared data-object model, whose semantics are similar to the shared variable model. Finally, we discuss the design of a new language for distributed programming, Orca, based on the shared data-object model. 1.

In this paper we introduce key features of a programming language for building DAI and other types of distributed applications requiring the transmission and manipulation of complex symbolic data. The language is high-level and yet also offers a simple and smooth interface to other programming languages such as "C". April is oriented to the implementation of multi-agent systems. However, April is NOT a `multiagent applications language'. It does not directly offer high level features such as: planners, problem solvers and knowledge representation systems that a multi-agent applications language might be expected to include. April is more an object based concurrent language with objects as processes. As argued in [10], this is a highly suitable base for extension to DAI and multi-agent application platforms. 1 What is April? Any programming language which is suitable for implementing multi-agent systems must span a diverse collection of areas. Clearly, it must have strong support for...

PARALLEL ARCHITECTURE : : : : : : : : : : : : : : : : 56 4.1 Thread Management : : : : : : : : : : : : : : : : : : : : : : : : : : : : 56 4.2 Resource Management : : : : : : : : : : : : : : : : : : : : : : : : : : : 64 4.3 Communication Management : : : : : : : : : : : : : : : : : : : : : : : 69 4.4 Configuration Management : : : : : : : : : : : : : : : : : : : : : : : : 72 4.5 Performance : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 76 4.6 Evaluation : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 78 4.7 Other Models and Implementations : : : : : : : : : : : : : : : : : : : : vii 5 META-PROGRAMMABILITY : : : : : : : : : : : : : : : : : : : : : : : : 85 5.1 Local Meta-programmability : : : : : : : : : : : : : : : : : : : : : : : : 85 5.2 Global Meta-programmability : : : : : : : : : : : : : : : : : : : : : : : 95 5.3 Evaluation : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 99 5.4 Other Models and Implementations : : : : : : ...