This project examines new language mechanisms for accessing message invocations in message passing based concurrent programming languages. Current mechanisms for invocation selection control, including those that allow invocation parameters to be examined, are not expressive enough. In particular, they do not provide a simple and efficient way to examine pending invocations of an operation or allow the selection decision to be based on more than just a single invocations parameters. Examining invocations and performing cross-invocation or cross-operation selection are important in a number of real applications, including debugging, visualization, and scheduling algorithms. Using current mechanisms to solve these problems results in cumbersome and inefficient code. This project involves a mix of conceptual, design, implementation, and evaluation work. To make the work concrete, the new language mechanisms are being defined and implemented as extensions to the SR concurrent programming language. The results of the qualitative and quantitative evaluations are being used to refine the semantics and implementations of the new language mechanisms. ***
