Abstract
The Q system provides interoperability support for multilingual, heterogenous component-
based software systems. Initial development of Q began in 1988, and was driven by
the very pragmatic need for a communication mechanism between a client program written
in Ada and a server written in C. The initial design was driven by language features
present in C, but not in Ada, or vice-versa. In time our needs and aspirations grew
and Q evolved to support other languages, such as C++, Lisp, Prolog, Java, and Tcl.
As a result of pervasive usage by the Arcadia SDE research project, usage levels and
modes of the Q system grew and so more emphasis was placed upon portability, reliability,
and performance. In that context we identified specific ways in which programming language
support systems can directly impede effective interoperablility. This necessitated
extensive changes to both our conceptual model and our implementation of the Q system.
We also discovered the need to support modes of interoperability far more complex than
the usual client-server. The continued evolution of Q has allowed the architecture of
Arcadia software to become highly distributed and component-based, exploiting components
written in a variety of languages. in addition to becoming an Aradia project mainstay,
Q has also been made available to over 100 other sites, and it is currently in use in
a variety of other projects. This paper summarizes key points that have been learned
from this considerable base of experience.
