Understanding tool integration in a Software Development Environment (SDE) is one of the key issues being addressed in current work on providing automated support for large-scale software production. Work has been taking place at both the conceptual level ("What is integration?") and the mechanistic level ("How do we provide integration?"). Many people see the answers to these questions as providing the cornerstone of real progress in the area.

Until recently, existing integration mechanisms have been very rigid in the support for integration that they provide. Users have been offered a fixed level of integration with little flexibility. However, one approach that has been recently implemented employs a control integration paradigm that appears to be flexible, supportive, and adaptable to a wide range of end-user needs. Implementations of this paradigm are based on the notion of "message passing" as the underlying communication mechanism between SDE services.

In this paper we examine the message passing approach to integration in an SDE, look at the general principles of the approach, describe some existing implementations, and discuss the use of such a mechanism as the basis for a more flexible environment that is open to experimentation with different approaches to integration.