Theme: Putting Systems Together

Software techniques have developed enormously over the past 30-40 years, from primitive machine languages to sophisticated programming languages and tools for system configuration. We are working on the next great leap, shifting focus from programming-at-the-module level to programming-at-the-system level. In the former activity we build programs in terms of procedures and abstract data types using simple module interconnection as a way to compose modules; in the latter activity, we build systems in terms of more sophisticated components, usually entire systems themselves, and connect them with more sophisticated abstractions for composition.

What makes the construction of composable systems different from programming?

1. We are liberating ourselves from thinking of the task as merely programming. We are not just building a program, we are building a system, where in the most general sense a system has not just software but hardware components as well.

2. Our units of manipulation are components and connectors. They may be primitive or composite. We describe components with interface specifications; connectors, protocol specifications. Our innovative claim is that connectors are first-class entities, just as components are, in a system. Program modules like procedures are just examples of primitive components. Procedure call is just one example of a primitive connector between two modules. Our connectors must support richer interactions than procedure call. We want to support composite components and connectors whose properties, via their interface and protocol specifications, are as understandable as their constituents.

3. We want to provide ways to talk precisely about common patterns of structures. This allows us to provide larger abstractions for system structuring. It also allows us to exploit special properties of particular composition idioms for analysis, design guidance, and efficient implementation.