Reusable components for evolving systems

Increasingly, the computers of the world are connecting to form large continuously evolving information systems. Ultimately there will be just one; perhaps this is already the case. Our quest for software reuse has an economic foundation: we want to do more for less cost. We want to build on what has gone before, we want new things to coexist with and leverage older things. Extensible or open systems can be augmented by adding components and new configurations, and this ability is open to everyone. However these systems must keep running while being extended in this way, and must preserve their integrity, through the change and thereafter. The four D´s of software are design, development, debugging and deployment. The first three get their fair share of attention from the research community, but deployment does not. The author describes requirements for reusable components and infrastructure, and talks about how deployment requirements feed back into the others and in some cases simplify them