Laws for dynamic systems

A dynamic system is one which changes its configuration as it runs. It is a system into which we can drop new components which then cooperate with the existing ones. Such systems are necessarily built from reusable components, since as soon as the system is reconfigured to use some new components, those new components must reuse the existing, still running, ones. Design of reusable components in this context is an important problem. We suggest three laws which such reusable components might be required to obey, if dynamic systems are to be effective and to be economically built. We illustrate our conjecture that the laws are effective by describing a generic architecture based on the familiar registry services of COM/OLE or CORBA and by describing a simple point of sale system built according to this architecture. We conclude, of course, that some interesting open questions remain. But we suggest that an approach to reuse based on refining the three laws is a promising direction for system architecture to take.