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Showing posts from December, 2010

(Software) Architecture = Policy + Mechanism

In the early 1980's Bob Kowalski made famous an interesting equation: Program = Logic + Control. The idea of that equation was that programming was essentially a combination of logic -- i.e., what you wanted done -- with algorithm -- how you wanted it done.

It is a fairly commonplace fact that any non-trivial program has a similar flavor to it: there is often a substantial amount of machinery that is used to deliver the value in the program; together with some form of policy statement/expression that governs the precise requirements for a particular execution of the program.

The larger the program, the more obvious it is that there is this layering into mechanisms and policies. For example, one could argue that a word processor's mechanisms are all the pieces need to implement text editing, formatting and so on. If the word processor supports styles, especially named styles, then these styles are a simple form of policy.

At larger scales, when considering networked applications f…