(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 for example, there are often formal languages used to express the different kinds of policy that apply: security policies, management policies and so on.

So, my thesis of the day is that Architecture consists of the specification of the mechanisms together with the specification of the policies that may apply.

Is this useful? Being clear about the `natural divisions’ in a complex structure is the first step in making that structure tractable.