"Software changes. This is a rather obvious statement, but it is a fact that must be ever present in the minds of developers and architects. Although we tend to think of software development as chiefly an engineering exercise, the analogy breaks down very quickly. When was the last time someone asked the designers of the Empire State building to add ten new floors at the bottom, put a pool on the top, and have all of this done before Monday morning? " : Doug Purdy, Microsoft Corporation