Consider a system to be made up of procedures, some of which are stateful and others which aren’t. We have already discussed the difficulties of understanding the bits which are stateful, but what we would hope is that the procedures which aren’t themselves stateful would be more easy to comprehend. Alas, this is largely not the case. If the procedure in question (which is itself stateless) makes use of any other procedure which is stateful — even indirectly — then all bets are off, our procedure becomes contaminated and we can only understand it in the context of state. If we try to do anything else we will again run the risk of all the classic state- derived problems discussed above. As has been said, the problem with state is that “when you let the nose of the camel into the tent, the rest of him tends to follow”.
As a result of all the above reasons it is our belief that the single biggest remaining cause of complexity in most contemporary large systems is state, and the more we can do to limit and manage state, the better.
“Out of the Tar Pit”, Ben Moseley and Peter Marks, 2006