Why legacy modernisation matters more than it looks

This is one of those topics that looks small until it costs you something. Legacy modernisation is easy to treat as a detail, and that is exactly why it is worth a few minutes of attention.

Architecture is the set of decisions that are expensive to reverse, which is the only reason they deserve the name. The practical test is whether someone new to the project could tell, in a minute, that it had been handled.

The reason this keeps coming up

Strangle the old system gradually rather than replacing it at once. Small and consistent beats large and occasional here. The version that survives contact with a real deadline is the simple one.

Big-bang rewrites are the most reliable way to lose two years. Getting it slightly wrong is survivable. Ignoring it entirely is not. The version that survives contact with a real deadline is the simple one.

Warning signs

Every migration needs a way back. Where this goes wrong is almost never a lack of knowledge. If two people in the business would answer this differently, that gap is the actual problem.

The short version

Most systems fail at the seams rather than inside any one component. Three things worth confirming about legacy modernisation before you move on:

• Someone can say what the current setup is without going to look
• Every migration needs a way back — and you know whether that is true here
• There is a way to tell whether the last change to this helped

Worth checking on your own setup before it becomes someone else's problem to fix.