A short guide to legacy modernisation

Every audit we run turns up some version of this. Everything we would tell a client about legacy modernisation in the time it takes to drink a coffee.

Architecture is the set of decisions that are expensive to reverse, which is the only reason they deserve the name. If two people in the business would answer this differently, that gap is the actual problem.

The reason this keeps coming up

Strangle the old system gradually rather than replacing it at once. The teams that handle this well are rarely the ones with the biggest budgets. Budget a little time for it every quarter and it never becomes a project of its own.

How we handle it

Big-bang rewrites are the most reliable way to lose two years. None of that requires a large budget, only a decision and someone to own it. Doing this properly once is usually cheaper than doing it approximately three times.

Warning signs

Every migration needs a way back. Where this goes wrong is almost never a lack of knowledge. Doing this properly once is usually cheaper than doing it approximately three times.

In practice

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
• Big-bang rewrites are the most reliable way to lose two years — and you know whether that is true here
• There is a way to tell whether the last change to this helped

If any of that sounds like a description of your current setup, it is fixable.