A practical checklist for legacy modernisation

Most teams know this matters. Fewer have decided who owns it. Run through this the next time legacy modernisation comes up.

Architecture is the set of decisions that are expensive to reverse, which is the only reason they deserve the name. Most teams find the first pass takes an afternoon and the maintenance takes minutes a month.

The checklist

• Strangle the old system gradually rather than replacing it at once
• Big-bang rewrites are the most reliable way to lose two years
• Every migration needs a way back
• Someone is named as the owner
• There is a date to review it again

Why this earns attention

Strangle the old system gradually rather than replacing it at once. There is a version of this that is over-engineered, and it is worth avoiding. It is worth deciding this deliberately rather than inheriting whatever the last person set up.

How to tell if yours is fine

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

The point is not perfection, it is knowing which of these you have consciously chosen to skip.