Why data migration matters more than it looks

This is cheap to get right at the start and expensive to retrofit. Data migration is easy to treat as a detail, and that is exactly why it is worth a few minutes of attention.

Data outlives the applications built on top of it, which is why the model deserves more thought than the screens. The failure mode is not doing it wrong, it is doing it once and assuming it stays done.

Why it matters

Migrations fail on the data nobody knew existed. Small and consistent beats large and occasional here. It rarely shows up as a line item, which is exactly why it slips.

Run it against a full copy before the real thing. Where this goes wrong is almost never a lack of knowledge. Write the reasoning down alongside the decision, because the reasoning is what changes first.

Warning signs

Always have a documented way back. In practice this is a scheduling problem more than a technical one. It is the sort of thing that looks like polish right up until it costs you an enquiry.

How to tell if yours is fine

Numbers get quoted in meetings long after anyone remembers how they were calculated. Three things worth confirming about data migration before you move on:

• Someone can say what the current setup is without going to look
• Always have a documented 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.