Three myths about data migration

We end up explaining this on discovery calls often enough that it deserved writing down. A few things about data migration that get repeated more often than they get checked.

Numbers get quoted in meetings long after anyone remembers how they were calculated. Write the reasoning down alongside the decision, because the reasoning is what changes first.

“It only matters for big sites”

Migrations fail on the data nobody knew existed. Getting it slightly wrong is survivable. Ignoring it entirely is not. Assume whoever inherits this will have half your context and none of your patience.

“We can deal with it after launch”

Sometimes true, usually expensive. Small and consistent beats large and occasional here.

“Our platform handles it”

Always have a documented way back. The reasoning matters more than the rule, because the rule has exceptions. It is worth deciding this deliberately rather than inheriting whatever the last person set up.

In practice

Data outlives the applications built on top of it, which is why the model deserves more thought than the screens. Three things worth confirming about data migration before you move on:

• Someone can say what the current setup is without going to look
• Migrations fail on the data nobody knew existed — and you know whether that is true here
• There is a way to tell whether the last change to this helped

If you want a second opinion on how yours is set up, ask.