Migrating a legacy system without stopping the business

The old system works and nobody wants to touch it. Here's how you replace it without switching anything off.

Almost every company we work with has a system that's been running for years, that nobody fully understands, and that can't be switched off for even a day.

Replacing it all at once — the big bang — is the option that most often goes wrong, because it puts all the risk into a single weekend.

What we do instead. The new system starts by reading from the old one and serving a small piece: a report, a screen nobody complains about. Once that piece is reliable, it takes over the writing too. And so on, module by module, until the old system has no users left and can be switched off without anyone noticing.

It's slower on paper and considerably faster in practice, because there's never a single night where everything hinges on it going right.