Splitting a payments monolith without a rewrite

Northwind had forty engineers and one deployable unit. Every attempt to split it had started with a service boundary and stalled in the database.

Ownership before code

We started with ownership instead: a written list of which service was allowed to write each of the ninety-one tables. Producing that list took nine days and settled four arguments that had been open for a year.

The mechanical split

From there the split was mechanical. Write permissions were revoked one table at a time, read views were issued to the previous writers, and only then did code move. The first service left the monolith in week seven. Nothing was rewritten.