The problem

An online shop runs on one Rails app of 300 000 lines. Four teams of 6 to 8 developers merge into it about 25 times a day. Last month, 18 merges from one team broke the tests of another.

The Billing team changed how a refund rounds cents. The Orders team's order summary test failed the next morning, because the summary called the refund code directly. Nobody on Orders knew that call existed.

The CTO asks for a plan. Someone says "microservices". Someone else says "fix the folders first". You have one quarter and you must name the first module to cut out.