The problem

A shop runs on one MySQL database of 500 GB. The team has decided to move to PostgreSQL. Every order, cart and invoice lives in that MySQL database today.

A dump and restore of 500 GB takes hours, and the shop cannot refuse orders for hours. The product owner gives you 5 minutes of refused writes, at 3 a.m., once.

So the copy has to happen while MySQL keeps taking writes, and the switch has to happen in one short window.