Zero-downtime Postgres migrations - the hard parts

A few months ago, we took around 15 seconds of unexpected API downtime during a planned database migration. We're always careful about deploying schema changes, so we were surprised to see one go so badly wrong. As a payments company, the uptime of our API matters more than most - if we're not accepting requests, our merchants are losing money. It's not in our nature to leave issues like this unex