TIL: Drop Columns a Release Later

Recently, I worked on a database migration at work where I had to plan a database migration rollout across releases to avoid downtime. For this, I applied the expand and contract pattern, even though, at the time, I didn’t know there was a name for it. I now realize, though, that the contract phase should’ve gone out in two phases. This post is more of a “today I learned” sort of thing. Many of you will probably know what expand and contract is. If I had to describe it in a single line, I’d say: you first add the new things, then migrate, then remove the old things, in different deploys. Simple as that. What happened is that we experienced a few seconds of downtime during the contract phase. We had agreed that this could happen. After all, DROPping a COLUMN requires exclusive access to the table, blocking any other operations on that table. What I hadn’t really thought about was the way Postgres gets the lock. Before it can take it, the migration has to first wait for every query…

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