Migration review

Answer every question before the migration is approved. "Probably fine" is not an answer to any of them.

Blast radius

• How many rows does this touch?
• What lock does each statement take, and for how long?
• What happens to in-flight queries during that lock?
• Is the busiest table involved? At what time will this run?

Compatibility

• Does the currently deployed application version still work after this lands?
• Does the new version work before it lands?
• If the answer to either is no, this migration must be split.

Reversibility

• Is there a down migration? Has it been run against a copy?
• If irreversible, is that written down, and is the reason acceptable?
• What is the recovery path if it fails halfway?

Data

• Is any data destroyed? Is it backed up somewhere that is not this database?
• Is the backfill batched and resumable?
• What happens if the backfill is interrupted at 50%?

Rehearsal

• Has this run against a restored production-sized copy?
• How long did it take there?