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# Migration review
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Answer every question before the migration is approved. "Probably fine" is not an
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answer to any of them.
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## Blast radius
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- [ ] How many rows does this touch?
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- [ ] What lock does each statement take, and for how long?
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- [ ] What happens to in-flight queries during that lock?
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- [ ] Is the busiest table involved? At what time will this run?
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## Compatibility
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- [ ] Does the currently deployed application version still work after this lands?
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- [ ] Does the new version work before it lands?
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- [ ] If the answer to either is no, this migration must be split.
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## Reversibility
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- [ ] Is there a down migration? Has it been run against a copy?
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- [ ] If irreversible, is that written down, and is the reason acceptable?
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- [ ] What is the recovery path if it fails halfway?
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## Data
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- [ ] Is any data destroyed? Is it backed up somewhere that is not this database?
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- [ ] Is the backfill batched and resumable?
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- [ ] What happens if the backfill is interrupted at 50%?
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## Rehearsal
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- [ ] Has this run against a restored production-sized copy?
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- [ ] How long did it take there?
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