Database-design skill: prove derivation changes against old-shape stores - #87
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Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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August 17, 2026 18:39
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Lesson. In an append-only store, rows written under yesterday's rules are immortal, so a change that re-rules what recorded events mean must be proven against state shaped by the old rules — a fresh-store test can never meet the rows where such a change breaks.
Evidence. PR #83 changed what a backfill failure row means (permission denials became their own outcome). Every gate, three review passes, and a live proof against a freshly seeded store all passed — yet on the first real deployment, 22 archived threads sat permanently
failed: their old-shape failure rows had spent the archived-thread final sweep under the old rules, and no new run would ever revisit them. Only a store carrying pre-change history could expose it; the fix (only a completed run spends the final sweep) landed asb7b2ebcafter a live install surfaced it.Why it clears the bar. This is structural to the append-only doctrine, not specific to backfills: any semantic re-ruling of recorded events meets every deployed store's oldest rows. One sentence of policy converts a class of only-found-in-production defects into a seedable unit test.
Not for merging by the session that opened it — this PR is for the maintainer's personal review.
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