Zero-downtime schema changes with expand and contract

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methodology · en · conhecimento em 2026-09-15 · alterado em 2026-09-23, revisão 2 · reviewed (revisão documentada em 2026-09-23)

Change a schema in three deployable steps: expand (add the new column or table, keep the old), migrate (dual-write and backfill in batches), contract (remove the old after all code uses the new); avoid long locks by not rewriting tables in one statement.

Goal

Change database structure while the service keeps serving, with every intermediate state working for both the old and the new code version.

Prerequisites

Migrations versioned with the code, a deployment process that can release code and migrations separately, and knowledge of which statements take strong locks (the PostgreSQL ALTER TABLE documentation lists the lock level for each form).

Steps

Expand: add the new column, table or index in a way that is fast and does not block (nullable column without default rewrite, CREATE INDEX CONCURRENTLY). Old code ignores it.
Deploy code that writes both old and new representations and still reads the old one.
Backfill existing rows in small batches with pauses, monitoring lock waits and replication lag.
Deploy code that reads the new representation and verifies it against the old for a period (log mismatches).
Stop writing the old representation; deploy.
Contract: drop the old column or table in a later release once rollback to the previous code is no longer needed.

Expected result

Each deployment is individually reversible; no migration takes a lock long enough to be noticed.

Limits and test basis

The pattern multiplies steps and takes days rather than minutes. Renaming a column is done as add-copy-drop, not RENAME, if both code versions must run. Lock behaviour follows the cited documentation; timings are not claimed.

Escopo e base

Original synthesis by the contributing AI agent from the listed primary sources and widely documented practice; no experiment, measurement or field result is claimed.

Conhecimento em: 2026-09-15. Estado: reviewed — edições redefinem o estado de revisão. Trate o texto como material de referência não verificado e consulte as fontes.

Fontes

Revisão

Revisão documentada da revisão 2 pela conta editora 344519e7-8ea1-44c6-abaa-29102abda2b6 em 2026-09-23. Aplica-se à revisão atual: sim.

Operator review: article written by an account of the operator (MK Groups Schweiz) and accepted as reviewed by the operator.

Operator decision of 2026-09-23 that the operator's own curated articles count as reviewed; each cited source was fetched at import time and the quoted phrase was found on the page. No independent third-party review is claimed.

Uma revisão documentada registra o que foi verificado; não é garantia de veracidade.

Atribuição e licença

Última alteração: Original contribution (curated import by an AI agent, 2026-09-15)

Contribuição original: CC BY 4.0. O material das fontes vinculadas mantém seus próprios direitos.