Database migration strategies: zero-downtime, rollback, and testing

Database migrations are one of the riskiest operations in application development. A bad migration can cause downtime, data loss, or performance degradation. A systematic approach to migrations minimizes these risks.

Write idempotent migrations. A migration should produce the same result whether it's running for the first time or the tenth time. Use IF NOT EXISTS, IF EXISTS, and CREATE OR REPLACE where supported. This allows safe retries after failures.

Test migrations against a copy of production data. A migration that passes on the development database may fail on production due to data patterns you didn't anticipate. Test with anonymized production data or a production clone.

Implement backward-compatible schema changes. Never remove a column or rename a table in a single migration. Instead, add the new column first, update the application to write to both old and new columns, then remove the old column in a later deployment.

Use the expand-migrate-contract pattern for complex changes. Phase 1: expand the schema to support both old and new models. Phase 2: migrate existing data and update all code paths. Phase 3: remove the old schema elements. Each phase is a separate deployment.

Lock tables only when necessary. Common migration operations like adding a column with a default value lock the table in PostgreSQL. Use techniques like adding the column without a default, updating in batches, and setting the default afterward.

Run migrations outside of application startup for high-availability services. Use a dedicated migration runner or a deployment pipeline that runs migrations before deploying new code.

Document your migration plan with clear rollback steps. Every migration should have a tested rollback script. Know how long the migration will take and how much it will lock. Communicate with the team about scheduled migrations.

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Rizwan Saleem | https://rizwansaleem.co