Strategies for zero-downtime database migrations

Downtime can hit users hard and hurt revenue. With careful planning, you can migrate databases with little or no interruption. The key is to combine non-blocking changes, continuous data sync, controlled cutover, and good monitoring. Start by mapping the most disruptive steps and then replace them with safer alternatives.

  • Use non-blocking schema changes: add new columns with default NULL, avoid long-running locks. In PostgreSQL, create indexes concurrently; in MySQL, tools like gh-ost or pt-online-schema-change help minimize locks.
  • Run dual writes and backfill data: keep old and new schema in sync during the transition. The app can write to both paths, then backfill existing rows in the background.
  • Leverage replication and read traffic shifts: use read replicas to absorb load during the migration. Streaming replication keeps backups ready for a quick switch.
  • Employ canary and blue-green rollout: run the new code path for a small user segment, then widen the exposure as confidence grows.
  • Cutover with feature flags and clear rollback: toggle the new behavior behind a flag, monitor metrics, and roll back if problems appear.
  • Validate with checks and safeguards: run row counts, checksums, and latency tests. Have a rollback plan and a tested, documented recovery path.

Example approach to a common change: adding a new nullable field and then using a view to unify reads.

  • Step 1: Add the new column as NULLABLE to the table. No data movement yet.
  • Step 2: Backfill the column in the background, keeping the rest of your queries unchanged.
  • Step 3: Create a simple view or alias that maps old fields to the new ones, so the app reads through a single path.
  • Step 4: Enable dual writes for a period, updating the old and new columns in tandem.
  • Step 5: Switch the app to the new path, then remove the old column after confirming stability.

Clear communication, tests in staging, and a well-practiced cutover plan are essential. With these practices, you can reduce risk and keep users online during migrations.

Key Takeaways

  • Plan non-disruptive changes, test thoroughly, and use staging mirrors.
  • Combine replication, dual writes, and feature flags to minimize risk.
  • Always have a rollback plan and concrete validation steps before going live.