Automating Database Schema Migrations Safely in Continuous Delivery

Never execute breaking schema changes (e.g. renaming or deleting a column) in a single deployment. Because old and new application versions run concurrently during zero-downtime rolling updates, the old version will immediately crash if a required column is renamed.

Follow the three-phase Expand and Contract pattern:

Phase 1 (Expand): Add the new column as nullable and update application code to write to both columns and read from the old.

Phase 2 (Backfill): Backfill existing data from the old column into the new column in batched background transactions.

Phase 3 (Contract): Switch application reads to the new column, deprecate the old column, and safely drop the old column in a subsequent release.

Run database migrations in isolated Kubernetes Jobs with Helm pre-install/pre-upgrade hooks before newly deployed web pods start up, ensuring transactions commit cleanly before traffic shifts.