Scenario

You need to rename a core column user_name to username in a User table that receives 10,000 requests per second. You cannot afford any downtime or maintenance window.

Question

How do you execute this migration safely ensuring both the old and new versions of the application code work during the transition?

Expected Answer

1. Phase 1 - Add: Add the new column username (nullable) to the DB.
2. Phase 2 - Dual Write: Deploy app version v1.1 that writes to both columns but reads from user_name.
3. Phase 3 - Backfill: Run a script to copy existing data from user_name to username.
4. Phase 4 - Switch Read: Deploy app v1.2 that reads from username (still writes to both for rollback safety).
5. Phase 5 - Cleanup: Remove user_name column and stop writing to it.
6. Key Concept: “Expand and Contract” pattern.