Q: A developer wants to perform a breaking schema change in a live production database (e.g., renaming a heavily used column or splitting a table) with zero downtime. Walk through the steps to achieve this safely.

You cannot simply run an ALTER TABLE RENAME command, as the live application will instantly crash when it queries the old name. The solution is the Expand and Contract pattern:

• Expand (Database): Add the *new* column alongside the old one. Do not delete the old one.
• Expand (Code - Write both): Deploy a new version of the application that writes data to *both* the old and the new columns but still reads from the old one.
• Backfill: Run a background script to migrate existing data from the old column to the new column.
• Transition (Code - Read new): Deploy a new version of the application that reads from the *new* column instead of the old one.

• Contract (Code - Stop writes): Deploy a version that stops writing to the old column completely.
• Contract (Database): Finally, safely drop the old column from the database.