Zero-Downtime Database Migrations (Expand and Contract Pattern).

In Simple Words: How to rename columns or alter schemas on live production databases with billions of rows without table locks or service downtime.

1. Expand: Add new nullable column new_column.
2. Dual-Write: Deploy application code that reads from old_column and writes to both old_column and new_column.
3. Backfill: Asynchronously copy legacy data from old_column to new_column in small batches.
4. Switch Read: Switch application reads to new_column.
5. Contract: Stop writing to old_column and drop it during a low-traffic window.