Zero-Downtime Database Migrations.

Changing database schemas in production without locking tables or causing application downtime.

It requires the Expand and Contract pattern. 1) Expand: Add the new column/table without modifying the old one. 2) Deploy app v2: Writes to both old and new, reads from old. 3) Backfill data from old to new. 4) Deploy app v3: Reads and writes only to new. 5) Contract: Drop the old column.