Database Migration

Safe patterns for evolving database schemas in production.

Migration Principles

1. Backward compatible - New code works with old schema
2. Reversible - Can rollback if needed
3. Tested - Verify on staging before production
4. Incremental - Small changes, not big-bang
5. Zero downtime - No service interruption

Safe Migration Pattern

Phase 1: Add New (Compatible)

-- Add new column (nullable initially)
ALTER TABLE users ADD COLUMN full_name VARCHAR(255) NULL;

-- Deploy new code that writes to both old and new