Originally published on TechSaaS Cloud Originally published on TechSaaS Cloud Zero-Downtime Database Migration: Shadow Writes, Dual-Read, and the 12-Second Cutover Database migrations are the scariest infrastructure change you can make. Your data is the one thing you absolutely cannot lose, corrupt, or make unavailable. We migrated a 2TB PostgreSQL database to CockroachDB for a SaaS client with zero downtime, zero data loss, and a cutover that took 12 seconds. Here's the complete playbook. Why Not Just pg_dump and Restore? For a 2TB database, pg_dump takes roughly 4-8 hours depending on your hardware. During that time, your application is either down or writing data that won't be in the dump. You'd need a maintenance window, and for a SaaS product with global users, "maintenance windows" mean lost revenue and broken SLAs. The shadow-write approach eliminates the maintenance window entirely.…