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Engineering3 min readWednesday, August 19, 2026
Zero-Downtime Migrations: A Practical Playbook
Migrating a legacy system without downtime is not about being clever during the cutover, it is about the boring groundwork you do weeks before it.
Every zero-downtime migration we have run without incident followed the same unglamorous structure, most of the risk gets removed weeks before the actual cutover happens.
The playbook
Run old and new systems in parallel behind a feature flag, mirror production traffic to the new system before it serves anything real, and load-test to a multiple of historical peak, not just current average load.
- Feature-flag the cutover so rollback is a config change, not a redeploy
- Mirror real production traffic before the new system serves live users
- Load-test to a multiple of peak, not average, historical traffic