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Running two identical environments (blue and green) and switching traffic between them for zero-downtime deployments.
Blue is the current live environment. Green is the new version. Deploy to green, run smoke tests, switch the load balancer. Rollback = switch back to blue. Zero downtime, instant rollback.
Requires infrastructure for two full environments and database migration compatibility. Session stickiness and in-flight requests must be handled during the switch. Works best with stateless services; stateful services need shared or migrated state.