Blue-green deployment

A blue-green deployment runs two complete environments. One serves live traffic while the other receives the new version; once the new one is verified, traffic is switched over. The old environment stays running, untouched.

The value is the rollback. If the new version misbehaves, traffic switches back in seconds to an environment that is already running and known good, rather than waiting for a redeployment of the previous release under pressure.

The cost is running two environments, and the complication is shared state: the database is not duplicated, so a release that changes the schema must remain compatible with both versions for the duration of the switch.