Blue-green deployment

Two identical production environments: the new version goes to the idle one, then traffic switches over, and back if something breaks.

Blue serves users while green gets the new release and is tested in place. Switching the router sends everyone to green; rollback is switching back, in seconds. The hard part is the database: both versions must work with the same schema during the switch, so schema changes go out in compatible steps first. A canary release is the gradual version: a small share of traffic first, then everyone.

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Related terms

CI/CD: Continuous integration and continuous delivery: every change is built and tested automatically and can be released at any time.
Feature flags: Switches in code that turn a feature on or off for some or all users without a new deploy.
Strangler fig pattern: Replacing a legacy system piece by piece, routing one feature at a time to new code until the old system can be switched off.

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