Zero-downtime деплой — как сохраняется состояние пользователя при переключении версий?

Zero-downtime deployment involves updating the application without interrupting user service and without losing their state.

To preserve user state when switching versions, the following approaches are usually used:

• External state storage — user sessions and data are stored outside the application, for example, in Redis, a database, or cookies. This way, the state is not lost when switching versions.

• Graceful shutdown — old application instances continue to serve current requests while new instances are being launched and start accepting traffic.

• Load balancer and rolling update — traffic is gradually shifted to the new version, and the old version is smoothly shut down.

• Sticky sessions — if a load balancer is used, it can direct requests from the same user to the same instance to maintain state.

In Go applications, graceful shutdown is often implemented using contexts and OS signals to properly finish processing requests before stopping the server.