Health checks

To make informed decisions, for example, during a rolling update of a swarm service whether or not the just-installed batch of new service instances is running OK or if a rollback is needed, the SwarmKit needs a way to know about the overall health of the system. On its own, SwarmKit (and Docker) can collect quite a bit of information. But there is a limit. Imagine a container containing an application. The container, as seen from outside, can look absolutely healthy and chuckle away just fine. But that doesn't necessarily mean that the application running inside the container is also doing well. The application could, for example, be in an infinite loop or be in a corrupt state, yet still running. But, as long as the application ...

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