The OOM watcher

The OOM watcher writes a warning in the server log each time the kernel kills a container for memory.

A container goes over its memory limit. The kernel kills it.

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The OOM watcher follows the oom events of Docker for the containers of this server.

Server log

instance a1b2c3 of service 'web' ran out of memory (limit 512 MiB) — raise the service's memory limit

Why it exists

The kernel does the OOM kill. Docker then restarts the container by itself. Then Docker clears the flag that tells about the kill.

Without the watcher, nobody sees a container that crashes again and again at its memory limit.

The route watcher

The route watcher follows the host port of each instance, each second.

At boot

At boot, the engine first closes what a previous process did not complete, then runs the boot pass.