Docker container health check

K8sCat 2021-01-24 14:02:29
docker container health check

Docker Container health check refers to Dockerfile Use in HEALTHCHECK The command checks the running state of the container ,
And in docker ps Of STATUS Column display healthy/unhealthy.

HEALTHCHECK Instructions come in two formats :

  • HEALTHCHECK [OPTIONS] CMD command( Check the health of the container by running commands inside the container )
  • HEALTHCHECK NONE( Disable inheriting any health checks from the underlying image )

HEALTHCHECK Instructions tell Docker How to test a container , To see if it's still working .
This can detect something , for example web The server is in an infinite loop , Unable to process new connection , Even if the server process is still running .

When the container specifies a health check , In addition to the normal state , It's also healthy . This state is initially starting.
As long as the health check-up passes , It will return to healthy( No matter what state it was in before ). After a certain number of consecutive failures , It will become unhealthy.

CMD The options that can appear before are :

  • --interval=DURATION( Default :30s)
  • --timeout=DURATION( Default :30s)
  • --start-period=DURATION( Default :0s)
  • --retries=N( Default :3)

The runtime check will first be performed after the container is started interval Run in seconds , And then after the last check interval Run again in seconds .

If a status check takes more than timeout second , The inspection failed .

The running state check of the container failed continuously retries It's not healthy until it's too late .

start period Provide initialization time for containers that need time to start . Detection failures during this period are not counted in the maximum number of retries .
however , If the health check is successful during startup , The container is considered to be started , The maximum number of retries will be calculated for all consecutive failures .

Dockerfile There can only be one of them HEALTHCHECK Instructions . If more than one is listed , Only the last one HEALTHCHECK Will take effect .

CMD The command after the keyword can be shell command ( for example HEALTHCHECK CMD /bin/check-running) or exec Array ( And others Dockerfile command , For more information , Please see the ENTRYPOINT).

command The exit state of represents the health state of the container . The possible value is :

  • 0: success -- The container is working well , have access to
  • 1: unhealthy —- The container is not working properly
  • 2: Retain -- Do not use this exit code

for example , Check every five minutes or so web Whether the server can serve the home page of the site in three seconds :

HEALTHCHECK --interval=5m --timeout=3s \
CMD curl -f http://localhost/ || exit 1

To help debug failed probes ,command Written in stdout or stderr Any output text on (UTF-8 code ) Will be stored in a healthy state , And through docker inspect The query .
Such output should be kept short ( At present, only the previous 4096 Bytes ).

When the health of the container changes , Will generate a... With a new state health_status event .

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