One of the new features in Docker 1.12 is how health check for a container can be baked into the image definition. And this can be overridden at the command line. Just like the CMD instruction, there can be multiple HEALTHCHECK instructions in Dockerfile but only the last one is effective.

This is a great addition because a container reporting status as Up 1 hour may return errors. The container may be up but there is no way for the application inside the container to provide a status. This instruction fixes that.

The Dockerfile that builds arungupta/couchbase image is:

It uses configure-node.sh script to configure the server using Couchbase REST API. The new instruction to notice here is HEALTHCHECK. This instruction can be specified as:

The can be:

  • --interval=DURATION (default 30s)
  • --timeout=DURATION (default 30s)
  • --retries=N (default 3)

The is the command that runs inside the container to check the health. If health check is enabled, then the container can have three states:

  • starting – Initial status when the container is still starting
  • healthy – If the command succeeds then the container is healthy
  • unhealthy – If a single run of the  takes longer than the specified timeout then it is considered unhealthy. If a health check fails then the  will run retries number of times and will be declared unhealthy if the  still fails.

The commands exit status indicates the health status of the container. The following values are allowed:

  • 0 – container is healthy
  • 1 – container is not healthy

In our instruction, /pools REST API is invoked using curl. If the command fails then an exit status of 1 is returned, and this marks the container unhealthy for that attempt. This command is invoked every 5 seconds. The container is marked unhealthy if the command does not return successfully within 3 seconds. Run the container as:

Check the status:

Notice how health: starting status is reported in the STATUS column. Checking after a few seconds shows the status:

And now its reported healthy. More details about this HEALTHCHECK instruction can be found on docs.docker.com. Now, if you are running an image that does not have HEALTHCHECK instruction then the docker run command can be used to specify similar values. An equivalent runtime command would be:

Last 5 health checks for a container can be obtained using the docker inspect command:

The output is shown as:

     

Posted by Arun Gupta, VP, Developer Advocacy, Couchbase

Arun Gupta is the vice president of developer advocacy at Couchbase. He has built and led developer communities for 10+ years at Sun, Oracle, and Red Hat. He has deep expertise in leading cross-functional teams to develop and execute strategy, planning and execution of content, marketing campaigns, and programs. Prior to that he led engineering teams at Sun and is a founding member of the Java EE team. Gupta has authored more than 2,000 blog posts on technology. He has extensive speaking experience in more than 40 countries on myriad topics and is a JavaOne Rock Star for three years in a row. Gupta also founded the Devoxx4Kids chapter in the US and continues to promote technology education among children. An author of several books on technology, an avid runner, a globe trotter, a Java Champion, a JUG leader, NetBeans Dream Team member, and a Docker Captain, he is easily accessible at @arungupta.

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