Restart Policies

 

  • Restart policies is used to control whether the containers starts automatically when they exit, or even when Docker restarts.
  • Restart policies tells Docker how to react when a container shuts down.
  • A restart policy only takes effect after a container starts successfully.
  • Starting successfully means that the container is up for at least 10 seconds and Docker has started monitoring it.
  • This prevents a container which does not start at all from going into a restart loop.
  • By using the --restart option with the docker run command you can specify a restart policy.
  • Types of restart policies
    • no
    • always
    • no-failure
    • unless-stopped

no

  • The no policy is the default restart policy and simply will not restart a container under any case.

always

  • If we wanted the container to be restarted no matter what exit code the command has, we can use the always restart policy.
  • The restart policy will always restart the container.
  • Whenever the Docker service is restarted, containers using the always policy will also be restarted.

on-failure

  • Restart your container whenever it exits with a non-zero exit status and not restart otherwise.
  • You can optionally provide a number of times for Docker to attempt to restart the container.
  • --restart=on-failure:@number
  • Docker uses a delay between restarting the container, to prevent flood-like protection.
  • This is an increasing delay; it starts with the value of 100 milliseconds, then Docker will double the previous delay.
  • If a container is successfully restarted, the delay is reset to the default value of 100 milliseconds

unless-stopped

  • Again, similar to always, if we want the container to be restarted regardless of the exit code, we can use unless-stopped.
  • It will restart the container regardless of the exit status, but do not start it on daemon startup if the container has been put to a stopped state before.
  • If you are using the app from Simple Spring App, make sure you are in dockerfile-initial branch.
  • We have couple of REST endpoints in the app
    • /test/ which kind of hello-world endpoint for us
    • /test/status-5xx which will return status 200 for first 5 times, but after that it will throw exception. Our app will still be running.
    • /test/exit-1 to exit the app with non-zero status code.
    • /test/exit-0 to exit the app with zero status code.

Restart Policies in Action

  • Let's start our app using on-failure
  • Here's a Dockerfile content
  • Run the image with --restart option.
docker run -d -p 9090:8080 --restart\=on-failure amantuladhar/doc-kub-ws:web-basic

IMAGE                                 STATUS              
amantuladhar/doc-kub-ws:web-basic     Up About a minute
  • Let's call /test/exit-1, which will end the process with non zero exit code.
IMAGE                                 STATUS                     
amantuladhar/doc-kub-ws:web-basic     Restarting (1) 1 second ago
  • Now let's call /test/exit-0, which will end the process with zero exit code.
  • If you list the running container, you won't see it now.

Update restart policies of running container

  • docker update --restart=new-policies @id/@name
  • Where @id/@name is of container