Init Containers

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• In a Multi-Container Pods, all containers continues to run as long as the Pod is running.
• Sometimes, you only want to run a containers once, like:
  • Pulling dependencies before the main app starts.
• For these kinds of one-time startup tasks, we use Init containers.
• If either container stops or fails, Kubelet restarts the containers based on restartPolicy
  • If restartPolicy is set to Always or OnFailure:
    • Kubelet will restart the failing init container until it succeeds.
  • If restartPolicy is set to Never:
    • Kubelet will not restart the failing init container and mark Pod as Failed

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How to create

• Init containers are just like regular containers, but they are placed in a special section in the pod YAML
• Their job is to finish a task and exit successfully.
• The main app container won’t start until all Init containers exit successfully.

initContainers:
- name: init-myservice
  image: busybox
  command: ['sh', '-c', 'git clone ;']

• This runs a Git clone command before the main app starts.

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Multiple Init Containers

• you can define more than one init container and they run in order and one at a time.
• The next init container won’t run until the previous one finishes successfully.
• For example:

initContainers:
- name: init-myservice-1
  image: busybox:1.28
  command: ['sh', '-c', 'do echo Hello; sleep 2; done;']
 
- name: init-myservice-2
  image: busybox:1.28
  command: ['sh', '-c', 'do echo World; sleep 2; done;']

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⬅️ Multi-Container Pods | Pod Lifecycle ➡️

References

1. https://kubernetes.io/docs/concepts/workloads/pods/init-containers/