Pod Lifecycle, Restart Policy

We will explore the Pod lifecycle, restart policies, and Probes for container health checks.

Pod Lifecycle #

A Pod's lifecycle is divided into four phases.

1. PENDING: The Pod has been accepted by the cluster, but its containers have not yet started, and it has not been scheduled onto a node.
2. RUNNING: The Pod has been accepted by the cluster, its containers have started, and it is scheduled onto a node.
3. SUCCEEDED: This status indicates that a finite (static) job on the node has completed. It's the status shown when a node terminates, meaning the job finished successfully.
4. FAILED: This status indicates that a finite (static) job on the node has completed. It's the status shown when a node terminates, meaning the job terminated abnormally.

In addition to these, there are additional status (.status.phase) values assigned to Pods only in special cases.

• unknown: The Pod's status cannot be determined. This usually occurs due to node network issues.
• terminating: This can be seen when a Pod is deleted. Generally, a Pod has a 30-second grace period when deleted. If you want to delete it without this grace period, you can add --force to the kubelet delete option.

The Pod's lifecycle and current status are periodically monitored by kubelet. This monitored status information can be seen in the STATUS field of the kubectl get pods output. It can also be checked in the .status.phase field of the Pod object, using the following command:

kubectl get pod <파드명> -o jsonpath='{.status.phase}'

Pod Restart Policy #

Containers within a Pod have a policy for restarting when an error occurs.

This policy is categorized into 3 types and can be directly assigned via spec.restartPolicy.

1. Always: Always restarts the container, whether it terminates normally or abnormally. (default)
2. Onfailure: Restarts the container only if it terminates abnormally.
3. Never: Never restarts.

Container restart attempts occur exponentially, starting at 10-second intervals, up to a maximum of 300 seconds.

After 10s, after 20s, after 40s, after 80s...