Determine the Reason for Pod Failure

This page shows how to write and read a Container termination message.

Termination messages provide a way for containers to write information about fatal events to a location where it can be easily retrieved and surfaced by tools like dashboards and monitoring software. In most cases, information that you put in a termination message should also be written to the general Kubernetes logs.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:

Writing and reading a termination message

In this exercise, you create a Pod that runs one container. The manifest for that Pod specifies a command that runs when the container starts:

apiVersion: v1
kind: Pod
  name: termination-demo
  - name: termination-demo-container
    image: debian
    command: ["/bin/sh"]
    args: ["-c", "sleep 10 && echo Sleep expired > /dev/termination-log"]
  1. Create a Pod based on the YAML configuration file:

    kubectl apply -f

    In the YAML file, in the command and args fields, you can see that the container sleeps for 10 seconds and then writes "Sleep expired" to the /dev/termination-log file. After the container writes the "Sleep expired" message, it terminates.

  2. Display information about the Pod:

    kubectl get pod termination-demo

    Repeat the preceding command until the Pod is no longer running.

  3. Display detailed information about the Pod:

    kubectl get pod termination-demo --output=yaml

    The output includes the "Sleep expired" message:

    apiVersion: v1
    kind: Pod
            containerID: ...
            exitCode: 0
            finishedAt: ...
            message: |
              Sleep expired          
  4. Use a Go template to filter the output so that it includes only the termination message:

    kubectl get pod termination-demo -o go-template="{{range .status.containerStatuses}}{{.lastState.terminated.message}}{{end}}"

If you are running a multi-container Pod, you can use a Go template to include the container's name. By doing so, you can discover which of the containers is failing:

kubectl get pod multi-container-pod -o go-template='{{range .status.containerStatuses}}{{printf "%s:\n%s\n\n" .name .lastState.terminated.message}}{{end}}'

Customizing the termination message

Kubernetes retrieves termination messages from the termination message file specified in the terminationMessagePath field of a Container, which has a default value of /dev/termination-log. By customizing this field, you can tell Kubernetes to use a different file. Kubernetes use the contents from the specified file to populate the Container's status message on both success and failure.

The termination message is intended to be brief final status, such as an assertion failure message. The kubelet truncates messages that are longer than 4096 bytes.

The total message length across all containers is limited to 12KiB, divided equally among each container. For example, if there are 12 containers (initContainers or containers), each has 1024 bytes of available termination message space.

The default termination message path is /dev/termination-log. You cannot set the termination message path after a Pod is launched.

In the following example, the container writes termination messages to /tmp/my-log for Kubernetes to retrieve:

apiVersion: v1
kind: Pod
  name: msg-path-demo
  - name: msg-path-demo-container
    image: debian
    terminationMessagePath: "/tmp/my-log"

Moreover, users can set the terminationMessagePolicy field of a Container for further customization. This field defaults to "File" which means the termination messages are retrieved only from the termination message file. By setting the terminationMessagePolicy to "FallbackToLogsOnError", you can tell Kubernetes to use the last chunk of container log output if the termination message file is empty and the container exited with an error. The log output is limited to 2048 bytes or 80 lines, whichever is smaller.

What's next