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Concepts

Detailed explanations of Kubernetes system concepts and abstractions.

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Pod Lifecycle

This page describes the lifecycle of a Pod.

Pod phase

A Pod’s status field is a PodStatus object, which has a phase field.

The phase of a Pod is a simple, high-level summary of where the Pod is in its lifecycle. The phase is not intended to be a comprehensive rollup of observations of Container or Pod state, nor is it intended to be a comprehensive state machine.

The number and meanings of Pod phase values are tightly guarded. Other than what is documented here, nothing should be assumed about Pods that have a given phase value.

Here are the possible values for phase:

Pod conditions

A Pod has a PodStatus, which has an array of PodConditions. Each element of the PodCondition array has a type field and a status field. The type field is a string, with possible values PodScheduled, Ready, Initialized, and Unschedulable. The status field is a string, with possible values True, False, and Unknown.

Container probes

A Probe is a diagnostic performed periodically by the kubelet on a Container. To perform a diagnostic, the kubelet calls a Handler implemented by the Container. There are three types of handlers:

Each probe has one of three results:

The kubelet can optionally perform and react to two kinds of probes on running Containers:

When should you use liveness or readiness probes?

If the process in your Container is able to crash on its own whenever it encounters an issue or becomes unhealthy, you do not necessarily need a liveness probe; the kubelet will automatically perform the correct action in accordance with the Pod’s restartPolicy.

If you’d like your Container to be killed and restarted if a probe fails, then specify a liveness probe, and specify a restartPolicy of Always or OnFailure.

If you’d like to start sending traffic to a Pod only when a probe succeeds, specify a readiness probe. In this case, the readiness probe might be the same as the liveness probe, but the existence of the readiness probe in the spec means that the Pod will start without receiving any traffic and only start receiving traffic after the probe starts succeeding.

If you want your Container to be able to take itself down for maintenance, you can specify a readiness probe that checks an endpoint specific to readiness that is different from the liveness probe.

Note that if you just want to be able to drain requests when the Pod is deleted, you do not necessarily need a readiness probe; on deletion, the Pod automatically puts itself into an unready state regardless of whether the readiness probe exists. The Pod remains in the unready state while it waits for the Containers in the Pod to stop.

Pod and Container status

For detailed information about Pod Container status, see PodStatus and ContainerStatus. Note that the information reported as Pod status depends on the current ContainerState.

Restart policy

A PodSpec has a restartPolicy field with possible values Always, OnFailure, and Never. The default value is Always. restartPolicy applies to all Containers in the Pod. restartPolicy only refers to restarts of the Containers by the kubelet on the same node. Failed Containers that are restarted by the kubelet are restarted with an exponential back-off delay (10s, 20s, 40s …) capped at five minutes, and is reset after ten minutes of successful execution. As discussed in the Pods document, once bound to a node, a Pod will never be rebound to another node.

Pod lifetime

In general, Pods do not disappear until someone destroys them. This might be a human or a controller. The only exception to this rule is that Pods with a phase of Succeeded or Failed for more than some duration (determined by the master) will expire and be automatically destroyed.

Three types of controllers are available:

All three types of controllers contain a PodTemplate. It is recommended to create the appropriate controller and let it create Pods, rather than directly create Pods yourself. That is because Pods alone are not resilient to machine failures, but controllers are.

If a node dies or is disconnected from the rest of the cluster, Kubernetes applies a policy for setting the phase of all Pods on the lost node to Failed.

Examples

Advanced liveness probe example

Liveness probes are executed by the kubelet, so all requests are made in the kubelet network namespace.

apiVersion: v1
kind: Pod
metadata:
  labels:
    test: liveness
  name: liveness-http
spec:
  containers:
  - args:
    - /server
    image: gcr.io/google_containers/liveness
    livenessProbe:
      httpGet:
        # when "host" is not defined, "PodIP" will be used
        # host: my-host
        # when "scheme" is not defined, "HTTP" scheme will be used. Only "HTTP" and "HTTPS" are allowed
        # scheme: HTTPS
        path: /healthz
        port: 8080
        httpHeaders:
          - name: X-Custom-Header
            value: Awesome
      initialDelaySeconds: 15
      timeoutSeconds: 1
    name: liveness

Example states

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