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pod-states.md

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The life of a pod

Updated: 9/22/2014

This document covers the intersection of pod states, the PodStatus type, the life-cycle of a pod, events, restart policies, and replication controllers. It is not an exhaustive document, but an introduction to the topics.

What is PodStatus?

While PodStatus represents the state of a pod, it is not intended to form a state machine. PodStatus is an observation of the current state of a pod. As such, we discourage people from thinking about "transitions" or "changes" or "future states".

Events

Since PodStatus is not a state machine, there are no edges which can be considered the "reason" for the current state. Reasons can be determined by examining the events for the pod. Events that affect containers, e.g. OOM, are reported as pod events.

TODO(@lavalamp) Event design

Controllers and RestartPolicy

The only controller we have today is ReplicationController. ReplicationController is only appropriate for pods with RestartPolicy = Always. ReplicationController should refuse to instantiate any pod that has a different restart policy.

There is a legitimate need for a controller which keeps pods with other policies alive. Both of the other policies (OnFailure and Never) eventually terminate, at which point the controller should stop recreating them. Because of this fundamental distinction, let's hypothesize a new controller, called JobController for the sake of this document, which can implement this policy.

Container termination

Containers can terminate with one of two statuses:

  1. success: The container exited voluntarily with a status code of 0.
  2. failure: The container exited with any other status code or signal, or was stopped by the system.

TODO(@dchen1107) Define ContainerStatus like PodStatus

PodStatus values and meanings

The number and meanings of PodStatus values are tightly guarded. Other than what is documented here, nothing should be assumed about pods with a given PodStatus.

pending

The pod has been accepted by the system, but one or more of the containers has not been started. This includes time before being schedule as well as time spent downloading images over the network, which could take a while.

running

The pod has been bound to a node, and all of the containers have been started. At least one container is still running (or is in the process of restarting).

succeeded

All containers in the pod have terminated in success.

failed

All containers in the pod have terminated, at least one container has terminated in failure.

Pod lifetime

In general, pods which are created do not disappear until someone destroys them. This might be a human or a ReplicationController. The only exception to this rule is that pods with a PodStatus of succeeded or failed for more than some duration (determined by the master) will expire and be automatically reaped.

If a node dies or is disconnected from the rest of the cluster, some entity within the system (call it the NodeController for now) is responsible for applying policy (e.g. a timeout) and marking any pods on the lost node as failed.

Examples

  • Pod is running, 1 container, container exits success

    • Log completion event
    • If RestartPolicy is:
      • Always: restart container, pod stays running
      • OnFailure: pod becomes succeeded
      • Never: pod becomes succeeded

  • Pod is running, 1 container, container exits failure

    • Log failure event
    • If RestartPolicy is:
      • Always: restart container, pod stays running
      • OnFailure: restart container, pod stays running
      • Never: pod becomes failed

  • Pod is running, 2 containers, container 1 exits failure

    • Log failure event
    • If RestartPolicy is:
      • Always: restart container, pod stays running
      • OnFailure: restart container, pod stays running
      • Never: pod stays running
    • When container 2 exits...
      • Log failure event
      • If RestartPolicy is:
        • Always: restart container, pod stays running
        • OnFailure: restart container, pod stays running
        • Never: pod becomes failed

  • Pod is running, container becomes OOM

    • Container terminates in failure
    • Log OOM event
    • If RestartPolicy is:
      • Always: restart container, pod stays running
      • OnFailure: restart container, pod stays running
      • Never: log failure event, pod becomes failed

  • Pod is running, a disk dies

    • All containers are killed
    • Log appropriate event
    • Pod becomes failed
    • If running under a controller, pod will be recreated elsewhere

  • Pod is running, its node is segmented out

    • NodeController waits for timeout
    • NodeController marks pod failed
    • If running under a controller, pod will be recreated elsewhere