Container Lifecycle Hooks

This page describes how kubelet managed Containers can use the Container lifecycle hook framework to run code triggered by events during their management lifecycle.

Overview

Analogous to many programming language frameworks that have component lifecycle hooks, such as Angular, Kubernetes provides Containers with lifecycle hooks. The hooks enable Containers to be aware of events in their management lifecycle and run code implemented in a handler when the corresponding lifecycle hook is executed.

Container hooks

There are two hooks that are exposed to Containers:

PostStart

This hook is executed immediately after a container is created. However, there is no guarantee that the hook will execute before the container ENTRYPOINT. No parameters are passed to the handler.

PreStop

This hook is called immediately before a container is terminated due to an API request or management event such as liveness probe failure, preemption, resource contention and others. A call to the preStop hook fails if the container is already in terminated or completed state. It is blocking, meaning it is synchronous, so it must complete before the signal to stop the container can be sent. No parameters are passed to the handler.

A more detailed description of the termination behavior can be found in Termination of Pods.

Hook handler implementations

Containers can access a hook by implementing and registering a handler for that hook. There are two types of hook handlers that can be implemented for Containers:

  • Exec - Executes a specific command, such as pre-stop.sh, inside the cgroups and namespaces of the Container. Resources consumed by the command are counted against the Container.
  • HTTP - Executes an HTTP request against a specific endpoint on the Container.

Hook handler execution

When a Container lifecycle management hook is called, the Kubernetes management system execute the handler according to the hook action, exec and tcpSocket are executed in the container, and httpGet is executed by the kubelet process.

Hook handler calls are synchronous within the context of the Pod containing the Container. This means that for a PostStart hook, the Container ENTRYPOINT and hook fire asynchronously. However, if the hook takes too long to run or hangs, the Container cannot reach a running state.

PreStop hooks are not executed asynchronously from the signal to stop the Container; the hook must complete its execution before the signal can be sent. If a PreStop hook hangs during execution, the Pod’s phase will be Terminating and remain there until the Pod is killed after its terminationGracePeriodSeconds expires. This grace period applies to the total time it takes for both the PreStop hook to execute and for the Container to stop normally. If, for example, terminationGracePeriodSeconds is 60, and the hook takes 55 seconds to complete, and the Container takes 10 seconds to stop normally after receiving the signal, then the Container will be killed before it can stop normally, since terminationGracePeriodSeconds is less than the total time (55+10) it takes for these two things to happen.

If either a PostStart or PreStop hook fails, it kills the Container.

Users should make their hook handlers as lightweight as possible. There are cases, however, when long running commands make sense, such as when saving state prior to stopping a Container.

Hook delivery guarantees

Hook delivery is intended to be at least once, which means that a hook may be called multiple times for any given event, such as for PostStart or PreStop. It is up to the hook implementation to handle this correctly.

Generally, only single deliveries are made. If, for example, an HTTP hook receiver is down and is unable to take traffic, there is no attempt to resend. In some rare cases, however, double delivery may occur. For instance, if a kubelet restarts in the middle of sending a hook, the hook might be resent after the kubelet comes back up.

Debugging Hook handlers

The logs for a Hook handler are not exposed in Pod events. If a handler fails for some reason, it broadcasts an event. For PostStart, this is the FailedPostStartHook event, and for PreStop, this is the FailedPreStopHook event. You can see these events by running kubectl describe pod <pod_name>. Here is some example output of events from running this command:

  1. Events:
  2.   FirstSeen  LastSeen  Count  From                                                   SubObjectPath          Type      Reason               Message
  3.   ---------  --------  -----  ----                                                   -------------          --------  ------               -------
  4.   1m         1m        1      {default-scheduler }                                                          Normal    Scheduled            Successfully assigned test-1730497541-cq1d2 to gke-test-cluster-default-pool-a07e5d30-siqd
  5.   1m         1m        1      {kubelet gke-test-cluster-default-pool-a07e5d30-siqd}  spec.containers{main}  Normal    Pulling              pulling image "test:1.0"
  6.   1m         1m        1      {kubelet gke-test-cluster-default-pool-a07e5d30-siqd}  spec.containers{main}  Normal    Created              Created container with docker id 5c6a256a2567; Security:[seccomp=unconfined]
  7.   1m         1m        1      {kubelet gke-test-cluster-default-pool-a07e5d30-siqd}  spec.containers{main}  Normal    Pulled               Successfully pulled image "test:1.0"
  8.   1m         1m        1      {kubelet gke-test-cluster-default-pool-a07e5d30-siqd}  spec.containers{main}  Normal    Started              Started container with docker id 5c6a256a2567
  9.   38s        38s       1      {kubelet gke-test-cluster-default-pool-a07e5d30-siqd}  spec.containers{main}  Normal    Killing              Killing container with docker id 5c6a256a2567: PostStart handler: Error executing in Docker Container: 1
  10.   37s        37s       1      {kubelet gke-test-cluster-default-pool-a07e5d30-siqd}  spec.containers{main}  Normal    Killing              Killing container with docker id 8df9fdfd7054: PostStart handler: Error executing in Docker Container: 1
  11.   38s        37s       2      {kubelet gke-test-cluster-default-pool-a07e5d30-siqd}                         Warning   FailedSync           Error syncing pod, skipping: failed to "StartContainer" for "main" with RunContainerError: "PostStart handler: Error executing in Docker Container: 1"
  12.   1m         22s       2      {kubelet gke-test-cluster-default-pool-a07e5d30-siqd}  spec.containers{main}  Warning   FailedPostStartHook

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