105 lines
3.2 KiB
Markdown
105 lines
3.2 KiB
Markdown
---
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title: Attach Handlers to Container Lifecycle Events
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content_type: task
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weight: 140
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---
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<!-- overview -->
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This page shows how to attach handlers to Container lifecycle events. Kubernetes supports
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the postStart and preStop events. Kubernetes sends the postStart event immediately
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after a Container is started, and it sends the preStop event immediately before the
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Container is terminated. A Container may specify one handler per event.
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## {{% heading "prerequisites" %}}
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{{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}
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<!-- steps -->
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## Define postStart and preStop handlers
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In this exercise, you create a Pod that has one Container. The Container has handlers
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for the postStart and preStop events.
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Here is the configuration file for the Pod:
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{{< codenew file="pods/lifecycle-events.yaml" >}}
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In the configuration file, you can see that the postStart command writes a `message`
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file to the Container's `/usr/share` directory. The preStop command shuts down
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nginx gracefully. This is helpful if the Container is being terminated because of a failure.
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Create the Pod:
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kubectl apply -f https://k8s.io/examples/pods/lifecycle-events.yaml
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Verify that the Container in the Pod is running:
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kubectl get pod lifecycle-demo
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Get a shell into the Container running in your Pod:
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kubectl exec -it lifecycle-demo -- /bin/bash
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In your shell, verify that the `postStart` handler created the `message` file:
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root@lifecycle-demo:/# cat /usr/share/message
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The output shows the text written by the postStart handler:
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Hello from the postStart handler
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<!-- discussion -->
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## Discussion
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Kubernetes sends the postStart event immediately after the Container is created.
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There is no guarantee, however, that the postStart handler is called before
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the Container's entrypoint is called. The postStart handler runs asynchronously
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relative to the Container's code, but Kubernetes' management of the container
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blocks until the postStart handler completes. The Container's status is not
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set to RUNNING until the postStart handler completes.
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Kubernetes sends the preStop event immediately before the Container is terminated.
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Kubernetes' management of the Container blocks until the preStop handler completes,
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unless the Pod's grace period expires. For more details, see
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[Pod Lifecycle](/docs/concepts/workloads/pods/pod-lifecycle/).
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{{< note >}}
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Kubernetes only sends the preStop event when a Pod is *terminated*.
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This means that the preStop hook is not invoked when the Pod is *completed*.
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This limitation is tracked in [issue #55087](https://github.com/kubernetes/kubernetes/issues/55807).
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{{< /note >}}
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## {{% heading "whatsnext" %}}
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* Learn more about [Container lifecycle hooks](/docs/concepts/containers/container-lifecycle-hooks/).
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* Learn more about the [lifecycle of a Pod](/docs/concepts/workloads/pods/pod-lifecycle/).
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### Reference
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* [Lifecycle](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#lifecycle-v1-core)
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* [Container](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#container-v1-core)
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* See `terminationGracePeriodSeconds` in [PodSpec](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#podspec-v1-core)
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