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Bjørn Erik Pedersen
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---
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title: Expose Pod Information to Containers Through Files
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content_template: templates/task
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---
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{{% capture overview %}}
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This page shows how a Pod can use a DownwardAPIVolumeFile to expose information
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about itself to Containers running in the Pod. A DownwardAPIVolumeFile can expose
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Pod fields and Container fields.
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{{% /capture %}}
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{{% capture prerequisites %}}
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{{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}
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{{% /capture %}}
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{{% capture steps %}}
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## The Downward API
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There are two ways to expose Pod and Container fields to a running Container:
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* [Environment variables](/docs/tasks/configure-pod-container/environment-variable-expose-pod-information/)
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* DownwardAPIVolumeFiles
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Together, these two ways of exposing Pod and Container fields are called the
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*Downward API*.
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## Store Pod fields
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In this exercise, you create a Pod that has one Container.
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Here is the configuration file for the Pod:
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{{< code file="dapi-volume.yaml" >}}
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In the configuration file, you can see that the Pod has a `downwardAPI` Volume,
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and the Container mounts the Volume at `/etc/podinfo`.
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Look at the `items` array under `downwardAPI`. Each element of the array is a
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[DownwardAPIVolumeFile](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#downwardapivolumefile-v1-core).
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The first element specifies that the value of the Pod's
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`metadata.labels` field should be stored in a file named `labels`.
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The second element specifies that the value of the Pod's `annotations`
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field should be stored in a file named `annotations`.
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{{< note >}}
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**Note:** The fields in this example are Pod fields. They are not
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fields of the Container in the Pod.
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{{< /note >}}
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Create the Pod:
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```shell
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kubectl create -f https://k8s.io/docs/tasks/inject-data-application/dapi-volume.yaml
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```
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Verify that Container in the Pod is running:
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```shell
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kubectl get pods
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```
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View the Container's logs:
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```shell
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kubectl logs kubernetes-downwardapi-volume-example
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```
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The output shows the contents of the `labels` file and the `annotations` file:
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```shell
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cluster="test-cluster1"
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rack="rack-22"
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zone="us-est-coast"
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build="two"
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builder="john-doe"
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```
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Get a shell into the Container that is running in your Pod:
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```
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kubectl exec -it kubernetes-downwardapi-volume-example -- sh
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```
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In your shell, view the `labels` file:
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```shell
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/# cat /etc/podinfo/labels
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```
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The output shows that all of the Pod's labels have been written
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to the `labels` file:
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```shell
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cluster="test-cluster1"
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rack="rack-22"
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zone="us-est-coast"
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```
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Similarly, view the `annotations` file:
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```shell
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/# cat /etc/podinfo/annotations
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```
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View the files in the `/etc/podinfo` directory:
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```shell
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/# ls -laR /etc/podinfo
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```
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In the output, you can see that the `labels` and `annotations` files
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are in a temporary subdirectory: in this example,
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`..2982_06_02_21_47_53.299460680`. In the `/etc/podinfo` directory, `..data` is
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a symbolic link to the temporary subdirectory. Also in the `/etc/podinfo` directory,
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`labels` and `annotations` are symbolic links.
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```
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drwxr-xr-x ... Feb 6 21:47 ..2982_06_02_21_47_53.299460680
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lrwxrwxrwx ... Feb 6 21:47 ..data -> ..2982_06_02_21_47_53.299460680
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lrwxrwxrwx ... Feb 6 21:47 annotations -> ..data/annotations
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lrwxrwxrwx ... Feb 6 21:47 labels -> ..data/labels
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/etc/..2982_06_02_21_47_53.299460680:
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total 8
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-rw-r--r-- ... Feb 6 21:47 annotations
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-rw-r--r-- ... Feb 6 21:47 labels
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```
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Using symbolic links enables dynamic atomic refresh of the metadata; updates are
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written to a new temporary directory, and the `..data` symlink is updated
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atomically using
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[rename(2)](http://man7.org/linux/man-pages/man2/rename.2.html).
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{{< note >}}
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**Note:** A container using Downward API as a
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[subPath](/docs/concepts/storage/volumes/#using-subpath) volume mount will not
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receive Downward API updates.
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{{< /note >}}
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Exit the shell:
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```shell
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/# exit
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```
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## Store Container fields
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The preceding exercise, you stored Pod fields in a DownwardAPIVolumeFile.
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In this next exercise, you store Container fields. Here is the configuration
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file for a Pod that has one Container:
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{{< code file="dapi-volume-resources.yaml" >}}
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In the configuration file, you can see that the Pod has a `downwardAPI` Volume,
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and the Container mounts the Volume at `/etc/podinfo`.
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Look at the `items` array under `downwardAPI`. Each element of the array is a
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DownwardAPIVolumeFile.
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The first element specifies that in the Container named `client-container`,
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the value of the `limits.cpu` field
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should be stored in a file named `cpu_limit`.
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Create the Pod:
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```shell
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kubectl create -f https://k8s.io/docs/tasks/inject-data-application/dapi-volume-resources.yaml
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```
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Get a shell into the Container that is running in your Pod:
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```
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kubectl exec -it kubernetes-downwardapi-volume-example-2 -- sh
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```
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In your shell, view the `cpu_limit` file:
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```shell
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/# cat /etc/podinfo/cpu_limit
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```
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You can use similar commands to view the `cpu_request`, `mem_limit` and
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`mem_request` files.
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{{% /capture %}}
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{{% capture discussion %}}
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## Capabilities of the Downward API
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The following information is available to containers through environment
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variables and `downwardAPI` volumes:
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* Information available via `fieldRef`:
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* `spec.nodeName` - the node’s name
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* `status.hostIP` - the node's IP
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* `metadata.name` - the pod’s name
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* `metadata.namespace` - the pod’s namespace
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* `status.podIP` - the pod’s IP address
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* `spec.serviceAccountName` - the pod’s service account name
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* `metadata.uid` - the pod’s UID
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* `metadata.labels['<KEY>']` - the value of the pod’s label `<KEY>` (for example, `metadata.labels['mylabel']`); available in Kubernetes 1.9+
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* `metadata.annotations['<KEY>']` - the value of the pod’s annotation `<KEY>` (for example, `metadata.annotations['myannotation']`); available in Kubernetes 1.9+
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* Information available via `resourceFieldRef`:
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* A Container’s CPU limit
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* A Container’s CPU request
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* A Container’s memory limit
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* A Container’s memory request
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In addition, the following information is available through
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`downwardAPI` volume `fieldRef`:
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* `metadata.labels` - all of the pod’s labels, formatted as `label-key="escaped-label-value"` with one label per line
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* `metadata.annotations` - all of the pod’s annotations, formatted as `annotation-key="escaped-annotation-value"` with one annotation per line
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{{< note >}}
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**Note:** If CPU and memory limits are not specified for a Container, the
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Downward API defaults to the node allocatable value for CPU and memory.
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{{< /note >}}
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## Project keys to specific paths and file permissions
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You can project keys to specific paths and specific permissions on a per-file
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basis. For more information, see
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[Secrets](/docs/concepts/configuration/secret/).
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## Motivation for the Downward API
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It is sometimes useful for a Container to have information about itself, without
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being overly coupled to Kubernetes. The Downward API allows containers to consume
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information about themselves or the cluster without using the Kubernetes client
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or API server.
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An example is an existing application that assumes a particular well-known
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environment variable holds a unique identifier. One possibility is to wrap the
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application, but that is tedious and error prone, and it violates the goal of low
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coupling. A better option would be to use the Pod's name as an identifier, and
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inject the Pod's name into the well-known environment variable.
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{{% /capture %}}
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{{% capture whatsnext %}}
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* [PodSpec](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#podspec-v1-core)
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* [Volume](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#volume-v1-core)
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* [DownwardAPIVolumeSource](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#downwardapivolumesource-v1-core)
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* [DownwardAPIVolumeFile](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#downwardapivolumefile-v1-core)
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* [ResourceFieldSelector](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#resourcefieldselector-v1-core)
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{{% /capture %}}
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