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environment-variable-expose-pod-information.md

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Expose Pod Information to Containers Through Environment Variables
task
30

This page shows how a Pod can use environment variables to expose information about itself to containers running in the Pod, using the downward API. You can use environment variables to expose Pod fields, container fields, or both.

In Kubernetes, there are two ways to expose Pod and container fields to a running container:

  • Environment variables, as explained in this task
  • Volume files

Together, these two ways of exposing Pod and container fields are called the downward API.

As Services are the primary mode of communication between containerized applications managed by Kubernetes, it is helpful to be able to discover them at runtime.

Read more about accessing Services here.

{{% heading "prerequisites" %}}

{{< include "task-tutorial-prereqs.md" >}}

Use Pod fields as values for environment variables

In this part of exercise, you create a Pod that has one container, and you project Pod-level fields into the running container as environment variables.

{{% code_sample file="pods/inject/dapi-envars-pod.yaml" %}}

In that manifest, you can see five environment variables. The env field is an array of environment variable definitions. The first element in the array specifies that the MY_NODE_NAME environment variable gets its value from the Pod's spec.nodeName field. Similarly, the other environment variables get their names from Pod fields.

{{< note >}} The fields in this example are Pod fields. They are not fields of the container in the Pod. {{< /note >}}

Create the Pod:

kubectl apply -f https://k8s.io/examples/pods/inject/dapi-envars-pod.yaml

Verify that the container in the Pod is running:

# If the new Pod isn't yet healthy, rerun this command a few times.
kubectl get pods

View the container's logs:

kubectl logs dapi-envars-fieldref

The output shows the values of selected environment variables:

minikube
dapi-envars-fieldref
default
172.17.0.4
default

To see why these values are in the log, look at the command and args fields in the configuration file. When the container starts, it writes the values of five environment variables to stdout. It repeats this every ten seconds.

Next, get a shell into the container that is running in your Pod:

kubectl exec -it dapi-envars-fieldref -- sh

In your shell, view the environment variables:

# Run this in a shell inside the container
printenv

The output shows that certain environment variables have been assigned the values of Pod fields:

MY_POD_SERVICE_ACCOUNT=default
...
MY_POD_NAMESPACE=default
MY_POD_IP=172.17.0.4
...
MY_NODE_NAME=minikube
...
MY_POD_NAME=dapi-envars-fieldref

Use container fields as values for environment variables

In the preceding exercise, you used information from Pod-level fields as the values for environment variables. In this next exercise, you are going to pass fields that are part of the Pod definition, but taken from the specific container rather than from the Pod overall.

Here is a manifest for another Pod that again has just one container:

{{% code_sample file="pods/inject/dapi-envars-container.yaml" %}}

In this manifest, you can see four environment variables. The env field is an array of environment variable definitions. The first element in the array specifies that the MY_CPU_REQUEST environment variable gets its value from the requests.cpu field of a container named test-container. Similarly, the other environment variables get their values from fields that are specific to this container.

Create the Pod:

kubectl apply -f https://k8s.io/examples/pods/inject/dapi-envars-container.yaml

Verify that the container in the Pod is running:

# If the new Pod isn't yet healthy, rerun this command a few times.
kubectl get pods

View the container's logs:

kubectl logs dapi-envars-resourcefieldref

The output shows the values of selected environment variables:

1
1
33554432
67108864

{{% heading "whatsnext" %}}

Read about Pods, containers and environment variables in the legacy API reference:

  • [PodSpec](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#podspec-v1-core)
  • [Container](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#container-v1-core)
  • [EnvVar](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#envvar-v1-core)
  • [EnvVarSource](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#envvarsource-v1-core)
  • [ObjectFieldSelector](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#objectfieldselector-v1-core)
  • [ResourceFieldSelector](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#resourcefieldselector-v1-core)