Expose Pod Information to Containers Through Environment Variables

This page shows how a Pod can use environment variables to expose information about itself to Containers running in the Pod. Environment variables can expose Pod fields and Container fields.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:

• Katacoda
• Play with Kubernetes

The Downward API

There are two ways to expose Pod and Container fields to a running Container:

• Environment variables
• Volume Files

Together, these two ways of exposing Pod and Container fields are called the Downward API.

Use Pod fields as values for environment variables

In this exercise, you create a Pod that has one Container. Here is the configuration file for the Pod:

apiVersion: v1
kind: Pod
metadata:
  name: dapi-envars-fieldref
spec:
  containers:
    - name: test-container
      image: k8s.gcr.io/busybox
      command: [ "sh", "-c"]
      args:
      - while true; do
          echo -en '\n';
          printenv MY_NODE_NAME MY_POD_NAME MY_POD_NAMESPACE;
          printenv MY_POD_IP MY_POD_SERVICE_ACCOUNT;
          sleep 10;
        done;
      env:
        - name: MY_NODE_NAME
          valueFrom:
            fieldRef:
              fieldPath: spec.nodeName
        - name: MY_POD_NAME
          valueFrom:
            fieldRef:
              fieldPath: metadata.name
        - name: MY_POD_NAMESPACE
          valueFrom:
            fieldRef:
              fieldPath: metadata.namespace
        - name: MY_POD_IP
          valueFrom:
            fieldRef:
              fieldPath: status.podIP
        - name: MY_POD_SERVICE_ACCOUNT
          valueFrom:
            fieldRef:
              fieldPath: spec.serviceAccountName
  restartPolicy: Never

In the configuration file, you can see five environment variables. The env field is an array of EnvVars. 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.

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:

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:

/# 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 Pod fields as the values for environment variables. In this next exercise, you use Container fields as the values for environment variables. Here is the configuration file for a Pod that has one container:

apiVersion: v1
kind: Pod
metadata:
  name: dapi-envars-resourcefieldref
spec:
  containers:
    - name: test-container
      image: k8s.gcr.io/busybox:1.24
      command: [ "sh", "-c"]
      args:
      - while true; do
          echo -en '\n';
          printenv MY_CPU_REQUEST MY_CPU_LIMIT;
          printenv MY_MEM_REQUEST MY_MEM_LIMIT;
          sleep 10;
        done;
      resources:
        requests:
          memory: "32Mi"
          cpu: "125m"
        limits:
          memory: "64Mi"
          cpu: "250m"
      env:
        - name: MY_CPU_REQUEST
          valueFrom:
            resourceFieldRef:
              containerName: test-container
              resource: requests.cpu
        - name: MY_CPU_LIMIT
          valueFrom:
            resourceFieldRef:
              containerName: test-container
              resource: limits.cpu
        - name: MY_MEM_REQUEST
          valueFrom:
            resourceFieldRef:
              containerName: test-container
              resource: requests.memory
        - name: MY_MEM_LIMIT
          valueFrom:
            resourceFieldRef:
              containerName: test-container
              resource: limits.memory
  restartPolicy: Never

In the configuration file, you can see four environment variables. The env field is an array of EnvVars. 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 Container fields.

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:

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

What's next

• Defining Environment Variables for a Container
• PodSpec
• Container
• EnvVar
• EnvVarSource
• ObjectFieldSelector
• ResourceFieldSelector