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.
You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. If you do not already have a cluster, you can create one by using Minikube, or you can use one of these Kubernetes playgrounds:
To check the version, enter kubectl version
.
There are two ways to expose Pod and Container fields to a running Container:
Together, these two ways of exposing Pod and Container fields are called the Downward API.
In this exercise, you create a Pod that has one Container. Here is the configuration file for the Pod:
pods/inject/dapi-envars-pod.yaml
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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.
Note: The fields in this example are Pod fields. They are not fields of the Container in the Pod.
Create the Pod:
kubectl create -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
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:
pods/inject/dapi-envars-container.yaml
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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 create -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
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