ConfigMap data propagation

README.md

Does ConfigMap data propagate to pods where it is used?

Experiment

Let's see. We first create a config map called testcm:

kubectl create configmap testcm --from-literal=somekey=INITIAL_VALUE

Now a pod that uses the testcm config map in an environment variable and via a volume mount:

kubectl apply -f podusecm.yaml

Let's have a look at the value of the environment variable created from the config map:

$ kubectl exec -it cmtest -- env | grep SOME_KEY
SOME_KEY=INITIAL_VALUE

And now at the volume mount created from the config map:

$ kubectl exec -it cmtest -- cat /tmp/somekey
INITIAL_VALUE

Now we change the value of the config map data, that is, we assign the somekey key in the testcm config map a new value:

kubectl patch configmap testcm --type merge  -p '{"data":{"somekey":"UPDATED_VALUE"}}'

And check again, first the environment variable:

$ kubectl exec -it cmtest -- env | grep SOME_KEY
SOME_KEY=INITIAL_VALUE

The value of the environment variable SOME_KEY in the pod, which we created from the config map HAS NOT changed.

Finally, we check the volume mount:

$ kubectl exec -it cmtest -- cat /tmp/somekey
UPDATED_VALUE

The value of the file /tmp/somekey in the pod, which we created from the config map HAS changed.

Conclusion

The config map data changes do propagate automatically if used via a volume mount, however not via environmment variables. See also Issue 22368 for details.

podusecm.yaml

apiVersion: v1
kind: Pod
metadata:
  name: cmtest
spec:
  containers:
    - name: main
      image: quay.io/mhausenblas/jump:0.2
      command: [ "sh", "-c", "sleep 10000" ]
      env:
        - name: SOME_KEY
          valueFrom:
            configMapKeyRef:
              name: testcm
              key: somekey
      volumeMounts:
        - name: config-volume
          mountPath: /tmp
  volumes:
    - name: config-volume
      configMap:
        name: testcm