Override env values defined in container spec

Question

I have a configmap where I have defined the following key-value mapping in the data section:

apiVersion: v1
kind: ConfigMap
metadata:
  namespace: test
  name: test-config
data:
  TEST: "CONFIGMAP_VALUE"

then in the definition of my container (in the deployment / statefulset manifest) I have the following:

        env:
        - name: TEST
          value: "ANOTHER_VALUE"
        envFrom:
        - configMapRef:
            name: test-config

When doing this I was expecting that the value from the configmap (TEST="CONFIGMAP_VALUE") will override the (default) value specified in the container spec (TEST="ANOTHER_VALUE"), but this is not the case (TEST always gets the value from the container spec). I couldn't find any relevant documentation about this - is it possible to achieve such env variable value overriding?

Answer 1

From Kubernetes api refernece : https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.13/#container-v1-core

envFrom : List of sources to populate environment variables in the container. The keys defined within a source must be a C_IDENTIFIER. All invalid keys will be reported as an event when the container is starting. When a key exists in multiple sources, the value associated with the last source will take precedence. Values defined by an Env with a duplicate key will take precedence. Cannot be updated.

So above clearly states the env will take precedence than envFrom.

When a key exists in multiple sources, the value associated with the last source will take precedence.

So, for overriding see below:

apiVersion: v1
kind: ConfigMap
metadata:
  namespace: default
  name: test-config
data:
  TEST: "CONFIGMAP_VALUE"
---
apiVersion: v1
kind: Pod
metadata:
  name: busy
  namespace: default
spec:
  containers:
  - name: busybox
    image: busybox
    env:
    - name: TEST
      value: "DEFAULT_VAULT"
    - name: TEST
      valueFrom:
        configMapKeyRef:
          name: test-config
          key: TEST
    command:
    - "sh"
    - "-c"
    - >
      while true; do
        echo "$(TEST)";
        sleep 3600;
      done

Check:

kubectl logs busy -n default
CONFIGMAP_VALUE