Understanding ConfigMap resources on Kubernetes

ConfigMap resources are similar to Secrets, but they are suitable to hold no sensitive information.

As the name suggest, they provide a way to inject configurations into containers in order to set those configuration data separately from the application code.

For example, you can use ConfigMaps to store information about server addresses to connect to.

Example.

1) Create a configmap, to store a server url:

kubectl create configmap backend-config --from-literal=serverAddress=171.24.21.41

Review the configmap:

kubectl get configmaps backend-config -o yaml

apiVersion: v1
data:
  key1: serverAddress=171.24.21.41 

kind: ConfigMap
metadata:
  creationTimestamp: 2020-05-15T16:03:31Z 

  name: backend-config
  namespace: my-namespace 

  resourceVersion: "4637" 

  ...

2) To use the ConfigMap inside a pod, write a definition like pod-definition.yaml:

apiVersion: v1

kind: Pod

metadata:

  name: mypod

  namespace: my-namespace

spec:

  containers:

    - name: myapp

      image: myapp:0.1.0

      env:

        - name: SERVER_ADDRESS

          valueFrom:

            configMapKeyRef:

              name: backend-config

              key: serverAddress

  restartPolicy: Always

3) Create the pod:

kubectl apply -f pod-definition.yaml

4) Now you can read the SERVER_ADDRESS configuration value inside you pod as an environment variable.