What are ConfigMaps?

A ConfigMap allows you to decouple environment-specific configuration from your container images, so that your applications are easily portable.

Note: ConfigMap does not provide secrecy or encryption. If the data you want to store are confidential, use a Secret rather than a ConfigMap

Imperative way to create configmaps:

kubectl create configmap <CONFIG_NAME> --from-literal=<KEY>=<VALUE>
kubectl create configmap <CONFIG_NAME> --from-file=<PATH_TO_FILE>

Declarative way to create configmaps:

# kubectl create -f <FILE>
apiVersion: v1
kind: ConfigMap
metadata:
    name: <CONFIG_NAME>
data:
    <KEY>: <VALUE>

To use config map in a pod, you need to mount it to the container. Refer the example below:

apiVersion: v1
kind: Pod
metadata:
  name: configmap-demo-pod
spec:
  containers:
    - name: demo
      image: alpine
      envFrom:              # Fetch complete config from a ConfigMap
        - configMapRef:
            name: <CONFIG_NAME>
      env:                  # Fetch specific key from a ConfigMap
        - name: <KEYNAME_TO_PASS_TO_POD>
          valueFrom:
            configMapKeyRef:
              name: <CONFIG_NAME>
              key: <KEYNAME_IN_CONFIG>

Notes

  1. To run a custom command in a container instead of standard command that comes with image, use command param in pod spec.

  2. To pass custom args to a container, use args param in pod spec.

  3. Remember, you CANNOT edit specifications of an existing POD other than the below.

        spec.containers[*].image

    spec.initContainers[*].image

    spec.activeDeadlineSeconds

    spec.tolerations

  4. For example you cannot edit the environment variables, service accounts, resource limits (all of which we will discuss later) of a running pod. But if you really want to, you have 2 options:

    1. Run the kubectl edit pod command. This will open the pod specification in an editor (vi editor). Then edit the required properties. When you try to save it, you will be denied. This is because you are attempting to edit a field on the pod that is not editable. A copy of the file with your changes is saved in a temporary location as shown above. You can then delete the existing pod by running the command: kubectl delete pod webapp Then create a new pod with your changes using the temporary file kubectl create -f /tmp/kubectl-edit-acvrq.yaml

    2. The second option is to extract the pod definition in YAML format to a file using the command kubectl get pod webapp -o yaml > my-new-pod.yaml Then make the changes to the exported file using an editor (vi editor). Save the changes. Then delete the existing pod kubectl delete pod webapp Then create a new pod with the edited file kubectl create -f my-new-pod.yaml

  5. Edit Deployments

    With Deployments you can easily edit any field/property of the POD template. Since the pod template is a child of the deployment specification, with every change the deployment will automatically delete and create a new pod with the new changes. So if you are asked to edit a property of a POD part of a deployment you may do that simply by running the command

    kubectl edit deployment my-deployment

Continue reading: Kubernetes: Core Concepts Primer - Part 4