ConfigMap

ConfigMap is used to store non-sensitive information as key-value pairs. Pods can consume ConfigMaps via environment variables, command-line arguments, or volume mounts.
ConfigMap decouples environment-specific configuration from your application code, which improves application portability.
Note that ConfigMap does not provide any encryption.

How to Write a ConfigMap Object

Below is an example of a ConfigMap object:

apiVersion: v1
kind: ConfigMap
metadata:
  name: configmap-demo
data:
  debug: enable
  index.html: |
    hello, world

Like every other Kubernetes API object, a ConfigMap has apiVersion, kind, and metadata fields, but it does not have a spec field. ConfigMap provides two fields — data and binaryData — for storing configuration data.
data: Stores configuration data. Values must contain only UTF-8 encoded data; non-UTF-8 data must use binaryData instead.
binaryData: Stores binary data.

There is also an immutable field that specifies whether the data stored in a ConfigMap can be modified.
Note that keys within the same ConfigMap must be unique. A key already used in data cannot be used in binaryData, and vice versa.

How to Use a ConfigMap

Containers in a Pod can consume a ConfigMap in four ways:

1. As command-line arguments to the container’s entrypoint
2. As environment variables for the container
3. As a read-only volume
4. By reading the ConfigMap directly via the Kubernetes API from within the application

Below is a simple example (also uploaded to GitHub):

apiVersion: v1
kind: Pod
metadata:
  name: configmap-demo-pod
spec:
  containers:
    - name: demo
      image: nginx
      env:
      # environment for container
        - name: DEBUG 
          valueFrom:
            configMapKeyRef:
              name: configmap-demo          
              key: debug
      volumeMounts:
      # read-only volume
      - name: config
        mountPath: "/usr/share/nginx/html"
        readOnly: true
  volumes:
    - name: config
      configMap:
        name: configmap-demo

Results:

1. Verify the environment variable

2. Verify the read-only file