Kubernetes Chronicles:(K8s#07)|K8s Series | PODs | Environment Variables.

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Kubernetes environment variables are dynamic key-value pairs that provide configuration settings to containers within the pods. These variables later can be used by the application during runtime.

Environment variables can be defined at both the Pod and individual container levels. Variables defined at the pod level are shared among all containers within a pod.

We can define the Kubernetes Environment variables using env or envFrom fields in the manifests file. It overrides any environment variables defined in the container image. We can also set dependent environment variables using ${VAR_NAME} under the value section.

env : allows to set environment variables by specifying key-value pairs directly for each variable.

envFrom : allows to set environment variables by referencing either a ConfigMap or Secret.

Using env keyword:

This is straightforward forward and we can easily define any environment variables in the deployment manifest using env keyword under spec.containerssection.

apiVersion: v1
kind: Pod
metadata:
  name: envar-demo
  labels:
    purpose: demonstrate-envars
spec:
  containers:
  - name: envar-demo-container
    image: gcr.io/google-samples/node-hello:1.0
    env:
    - name: DEMO_GREETING
      value: "Hello from the environment"
    - name: DEMO_FAREWELL
      value: "Such a sweet sorrow"

• configMapKeyRef: To define individual key-value pairs as environment variables from configmap.

apiVersion: v1
kind: Pod
metadata:
  name: dapi-test-pod
spec:
  containers:
    - name: test-container
      image: registry.k8s.io/busybox
      command: [ "/bin/echo", "$(SPECIAL_LEVEL_KEY) $(SPECIAL_TYPE_KEY)" ]
      env:
        - name: SPECIAL_LEVEL_KEY
          valueFrom:
            configMapKeyRef:
              name: special-config
              key: SPECIAL_LEVEL
        - name: SPECIAL_TYPE_KEY
          valueFrom:
            configMapKeyRef:
              name: special-config
              key: SPECIAL_TYPE
  restartPolicy: Never

• secretKeyRef: To define individual secret key-value pairs as environment variables from Kubernetes secret object.

---
apiVersion: apps/v1
kind: Deployment
(...)
      containers:
        - name: example-app-prod
          image: [yourimage]
          env:
            # Inject variables from a Kuberentes secret
            - name: secret_variables
              valueFrom:
                secretKeyRef:
                  name: secret_data
                  key: password

Using envFrom keyword.

• ConfigMapRef: To define all Configmap’s data as the environment variable.

apiVersion: v1
kind: Pod
metadata:
  name: dapi-test-pod
spec:
  containers:
    - name: test-container
      image: registry.k8s.io/busybox
      command: [ "/bin/sh", "-c", "env" ]
      envFrom:
      - configMapRef:
          name: my-env-config
  restartPolicy: Never

• secretRef: To define all secrets i.e... sensitive configuration data as the environment variable. In this way, we only need to specify the Kubernetes secret name, which in turn load all data as environment variables automatically.

---
apiVersion: apps/v1
kind: Deployment
(...)
      containers:
        - name: example-app-prod
          image: [yourimage]
          env:
            # Inject variables from a Kuberentes secret
            - name: secret_variables
              valueFrom:
                secretRef:
                  name: secret_data

Summary:

In this article, we have learned how to define environment variables in multiple ways depending on application-specific use cases. We can use env and envFrom by leveraging the ConfigMap and Secret objects to load the environment variables into application runtime dynamically.

In the next upcoming article, we will learn about Pod ConfigMaps. Stay Tuned.

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