Kubernetes ConfigMaps Explained: What You Need to Know

What is a ConfigMap in Kubernetes?

A ConfigMap is an API object that lets you store configuration data as a key-value pairs. This data is then consumed by other kubernetes resources such as pods. It is mainly used to store non-confidential data, as it does not provide any sort of secrecy or encryption.

A ConfigMap is not designed to hold large amount of data (1 MiB Limit). For larger data storage you can use multiple ConfigMaps or use volume instead of ConfigMaps.

Why would you use a ConfigMap in Kubernetes?

You can use a ConfigMap to keep your application code separate from your configuration. This lets you change easily your configuration depending on your environment such as dev or prod and can dynamically change configuration at runtime.

When you work locally on your laptop, you set the environment variable in you shell, so when it comes to kubernetes deployments, you reference values in a ConfigMap.

How to use Kubernetes ConfigMap using YAML?

apiVersion: v1
kind: ConfigMap
metadata:
  name: test-cm
data:
  db-port: "3306"

• kind: Specifies the kind of resource, in this case its a ConfigMap.

• data: Contains the actual configuration data as key-value pairs.

Create the ConfigMap using the below command:

Kubectl apply -f cm.yaml

We will look at two different ways to use a ConfigMap to configure a container inside a Pod:

1. Using environment variables

2. Using volume mounts

Example of ConfigMap as pod environment variable

apiVersion: v1
kind: Pod
metadata:
  name: demo-cm
spec:
  containers:
  - name: demo-app
    image: nginx
    env:
    - name: DB-PORT
      valueFrom:
        configMapKeyRef:
          name: test-cm
          key: db-port

• DB-PORT - Name of the env variable

• valueFrom: Get the value from configMap

• configMapKeyRef: ConfigMap reference

• name: Name of the ConfigMap

• key: value of key inside the ConfigMap

kubectl apply -f pod.yaml

Once the pods are running, exec into the pods and run the below command.

kubectl exec -it (name of the pod) -- /bin/bash

Now we are inside the container, next run the below command to check the env variable.

env | grep DB

This is how you retrieve the environment variable from the ConfigMap.

Example of ConfigMap as volume mounts

apiVersion: v1
kind: pod
metadata:
  name: demo
spec:
  containers:
  - name: demo-python-app
    image: python-app
    volumeMounts:
     - name: db-connections
       mountPath: /opt
  volumes:
  - name: db-connections
    configMap:
      name: test-cm

• volumes: - name: db-connections - Name of the volume.

• configMap: - name: test-cm - Read the information from the ConfigMap named test-cm.

• volumeMounts: Mounting the volume named db-connections.

• mountPath: Path for the volume mount.

kubectl apply pod1.yaml

Once the pods are running, exec into the pods and run the below command.

kubectl exec -it (name of the pod) -- /bin/bash

Now we are inside the container, next run the below command to check the env variable.

cat /opt/db-port | more

The port number is 3308, since we have updated the cm.yml file which you can see below.

Understanding ConfigMap updates

When a ConfigMap is set with the environment variable they won't update the value automatically until the containers are restarted. The same ConfigMap when set using the volumes, it will receive the new ConfigMaps value automatically without restarting the containers.

Conclusion:

In the above guide we have covered both the ways to use the ConfigMaps - either by using them as environment variables or mounting them into a file.

ConfigMaps are stored in plain text so you should not use them for sensitive values. Kubernetes secrets are the alternative to ConfigMaps to store sensitive information with encryption support, which we will cover in our next part.

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