In this post, i am going to go over ConfigMaps. I assume you have a basic understanding of Kubernetes and its use cases. It is also important to have a basic understanding of what a pod is. To follow along with the demo example, you will need kubectl and minikube installed.
What is a ConfigMap?
When working with 12 factor apps one of the factor is configs so that mean when dealing with microservices, you need to figure out how the configurations will be applied. In situations where you want to deploy to multiple environments like stage, dev and prod, it is not ideal to bake the configs into the application because of the difference in environments. Ideally you want separate configurations to match the environment you are deploying to. This is where ConfigMaps comes into play. It allows you to decouple configuration artifacts from image content. This allows containerized application to become portable without worrying about configurations. ConfigMap is similar to Secrets, but provides a way of working with strings that don’t contain sensitive information. Users and system components can store configuration data in ConfigMap.
How to create ConfigMap?
Creating ConfigMaps is pretty simple and straightforward. You can create it with directories, file or literal values. Let us see each in action. The rest of the blog post will use a simple example to demonstrate how to work with a configmap.
From a directory
To create configmap from a directory, we must create or have an existing directory with our configs in there.
$ mkdir configmap-demo
wget the configs into our configmap-demo directory.
$ wget https://k8s.io/docs/tasks/configure-pod-container/configmap/kubectl/game.properties -O configmap-demo
$ wget https://k8s.io/docs/tasks/configure-pod-container/configmap/kubectl/ui.properties -O configmap-demo
What we just did was download test config files into our directory. We can then create a configmap from our directory that will have all the files we downloaded.
$ kubectl create configmap demo-configmap --from-file=configmap-demo configmap "demo-configmap" created
If we describe our configmap, we will see both files as data entries with their contents.
$ kubectl describe configmap demo-configmap Name: demo-configmap Namespace: default Labels: <none> Annotations: <none> Data ==== game.properties: ---- enemies=aliens lives=3 enemies.cheat=true enemies.cheat.level=noGoodRotten secret.code.passphrase=UUDDLRLRBABAS secret.code.allowed=true secret.code.lives=30 ui.properties: ---- color.good=purple color.bad=yellow allow.textmode=true how.nice.to.look=fairlyNice Events: <none>
From a file
Creating from a file is very similar to creating from directory. All we need to do is pass in the name of the file to –from-file argument. When creating configmap this way, you can pass in as many files as you want to the –from-file argument and it will add it to the configmap.
From literal value
Creating configmap this way mean you can specify your configuration directly from command line without creating any file or directory. For example “kubectl create configmap special-config –from-literal=special.how=very –from-literal=special.type=charm”. You can have multiple key-value pairs if needed.
Using configmap in a pod
Lets create a test configmap
$ kubectl create configmap special-config --from-literal=special.how=very configmap "special-config" created
Then create a pod to use the configmap as an env variable.
apiVersion: v1 kind: Pod metadata: name: configmap-demo-pod spec: containers: - name: test-container image: k8s.gcr.io/busybox command: [ "/bin/sh", "-c", "env" ] env: # Define the environment variable - name: SPECIAL_LEVEL_KEY valueFrom: configMapKeyRef: # The ConfigMap containing the value you want to assign to SPECIAL_LEVEL_KEY name: special-config # Specify the key associated with the value key: special.how restartPolicy: Never
$ Kubectl create -f demo-pod.yaml pod "configmap-demo-pod" created
We can now see that there is an environment variable set in the pod with our defined configmap value.
$ kubectl logs configmap-demo-pod | grep SPECIAL_LEVEL_KEY SPECIAL_LEVEL_KEY=very
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