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Exercise Explanation

ConfigMaps Part 2

The problem with the ConfigMap described in 40-pod-with-config-map-env.yaml is not obvious. In fact, it's one of the harder problems to identify as the YAML file seems semantically valid.

In order to understand the problem it is necessary to remember that ConfigMaps are key value pairs.

Look again at the ConfigMap config-example-1:

kubectl get configmaps config-example-1 -o yaml

The output is something like:

apiVersion: v1
data:
  20-config-file.conf: |-
    # This is an exemplary config file

    number-of-requests = 20
    very-import-switch = true
kind: ConfigMap
metadata:
  creationTimestamp: '2020-03-04T17:54:19Z'
  name: config-example-1
  namespace: k8s-training
  resourceVersion: '6128955'
  selfLink: /api/v1/namespaces/k8s-training/configmaps/config-example-1
  uid: 2ef0fc0d-e128-47d7-be57-766d2b023612

Now compare it to the output of config-example-2. Can you see the subtle difference?

kubectl get configmaps config-example-2 -o yaml

The output looks like this:

apiVersion: v1
data:
  number-of-requests: '20'
  very-import-switch: 'true'
kind: ConfigMap
metadata:
  creationTimestamp: '2020-03-04T17:40:48Z'
  name: config-example-2
  namespace: k8s-training
  resourceVersion: '6126817'
  selfLink: /api/v1/namespaces/k8s-training/configmaps/config-example-2
  uid: 3366244e-da8e-4ac8-8cf4-dc21fca7ff56

If you compare the indentation you may recognize that in config-example-1 the section data contains only one key: 20-config-file.conf. This implies that for Kubernetes this ConfigMap has one key and its value is a string:

# This is an exemplary config file

number-of-requests = 20
very-import-switch = true

This string is put into the ConfigMap value literally corresponding to the key 20-config-file.conf. A sign for this is the |- token indicating that a multi-line string follows and that newlines at the end of the string are to be stripped [1]. Consequently, Kubernetes does not parse the value associated with the key 20-config-file.conf. It doesn't know about its structure, although it is obvious to the human mind. Exactly that's the gotcha.

In contrast to this, in config-example-2 the structure is different. There is no top-level key named 20-config-file.conf. Instead, there are two keys number-of-requests and very-import-switch. As Kubernetes understands ConfigMap keys and values they can be used as expected in the Pod definition to define environment variables.

Here is how the fixed ConfigMap should look like:

apiVersion: v1
kind: Pod
metadata:
  name: busybox-config
spec:
  containers:
    - image: busybox
      name: busybox-config-container
      command:
        - 'env'
      env:
        - name: NUMBER_OF_REQUESTS
          valueFrom:
            configMapKeyRef:
              name: config-example-2
              key: number-of-requests
        - name: VERY_IMPORTANT_SWITCH
          valueFrom:
            configMapKeyRef:
              name: config-example-2
              key: very-important-switch
  restartPolicy: Never

So why Go Through This Hassle?​

Sometimes you want to access several parameters individually. In this case the definition of distinct key-value pairs is necessary. However, if you want to store an entire config file as a value, reading the config file into a ConfigMap associated with a filename-like key such as config-file.conf is helpful.

  1. YAML Multiline, https://yaml-multiline.info/