Using Files as ConfigMaps in Kubernetes


The canonical example for ConfigMaps in K8S looks something like this

apiVersion: v1
kind: ConfigMap
metadata:
  name: special-config
  namespace: default
data:
  SPECIAL_LEVEL: very
  SPECIAL_TYPE: charm

Personally, I have some issues with this:-

  1. When a developer manages the properties, I don't want the dev to accidentally mess with the other metadata that exists in the file.
  2. It's YAML. YAML with its indentation based namespacing is super easy to get wrong.
  3. When you have a large number of properties, or multiple properties files, you have either multiple ConfigMaps or a really large ConfigMap YAML file.

Configuration changes based on environment, so you're probably using some form of templatization over the YAML. The following is an example in Helm

apiVersion: v1
kind: ConfigMap
metadata:
  name: special-config
  namespace: default
data:
  SPECIAL_LEVEL: { { .Values.special_level } }
  SPECIAL_TYPE: { { .Values.special_type } }

So my goals were to:-

  1. Have propeties files seperate from the ConfigMap definition itself.
  2. It should be templatized.
  3. It should not be YAML.

So this is my solution!

File directory structure:

application-charts
|
|---config
|   |--- file.properties
|   |--- file2.properties
|---templates
|   |--- configmap.yaml
|   |--- deployment.yaml
|   |--- ...
|---values.production.yaml
|---values.qa.yaml
|--- ...

My ConfigMap.yaml looks like this

apiVersion: v1
kind: ConfigMap
metadata:
  name: special-config
data: { { - (tpl (.Files.Glob "config/*").AsConfig . ) | nindent 2 } }

And my properties file looks like this

somePropertyThatIsConstant=HELLO
somePropertyThatChanges={{ .Values.some_property_that_changes }}

Explanation

.Files.Glob is a helper that Helm provides to read directories inside the Helm chart folder structure, and .AsConfig is a helper that reads the files as configuration. We then pass those files to the tpl function to replace the template strings in the files. We pipe the result to nindent 2 to correctly indent it with 2 spaces.