Helm provides a nice way to distribute Kubernetes applications, by allowing providers to define a set a templates (Charts) that make use of overridable inputs. Charts can then be instantiated and customized (as Releases), using inputs that can be set in various ways, either via the command line, or via one or more Values files.

At some point however, one might want to merge dynamic data, i.e. end-user values not known in advance by the Chart developers. For example, I had a simple case where, given a YAML object representing a base configuration to be used as a Kubernetes ConfigMap, I wanted users to be able to override part of this configuration or append new properties to it, without me knowing the exact keys in the first place. This was needed for example in a SpringBoot application, which could define a lot of Configuration properties; and I did not want to explicitly list all possible properties in the default Chart Values file.

In other words, the Helm Chart would provide certain predefined configuration data, while users might specify other configuration data. And all these data would then get merged into a single Kubernetes ConfigMap or Secret, so as to be consumed in the Application Pods.

This is the point of this article, where I wanted to share how I stumbled upon a nice builtin template function in Helm that I thought would be useful in similar contexts.

As usual, you can find the repository that backs this article on GitHub://rm3l/demo-merging-dynamic-configuration-in-helm

A picture being worth a thousand words, let’s jump right in with a big picture of what we are trying to achieve here:

Sample Chart

Let’s quickly get started by generating a sample Helm Chart using the “helm create” command:

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helm create demo-merging-dynamic-configuration-in-helm

This command creates a demo-merging-dynamic-configuration-in-helm directory, along with the common files and directories typically used in a Chart:

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❯ tree demo-merging-dynamic-configuration-in-helm 
demo-merging-dynamic-configuration-in-helm
├── charts
├── Chart.yaml
├── templates
│ ├── deployment.yaml
│ ├── _helpers.tpl
│ ├── hpa.yaml
│ ├── ingress.yaml
│ ├── NOTES.txt
│ ├── serviceaccount.yaml
│ ├── service.yaml
│ └── tests
│     └── test-connection.yaml
└── values.yaml

3 directories, 10 files

At any time, we can check locally how Helm would render those templates using the “helm template” command and inspecting its output, like so:

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helm template demo-merging-dynamic-configuration-in-helm

To limit the rendering to a single template, we can use the “-s relative/path/to/template/file” option, e.g.:

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helm template demo-merging-dynamic-configuration-in-helm \
  -s templates/deployment.yaml

Configuration

Predefined non-overridable configuration

This can be done by leveraging the Chart Helper named template to define a new element. The Helper template can perform more complex work, but we will keep it simple for now. It simply defines our base configuration data as a YAML object.

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diff --git a/templates/_helpers.tpl b/templates/_helpers.tpl
index 5abdb85..1e8fff1 100644
--- a/templates/_helpers.tpl
+++ b/templates/_helpers.tpl
@@ -60,3 +60,12 @@ Create the name of the service account to use
 {{- default "default" .Values.serviceAccount.name }}
 {{- end }}
 {{- end }}
+
+{{/*
+Default configuration
+*/}}
+{{- define "demo-merging-dynamic-configuration-in-helm.baseConfig" -}}
+myConfig1:
+  nonUpdatableParameter1: some-value
+nonUpdatableOption1: value1
+{{- end }}

Overridable configuration

Properties that can be overridden are typically defined in the Chart values.yaml file, like so:

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diff --git a/values.yaml b/values.yaml
index f918f2d..e5ee9fd 100644
--- a/values.yaml
+++ b/values.yaml
@@ -80,3 +80,9 @@ nodeSelector: {}
 tolerations: []
 
 affinity: {}
+
+config:
+  myConfig1:
+    parameter11: value11
+    parameter12: value12
+

This provides providing a single source of truth and documentation for all accepted inputs. This way, they may also end up in some sort of documentation for the Helm Chart, which could be generated by tools like helm-docs.

But bear in mind that users can specify any other data under the config element, even if they were not explicitly declared in the values.yaml file.

Approaches for meeting the requirements

Now suppose you want to inject a ConfigMap (representing the target application configuration) into this Chart, and mount it under a Volume in any of the Pods part of the Deployment. This last point is not covered in this article, but left as an exercise to the reader. See https://kubernetes.io/docs/tasks/configure-pod-container/configure-pod-configmap/ for more details.

Predefined configuration data defined in the _templates/helpers.tpl file are considered mandatory, and should not change. But all other configuration data exposed in values.yaml under the config field are customizable.

Approach #1 — Using static values

A very simple way to do this could be to first declare all of the properties expected in the ConfigMap in the default values.yaml file (so they can be overridden by users installing this Chart), and then update our ConfigMap template file to manually define where each property comes from.

The code for this approach can be found in the approach_1_values_containing_all_confimap_properties branch.

Here is the ConfigMap template (in a templates/configmap.yaml file):

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apiVersion: v1
kind: ConfigMap
metadata:
  name: {{ .Release.Name }}-configmap
data:
  myconfig.properties: |
    myConfig1:
      nonUpdatableParameter1: {{ include "demo-merging-dynamic-configuration-in-helm.baseConfig.myConfig1.nonUpdatableParameter1" . }}
      parameter11: {{ .Values.config.myConfig1.parameter11 }}
      parameter12: {{ .Values.config.myConfig1.parameter12 }}
    myConfig2:
      parameter21: {{ .Values.config.myConfig2.parameter21 }}
    nonUpdatableOption1: {{ include "demo-merging-dynamic-configuration-in-helm.baseConfig.nonUpdatableOption1" . }}
    option2: {{ .Values.config.option2 }}    

And the diff for the other files:

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diff --git a/templates/_helpers.tpl b/templates/_helpers.tpl
index 1e8fff1..cc5e49e 100644
--- a/templates/_helpers.tpl
+++ b/templates/_helpers.tpl
@@ -64,8 +64,9 @@ Create the name of the service account to use
 {{/*
 Default configuration
 */}}
-{{- define "demo-merging-dynamic-configuration-in-helm.baseConfig" -}}
-myConfig1:
-  nonUpdatableParameter1: some-value
-nonUpdatableOption1: value1
+{{- define "demo-merging-dynamic-configuration-in-helm.baseConfig.myConfig1.nonUpdatableParameter1" -}}
+some-value
+{{- end }}
+{{- define "demo-merging-dynamic-configuration-in-helm.baseConfig.nonUpdatableOption1" -}}
+value1
 {{- end }}
diff --git a/values.yaml b/values.yaml
index e5ee9fd..30433ca 100644
--- a/values.yaml
+++ b/values.yaml
@@ -85,4 +85,7 @@ config:
   myConfig1:
     parameter11: value11
     parameter12: value12
-
+  myConfig2:
+    parameter21: value21
+  option1: value1
+  option2: value2

We can check that we get the expected result:

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❯ helm template . \
    -s templates/configmap.yaml \
    --set config.myConfig1.nonUpdatableParameter1="my-value-that-should-not-get-changed" \
    --set config.myConfig1.parameter11="my-value11-overridden" \
    --set config.myConfig2.parameter21="my-value21" \
    --set config.option2="my-value2" \
    --set config.userDefinedDynamicOption="my value"

---
# Source: demo-merging-dynamic-configuration-in-helm/templates/configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: RELEASE-NAME-configmap
data:
  myconfig.properties: |
    myConfig1:
      nonUpdatableParameter1: some-value
      parameter11: my-value11-overridden
      parameter12: value12
    myConfig2:
      parameter21: my-value21
    nonUpdatableOption1: value1
    option2: my-value2

As you can guess, this does not quite meet our initial requirement, because it requires all configuration keys to be known in advance and declared in the default Chart values.yaml file.

Also, additional user-defined values (like config.userDefinedDynamicOption), not explicitly declared in the Chart Values file and consumed in the ConfigMap template, will just get ignored.

Approach #2 (recommended) — Allowing users to inject dynamic data

The code for this approach can be found in the approach_2_dynamic_data branch.

This approach, which is the recommended one, builds upon 2 simple principles:

• The initial state is that all mandatory configuration data that should not be customized are defined in a baseConfig Helper variable, as a complete YAML object. Configuration that are explicitly customizable are defined in the Chart values.yaml file.
• Any other configuration data passed at runtime (nested under the config field) by end-users is just merged as is to construct the final ConfigMap.

Without further ado, here is the ConfigMap template (in templates/configmap.yaml):

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{{- $baseConfig := include "demo-merging-dynamic-configuration-in-helm.baseConfig" . | fromYaml -}}
{{- $mergedConfig := mustMergeOverwrite (dict) .Values.config $baseConfig -}}
apiVersion: v1
kind: ConfigMap
metadata:
  name: {{ .Release.Name }}-configmap
data:
  myconfig.properties: |
        {{- toYaml $mergedConfig | nindent 4 }}

We can check that we also get the expected result:

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❯ helm template . \
    -s templates/configmap.yaml \
    --set config.myConfig1.nonUpdatableParameter1="my-value-that-should-not-get-changed" \
    --set config.myConfig1.parameter11="my-value11-overridden" \
    --set config.myConfig2.parameter21="my-value21" \
    --set config.option2="my-value2" \
    --set config.userDefinedDynamicOption="my value"
    
---
# Source: demo-merging-dynamic-configuration-in-helm/templates/configmap.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: RELEASE-NAME-configmap
data:
  myconfig.properties: |
    myConfig1:
      nonUpdatableParameter1: some-value
      parameter11: my-value11-overridden
      parameter12: value12
    myConfig2:
      parameter21: my-value21
    nonUpdatableOption1: value1
    option2: my-value2
    userDefinedDynamicOption: my value

We can see that :

• config.myConfig1.nonUpdatableParameter1 was set to be overridden, but its value finally did not change in the resulting ConfigMap. This means that our baseConfig variable has precedence over user-defined configuration.
• Unlike the approach #1 above, config.myConfig2, config.option2 and config.userDefinedDynamicOption were not defined anywhere in the Chart, but set at runtime. They ended up being merged to construct the resulting ConfigMap. And this provides much more flexibility.

Why approach #2 works

Approach #2 leverages the mustMergeOverwrite template function to work with dictionaries in Helm Charts.

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{{- $baseConfig := include "demo-merging-dynamic-configuration-in-helm.baseConfig" . | fromYaml -}}
{{- $mergedConfig := mustMergeOverwrite (dict) .Values.config $baseConfig -}}

This is quite useful in the scope of this article, as it allows to merge two or more dictionaries into one, giving precedence from one dictionary to another.

That’s it for this post. As usual, feel free to share your valuable thoughts in the comments.