This content originally appeared on DEV Community and was authored by Benoit COUETIL 💫

1. What is Kustomize?
2. Why variable substitution matters
3. Limitations and advantages of kustomize by default
4. Implementing custom variable substitution
- No compromise solution using renvsubst as a transformer
Wrapping up
Further reading

1. What is Kustomize?

Kustomize is one of the prominent tools alongside Helm used for managing and configuring Kubernetes deployments across multiple environments. Unlike Helm, which utilizes templates, Kustomize simplifies management by primarily focusing on plain manifests without direct comparison to Helm’s approach.

2. Why variable substitution matters

Variable substitution becomes crucial when environments are ephemeral, as in the case of feature branches, or when configuration settings like DNS entries need to be determined dynamically at deployment time. Kustomize assumes a fixed environment list (e.g., dev/staging/prod), but practical Kubernetes usage often demands more flexibility.

3. Limitations and advantages of kustomize by default

The biggest advantage of Kustomize lies in its straightforward approach to managing manifests. It allows for easy manipulation of fields such as annotations, image names, and replicas across environments. However, Kustomize is strict about variable substitution, preferring a predefined set of options which can be limiting.

To circumvent these limitations, the following official method is often employed within a kustomization.yml file, after creating an environment-specific file:

resources:
  - deployment.yml
  - ingress.yml
  - service.yml
  - sa.yml
  - role.yml
  - role-binding.yml

generatorOptions:
  disableNameSuffixHash: true

configMapGenerator:
- name: environment-variables
  envs: [rendered/api-gateway.env]
  behavior: create

replacements:
- source:
    kind: ConfigMap
    version: v1
    name: environment-variables
    fieldPath: data.API_GATEWAY_URL
  targets:
  - select:
      kind: Ingress
      name: api-gateway
    fieldPaths:
    - spec.rules.0.host
    - spec.tls.0.hosts.0
  - select:
      kind: Deployment
      name: api-gateway
    fieldPaths:
    - spec.template.spec.containers.0.env.[name=API_GATEWAY_URL].value

So much for such a simple need like variable substitution. People will even find Helm appealing at this step. Let's explain why and how to find a simpler solution.

4. Implementing custom variable substitution

While Kustomize apply workflow supports basic variable substitution using tools like envsubst on top of kustomize build, more complex scenarios require custom solutions. One more refined approach involves using renvsubst, to selectively replace variables based on specific criteria (e.g., K_* environment-specific or CI_* CI-specific variables).

We could then use in our pipelines, in ascending order of complexity :

For simple use cases : kustomize build . | envsubst
To avoid replacing some variables, define it in manifests as ${DOLLAR}MY_VARIABLE and then:
- kustomize build . | DOLLAR=$ envsubst
Use renvsubst to choose variables with prefixes:
- kustomize build . | renvsubst --prefix="K_" --prefix="CI_"

But all these solutions have some drawbacks. You have to pipe the kustomize build result before applying manifests; it breaks some convenient wrapping tools features like intelligent local updates using tilt.dev.

No compromise solution using renvsubst as a transformer

For a no compromise solution using a Kustomize transformer exec plugin using renvsubst, follow these steps.

Install renvsubst: Ensure renvsubst is installed from here.
set the KUSTOMIZE_PLUGIN_HOME variable for kustomize to find your plugin, for example KUSTOMIZE_PLUGIN_HOME=$PWD/devops/k8s/scripts/
Set up Transformer Script:

Create a transformer script (renvsubst) at $KUSTOMIZE_PLUGIN_HOME/transformers/renvsubst/renvsubst:

#!/bin/sh

RENVSUBST=${RENVSUBST_PATH:-$(which renvsubst)}

if [ -z "$RENVSUBST" ]; then
    echo "Error: renvsubst command not found. Please install renvsubst from https://github.com/containeroo/renvsubst/releases or set RENVSUBST_PATH." >&2
    exit 1
fi

$RENVSUBST --prefix="K_" --prefix="CI_" -i -

Configure Custom Resource:

Define a custom resource (renvsubst.transformer.yml) next to your kustomization.yml using it:

apiVersion: transformers
kind: renvsubst
metadata:
  name: renvsubst

Integrate with Kustomize:

Update your kustomization.yml to include the custom resource:

resources:
  - ...

transformers:
  - renvsubst.transformer.yml

By following these steps, you can enhance Kustomize's capabilities to handle dynamic environments and complex configuration needs effectively.

Wrapping up

While Kustomize offers simplicity in managing Kubernetes manifests, its default limitations in variable substitution can be overcome through strategic customization. By leveraging custom transformers and renvsubst, Kubernetes deployments with customize can become more adaptable to diverse and evolving environments, ensuring smoother and more efficient operations.

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