Kustomize

Introduction to Kustomize

TL;DR

  • Kustomize helps customizing config files in a template free way.
  • Kustomize provides a number of handy methods like generators to make customization easier.
  • Kustomize uses patches to introduce environment specific changes on an already existing standard config file without disturbing it.

Kustomize provides a solution for customizing Kubernetes resource configuration free from templates and DSLs.

Kustomize lets you customize raw, template-free YAML files for multiple purposes, leaving the original YAML untouched and usable as is.

Kustomize targets kubernetes; it understands and can patch kubernetes style API objects. It’s like make, in that what it does is declared in a file, and it’s like sed, in that it emits edited text.

Usage

1) Make a kustomization file

In some directory containing your YAML resource files (deployments, services, configmaps, etc.), create a kustomization file.

This file should declare those resources, and any customization to apply to them, e.g. add a common label.

File structure:

  1. ~/someApp
  2. ├── deployment.yaml
  3. ├── kustomization.yaml
  4. └── service.yaml

The resources in this directory could be a fork of someone else’s configuration. If so, you can easily rebase from the source material to capture improvements, because you don’t modify the resources directly.

Generate customized YAML with:

  1. kustomize build ~/someApp

The YAML can be directly applied to a cluster:

  1. kustomize build ~/someApp | kubectl apply -f -

2) Create variants using overlays

Manage traditional variants of a configuration - like development, staging and production - using overlays that modify a common base.

File structure:

  1. ~/someApp
  2. ├── base
  3. │   ├── deployment.yaml
  4. │   ├── kustomization.yaml
  5. │   └── service.yaml
  6. └── overlays
  7.     ├── development
  8.     │   ├── cpu_count.yaml
  9.     │   ├── kustomization.yaml
  10.     │   └── replica_count.yaml
  11.     └── production
  12.         ├── cpu_count.yaml
  13.         ├── kustomization.yaml
  14.         └── replica_count.yaml

Take the work from step (1) above, move it into a someApp subdirectory called base, then place overlays in a sibling directory.

An overlay is just another kustomization, referring to the base, and referring to patches to apply to that base.

This arrangement makes it easy to manage your configuration with git. The base could have files from an upstream repository managed by someone else. The overlays could be in a repository you own. Arranging the repo clones as siblings on disk avoids the need for git submodules (though that works fine, if you are a submodule fan).

Generate YAML with

  1. kustomize build ~/someApp/overlays/production

The YAML can be directly applied to a cluster:

  1. kustomize build ~/someApp/overlays/production | kubectl apply -f -