Configuring Applications and Components as Code with Kustomize

If you need to define a large application with many components, easily duplicate or update components, or ensure consistent component and application states, consider configuring your Konflux applications and components as code using Kustomize. This document provides an overview of a basic application and component configuration that can be replicated and duplicated using Kustomize bases and overlays.

For more information about Kustomize, visit the Kustomize official site. This document is based on the configuration defined in the Konflux CI repository, where you can see the configuration first-hand.

Key steps include:

Create a basic file structure for your bases and overlays: Prepare a space and structure for your application and component bases and overlays.
Create a base for your components: Prepare a base that contains all the default or common values for every component you create.
Create a base for your application: Prepare a base that contains all the default or common values for every application you create.
Create an overlay for each component: Prepare an overlay that patches in component-specific data for each component in your application.
Create an overlay for your application: Prepare an overlay that patches in specific data for each application you want to create.
Create patches for application-specific component configurations: Prepare application-specific patches for all components in a given application, accommodating variations in specific patches.
Bonus - Duplicating or expanding your application: Discuss the general philosophy of Configuration-as-Code and how to extend, duplicate, or customize your application, components, and workspace as code.

Throughout this document, substitute your application and component names for placeholder values like application-a, application-b, component-a, etc.

Create a File Structure

As with many Kustomize projects, you’ll need a space to define a base and some overlay(s). Within your overlay(s), define a folder for a base set of components and a folder for each application-specific override for that base set of components.

Create a file structure as follows:

├── base
│   ├── component.yaml
│   └── kustomization.yaml
├── overlay
│   └── application-a
│       ├── base
│       │   ├── application.yaml
│       │   ├── component-a
│       │   │   ├── component-a-override.yaml
│       │   │   └── kustomization.yaml
│       │   ├── component-b
│       │   │   ├── component-b-override.yaml
│       │   │   └── kustomization.yaml
│       │   └── kustomization.yaml
│       └── v1-overlay
│           ├── application-patch.yaml
│           ├── component-patch.yaml
│           ├── exception-component-patch-1.yaml
│           └── kustomization.yaml
└── README.md

You can omit component-b if you only wish to define a single component.

In this file structure, you will find:

base: A directory housing your base component YAML and a Kustomize file to point to the base.
overlay: Contains one or more application variants, useful when defining multiple different applications.
overlay/application-a/base: A base that defines one or more component overrides, one per component in the application, and a base for your application.
overlay/application-a/v1-overlay: An override for your application and application-specific patches for your component(s).

Create a Base for Your Components

If you are creating more than one component, it is likely that your components will share some, if not most, of their configuration. These default or global values can be defined in the base component and overridden on a case-by-case basis in your component overlays.

Creating Your Base Component

Define this base in base/component.yaml. It typically follows a pattern similar to the following:

apiVersion: appstudio.redhat.com/v1alpha1
kind: Component
metadata:
  annotations:
    image.redhat.com/generate: "true"
    build.appstudio.openshift.io/request: configure-pac
  name: example-component
spec:
  componentName: example-component
  application: appName
  targetPort: 8080
  source:
      git:
        url: gitUrl
        context: ./
        dockerfileUrl: ContainerFileLocation
        revision: defaultBranch

This creates a component with a custom build pipeline. If you wish to use the default build, omit the build.appstudio.openshift.io/request: configure-pac annotation.

Creating Your Base Kustomization

With the base component defined, tell Kustomize it exists for later consumption in overlays. Set up a basic base/kustomization.yaml as follows:

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

resources:
  - component.yaml

Create a Base for Your Application

Each application will have its own base and one or more overrides.

You can think of each application variant as the base that defines the structure of your application and all components in that application. The concrete application is an implementation of that variant that defines any differentiating specs, such as application name, version, and component branches.

Create your base application at overlay/application-a/application-a-base/application.yaml like the following:

apiVersion: appstudio.redhat.com/v1alpha1
kind: Application
metadata:
  name: base
spec:
  description: base
  displayName: base

And its Kustomization file at overlay/application-a/application-a-base/kustomization.yaml:

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

resources:
  - application.yaml

Create an Overlay for Each Component

For each application, define every component in the application as an overlay. These overlays should contain all component-specific information that is consistent across all versions of the application, in case you have more than one version of the application.

For example, you’ll typically have more than one version defined if you’re developing a versioned operator and have consistent component names across every version of an application, but different branches (this is the OpenShift / OpenShift CI Model with release-versioned branches).

Define these components as folders in overlay/application-a/base. Each folder should be named after its component name, e.g., component-a, and contain an override.yaml and kustomization.yaml that look like:

overlay/application-a/base/component-a/component-a-override.yaml (replacing name, URL, and Dockerfile):

- op: replace
  path: /metadata/name
  value: component-a-name
- op: replace
  path: /spec/componentName
  value: component-a-name
- op: replace
  path: /spec/source/git/url
  value: https://myvcs.com/myorg/component-a
- op: replace
  path: /spec/source/git/dockerfileUrl
  value: "Dockerfile"

overlay/application-a/base/component-a/kustomization.yaml:

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

resources:
  - ../../../../base # Path to base component

patches:
  - path: component-a-override.yaml # Path to override file
    target:
      kind: Component

You can repeat this pattern for every component in your application.

Create an Overlay for Your Application and Application-Specific Component Configuration

For each version or variant of your application, as configured in the prior steps, define an application overlay and any additional application-specific component patches.

Version this concrete application and set of patches in its own overlay folder in the application folder. In this case, overlay/application-a/v1-overlay will hold:

application-patch.yaml: Our application overlay
component-patch.yaml: A version-specific patch for all components, typically a branch name
exception-component-patch.yaml: An example version-specific patch for a specific component or set of components
kustomization.yaml: A Kustomization file that defines how the patches are applied to components

Let’s start with our application-patch.yaml at overlay/application-a/v1-overlay/application-patch.yaml (replacing values with your own):

- op: replace
  path: /metadata/name
  value: application-a-v1
- op: replace
  path: /spec/description
  value: "Pipeline for application-a v1"
- op: replace
  path: /spec/displayName
  value: "application-a v1"

Followed by our override for components at overlay/application-a/v1-overlay/component-patch.yaml:

- op: replace
  path: /spec/application
  value: application-a-v1 # Must match /metadata/name in application-patch.yaml
- op: replace
  path: /spec/source/git/revision
  value: release-v1 # Replace with your target branch for all components

If you have any patches specific to this application revision that only impact a certain component or set of components, define another patch as shown in overlay/application-a/v1-overlay/exception-component-patch-1.yaml:

- op: replace
  path: /spec/source/git/revision
  value: main # In this example, one of our components will build off of main, so we set it in a separate patch.

Finally, set up your overlay/application-a/v1-overlay/kustomization.yaml to apply these patches correctly:

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

nameSuffix: v1 # Add a suffix to all resource names in the application for uniqueness
resources:
  - ../base

patches:
  - target:
      kind: Application
    path: application-patch.yaml
  - target:
      kind: Component
    path: component-patch.yaml
  - target:
      kind: Component
      name: component-b
    path: exception-component-patch-1.yaml

This Kustomization applies a suffix to all resources. We recommend doing this to ensure uniqueness and make it easier to identify components and applications.

You can define more than one exceptional patch and match component names through regex.

Defining Multiple Versions or Variants of an Application

If multiple versions of an application exist (as in versioned operators) or variants of applications that share some or all components, define multiple application overlays following the same pattern as above.

This results in a configuration that looks something like the following, with a folder for each version:

├── base
│   ├── component.yaml
│   └── kustomization.yaml
├── overlay
│   └── application-a
│       ├── base
│       │   ├── application.yaml
│       │   ├── component-a
│       │   │   ├── component-a-override.yaml
│       │   │   └── kustomization.yaml
│       │   ├── component-b
│       │   │   ├── component-b-override.yaml
│       │   │   └── kustomization.yaml
│       │   └── kustomization.yaml
│       ├── v1-overlay
│       │   ├── application-patch.yaml
│       │   ├── component-patch.yaml
│       │   ├── exception-component-patch-1.yaml
│       │   └── kustomization.yaml
│       └── v2-overlay
│           ├── application-patch.yaml
│           ├── component-patch.yaml
│           ├── exception-component-patch-1.yaml
│           └── kustomization.yaml
└── README.md

Defining Multiple Applications

If you wish to define multiple applications with different sets of components and versions for each application, replicate the above configuration for application-a for a second application and include it as an additional application base and overlays in the overlay directory.

This approach is preferred for defining multiple applications within an application category (such as operators) or a family/product organization as code. It allows you to make bulk configurations to your base component YAML (such as enabling multi-architecture support or setting labels and ownership) in a single place — the component base — rather than in multiple places.

If you follow this method to create an application-b composed of component-c and component-d, then your directory structure will look something like:

├── base
│   ├── component.yaml
│   └── kustomization.yaml
├── overlay
│   ├── application-a
│   │   ├── base
│   │   │   ├── application.yaml
│   │   │   ├── component-a
│   │   │   │   ├── component-a-override.yaml
│   │   │   │   └── kustomization.yaml
│   │   │   ├── component-b
│   │   │   │   ├── component-b-override.yaml
│   │   │   │   └── kustomization.yaml
│   │   │   └── kustomization.yaml
│   │   ├── v1-overlay
│   │   │   ├── application-patch.yaml
│   │   │   ├── component-patch.yaml
│   │   │   ├── exception-component-patch-1.yaml
│   │   │   └── kustomization.yaml
│   │   └── v2-overlay
│   │       ├── application-patch.yaml
│   │       ├── component-patch.yaml
│   │       ├── exception-component-patch-1.yaml
│   │       └── kustomization.yaml
│   └── application-b
│       ├── base
│       │   ├── application.yaml
│       │   ├── component-c
│       │   │   ├── component-c-override.yaml
│       │   │   └── kustomization.yaml
│       │   ├── component-d
│       │   │   ├── component-d-override.yaml
│       │   │   └── kustomization.yaml
│       │   └── kustomization.yaml
│       └── v1-overlay
│           ├── application-patch.yaml
│           ├── component-patch.yaml
│           ├── exception-component-patch-1.yaml
│           └── kustomization.yaml
└── README.md

You can also modify the project structure to fit your own needs by moving application bases and component definitions to different levels, but this configuration offers the most layered encapsulation across applications and application versions.

Managing multiple related components and applications can be challenging. Refer https://redhat-appstudio.github.io/docs.appstudio.io/Documentation/main/how-to-guides/proc_multiversion/ [Managing multiple software versions] to manage multiple related components and applications.