Kustomize Use Cases

Patch replicas

To show you how Kustomize can be helpful, let's analyze a simple use case where we have a project and want to change the number of static replicas only for the production environment.

As a starting point, let's assume we have a project containing a microservice called hello-world with a deployment file similar to the following:

# file: ./configuration/helloworld.deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: hello-world
spec:
  replicas: 1
  template:
    spec:
      containers:
      - image: hello-world:latest

The base configuration sets the number of replicas to 1. Now, let's define an overlay for the production environment that changes the number of replicas from 1 to 2.

Add the following file:

# file: ./overlays/production/helloworld.patch.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: hello-world
spec:
  replicas: 2

This way, when the production environment will be deployed, there will be two static replicas of the hello-world service. Notice that ./overlays/production/kustomization.yaml is automatically generated and can be left empty.

Patch all deployments

In addition, Kustomize allows you to specify a list of targets in the kustomization.yaml files included in overlays. This feature allows applying patches to multiple resources at once.

The snippet below includes all the available targets:

patches:
  - path: <relative path to file containing patch>
    target:
      group: <optional group>
      version: <optional version>
      kind: <optional kind>
      name: <optional name or regex pattern>
      namespace: <optional namespace>
      labelSelector: <optional label selector>
      annotationSelector: <optional annotation selector>

For example, imagine you want to inject a sidecar proxy into all your Kubernetes deployments.

Let's suppose you already have a couple of microservices with the following deployments:

# file: ./configuration/helloworld.deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: hello-world
spec:
  replicas: 1
  template:
    spec:
      containers:
      - image: hello-world:latest
---
# file: ./configuration/goodbyeworld.deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: goodbye-world
spec:
  replicas: 1
  template:
    spec:
      containers:
      - image: hello-world:latest

First off, you need to create the patch containing the sidecar container specification in the environment of interest (e.g. development).

# file: ./overlays/development/sidecar.patch.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: not-important
spec:
  template:
    spec:
      containers:
        - name: istio-proxy
          image: docker.io/istio/proxyv2
          args:
          - proxy
          - sidecar

Then, to apply the patch to all your Deployments, you have to manually edit the kustomization.yaml file in the selected overlay directory. For instance, if you want to inject the sidecar in the development environment, the corresponding kustomization.yaml would look like this:

# file: ./overlays/development/kustomization.yaml
resources:
- deployments.yaml

patches:
- path: sidecar.patch.yaml
  target:
    kind: Deployment

info

When using target, the metadata.name of the patch will be ignored.

Once deployed, you will see the two Deployment resources with an additional istio-proxy container.