Kustomize Use Cases

In this page you can see examples of how you can use Kustomize in order to manage some 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.

tip

Sometimes you could need to patch keys containing a / character. The / character is often used (e.g. for inline operations) by Kustomize as separator, consequently you need to replace it with the ~1 value.

For example, if you want to add the app.kubernetes.io/component label to one or more manifests, you should escape the slash as following:

- patch: |-
      - op: replace
        path: /metadata/labels/app.kubernetes.io~1component
        value: monitoring

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:
- ../../configuration

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.

You can choose to add other conditions to find the targets to patch instead patching all deployments. For example, you can patch only deployments with a specific label or annotation:

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

patches:
- path: sidecar.patch.yaml
  target:
    kind: Deployment
    labelSelector: myLabel=labelValue,otherLabel=otherValue
    annotationSelector: myAnnotation=annotationValue,otherAnnotation=otherValue

With the example above, the istio-proxy container will be added to deployments with all the specified labels and annotations.

info

You can apply labels to microservices inside the Design section of the Mia-Platform Console, or, in case of Self-Hosted installation, you can set default labels to the microservices created from Marketplace.