Adding items to a list with kubectl kustomize

2 min read 05-10-2024

Adding Items to a List with `kubectl kustomize`

The Problem:

You're working with Kubernetes YAML files, and you need to add items to a list within a specific resource. You want to use kubectl kustomize to manage these changes, but you're unsure how to modify lists without overwriting existing values.

Understanding the Scenario:

Imagine you have a Kubernetes Deployment resource with a list of containers. You want to add a new container to the existing list without modifying the existing containers. Here's an example of a deployment.yaml file:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app
spec:
  replicas: 3
  selector:
    matchLabels:
      app: my-app
  template:
    metadata:
      labels:
        app: my-app
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2
        ports:
        - containerPort: 80

You want to add a second container for a sidecar like this:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app
spec:
  replicas: 3
  selector:
    matchLabels:
      app: my-app
  template:
    metadata:
      labels:
        app: my-app
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2
        ports:
        - containerPort: 80
      - name: sidecar
        image: busybox:1.31
        command: ["sleep", "infinity"]

Using kubectl kustomize to Add List Items

kubectl kustomize provides a powerful way to manage these changes without modifying the original YAML file. Here's how to use it:

Create a kustomization.yaml file: This file will specify the changes you want to apply. Place it in the same directory as your deployment.yaml.
Use the patchesStrategicMerge option: This allows you to strategically merge patches into your resource. In our case, we'll use it to append a new container to the containers list.

Here's an example of a kustomization.yaml file:

resources:
- deployment.yaml

patchesStrategicMerge:
- patch.yaml

Create a patch.yaml file: This file will contain the patch you want to apply. In this case, it will add a new container to the containers list.

Here's an example of a patch.yaml file:

spec:
  template:
    spec:
      containers:
      - name: sidecar
        image: busybox:1.31
        command: ["sleep", "infinity"]

Apply the changes: Use kubectl kustomize to apply the changes to your resource.

kubectl apply -k .

This will create a new Deployment with the added sidecar container.

Important Considerations:

Strategic Merge: patchesStrategicMerge works by taking the existing values in your resource and merging them with the values in the patch file. This ensures that your existing configurations are not overwritten.
Order: The order of the patches in the kustomization.yaml file matters. Patches are applied in the order they are listed.
Deep Merging: You can target specific elements within your resource by providing a path to the element you want to modify. For example, you could use spec.template.spec.containers[0] to target the first container in the Deployment.

Additional Benefits:

Version Control: kubectl kustomize integrates seamlessly with version control systems, making it easy to track changes and revert to previous versions.
Reusability: You can create multiple kustomization.yaml files and patch files, allowing you to easily manage different configurations and apply them to your resources.

Conclusion:

kubectl kustomize offers a powerful and flexible way to manage Kubernetes resources and add items to lists without overwriting existing values. By utilizing strategic merge patches, you can ensure that your changes are applied in a controlled and predictable manner. This approach promotes code organization, maintainability, and collaboration within your Kubernetes development workflow.

Adding items to a list with kubectl kustomize