KodeKloud Notes

Kustomize patches provide a surgical way to update individual Kubernetes resources without affecting all objects in your overlay. While common transformers are ideal for broad changes—such as applying a label or namespace to every resource—patches let you target one or a few objects with precise modifications. For instance, bumping the replica count in a Deployment is best handled via a dedicated patch.

Key Parameters of a Patch

Every patch in Kustomize requires three core parameters:

Operation type:
- add → append a new field or item
- remove → delete an existing field or item
- replace → swap an existing value for a new one
Target: selection criteria to identify the resource(s) you want to patch
Value: the data to add or replace (not needed for remove)

The image is a slide explaining how Kustomize patches modify Kubernetes configurations, highlighting the need for three parameters: operation type, target resource, and value.

Operation	Description	Example
add	Append a field or container	Add a sidecar container to a Pod
remove	Delete a field or label	Remove an unwanted annotation
replace	Update an existing value	Change replica count from 5 to 10

When to Use Patches

Use patches for fine-grained updates that shouldn’t apply globally. For bulk changes—like adding a common label—stick to transformers.

Defining the Target

Specify one or more match criteria under target to pinpoint resources:

target:
  kind: Deployment
  apiVersion: apps/v1
  name: api-deployment
  namespace: production
  labelSelector: "app=frontend"

Combine kind, apiVersion, name, namespace, labelSelector, or annotationSelector for exact control.

JSON 6902 Patch Example

Below is a basic Deployment manifest:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: api-deployment
spec:
  replicas: 1
  selector:
    matchLabels:
      component: api
  template:
    metadata:
      labels:
        component: api
    spec:
      containers:
        - name: nginx
          image: nginx

Add this inline JSON 6902 patch in your kustomization.yaml to rename the Deployment:

patches:
  - target:
      kind: Deployment
      name: api-deployment
    patch: |-
      - op: replace
        path: /metadata/name
        value: web-deployment

After running kustomize build, the output changes metadata.name to web-deployment:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: web-deployment
spec:
  replicas: 1
  selector:
    matchLabels:
      component: api
  template:
    metadata:
      labels:
        component: api
    spec:
      containers:
        - name: nginx
          image: nginx

JSON 6902 Format

JSON 6902 patches consist of an array of operations (op, path, value). Refer to the RFC 6902 for full details.

Updating Replicas

To adjust the replica count, update the path to /spec/replicas and set the desired value:

patches:
  - target:
      kind: Deployment
      name: api-deployment
    patch: |-
      - op: replace
        path: /spec/replicas
        value: 5

After kustomize build, you’ll see replicas: 5 in the generated manifest.

Strategic Merge Patch

Strategic merge patches let you describe only the fields to modify, using standard Kubernetes YAML:

patches:
  - patch: |
      apiVersion: apps/v1
      kind: Deployment
      metadata:
        name: api-deployment
      spec:
        replicas: 5

Here, Kustomize locates the Deployment by apiVersion, kind, and metadata.name, then merges spec.replicas: 5 into the base resource.

Mixing Patch Types

You can combine JSON 6902 and strategic merge patches in the same kustomization.yaml. Use whichever format fits your use case.

Links and References

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