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When customizing Kubernetes resources, you often need to modify lists—such as the containers array in a Deployment. Kustomize supports two patch strategies:
  • JSON 6902 patches: precise, index-based operations (add, replace, remove).
  • Strategic Merge Patches (SMP): declarative, merge-key based edits.
Below is a quick comparison:
OperationJSON 6902 PatchStrategic Merge Patch
Replace itemTargets an explicit indexMatches on name merge key
Add itemUses add with /- or an indexAppends new entries by default
Remove itemUses remove with a specific indexUses $patch: delete on the merge key

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Base Deployment Example

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
Every entry in spec.template.spec.containers is a YAML list item, so it begins with -.

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1. Replacing a Container

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1.1 JSON 6902 Patch

Add to your kustomization.yaml: 
patches:
  - target:
      kind: Deployment
      name: api-deployment
    patch: |-
      - op: replace
        path: /spec/template/spec/containers/0
        value:
          name: haproxy
          image: haproxy
Resulting section: 
spec:
  template:
    spec:
      containers:
        - name: haproxy
          image: haproxy

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1.2 Strategic Merge Patch

Create a file replace-container.yaml: 
apiVersion: apps/v1
kind: Deployment
metadata:
  name: api-deployment
spec:
  template:
    spec:
      containers:
        - name: nginx
          image: haproxy
Reference it in kustomization.yaml: 
patches:
  - replace-container.yaml
Kustomize finds the item with name: nginx and updates its image.

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2. Adding a Container

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2.1 JSON 6902 Patch

To append at the end: 
patches:
  - target:
      kind: Deployment
      name: api-deployment
    patch: |-
      - op: add
        path: /spec/template/spec/containers/-
        value:
          name: haproxy
          image: haproxy
Result: 
spec:
  template:
    spec:
      containers:
        - name: nginx
          image: nginx
        - name: haproxy
          image: haproxy
You can also insert at a specific index by replacing - with 1, 2, etc.
Using path: /containers/- always appends. For precise position, specify the index.

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2.2 Strategic Merge Patch

File add-container.yaml: 
apiVersion: apps/v1
kind: Deployment
metadata:
  name: api-deployment
spec:
  template:
    spec:
      containers:
        - name: haproxy
          image: haproxy
In your kustomization.yaml: 
patches:
  - add-container.yaml
The new haproxy container is appended automatically.

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3. Removing a Container

Assume this deployment: 
apiVersion: apps/v1
kind: Deployment
metadata:
  name: api-deployment
spec:
  replicas: 1
  template:
    spec:
      containers:
        - name: web
          image: nginx
        - name: database
          image: mongo

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3.1 JSON 6902 Patch

To remove the second container (index 1): 
patches:
  - target:
      kind: Deployment
      name: api-deployment
    patch:
      - op: remove
        path: /spec/template/spec/containers/1
After applying, only web remains.

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3.2 Strategic Merge Patch

Create remove-database.yaml: 
apiVersion: apps/v1
kind: Deployment
metadata:
  name: api-deployment
spec:
  template:
    spec:
      containers:
        - $patch: delete
          name: database
Include it in kustomization.yaml: 
patches:
  - remove-database.yaml
Kustomize deletes the container with name: database.

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