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What are strategic merge patches versus JSON 6902 patches in Kustomize?

Infrastructure as Code (Terraform, Ansible) · Advanced level

Answer

Strategic merge patches are Kubernetes-aware YAML patches that merge fields using Kubernetes schema behavior. JSON 6902 patches are explicit operation lists such as add, replace, and remove against JSON paths. Strategic merge is often simpler for Kubernetes objects; JSON 6902 is precise for targeted edits.

Technical explanation

Strategic merge depends on Kubernetes merge semantics and may not work for every custom resource the same way.

JSON 6902 is explicit and works well when you need precise path operations.

Choose the patch type that is easiest to review and least surprising.

Keep source manifests or IaC definitions readable enough that reviewers can understand the final desired state.

Use overlays, modules, or roles for reuse, but keep environment-specific differences explicit and reviewable.

Validate generated output in CI before applying it through kubectl, Argo CD, Terraform, or Ansible.

Hands-on example

1. Create a Kustomize base and overlay for: What are strategic merge patches versus JSON 6902 patches in Kustomize?

2. Base files:

base/deployment.yaml

base/service.yaml

base/kustomization.yaml

base/kustomization.yaml:

resources:

- deployment.yaml

- service.yaml

commonLabels:

app.kubernetes.io/name: payments

3. prod overlay:

resources:

- ../../base

namePrefix: prod-

namespace: payments-prod

images:

- name: ghcr.io/company/payments

newTag: 1.8.4

configMapGenerator:

- name: app-config

literals:

- LOG_LEVEL=info

patches:

- path: replica-patch.yaml

4. Render and apply:

kubectl kustomize overlays/prod

kubectl diff -k overlays/prod

kubectl apply -k overlays/prod

5. In GitOps, point Argo CD at overlays/prod and let it render, compare, and sync the desired state.

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