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name: kustomize description: This skill provides patterns and best practices for using Kustomize to manage Kubernetes configurations, including overlays, patches, and resource organization.

Kustomize Patterns and Best Practices

Directory Structure

Always organize kustomize projects using this standard structure:

base/
  kustomization.yaml
  deployment.yaml
  service.yaml
overlays/
  dev/
    kustomization.yaml
    patches/
  staging/
    kustomization.yaml
    patches/
  prod/
    kustomization.yaml
    patches/
components/
  monitoring/
    kustomization.yaml
  security/
    kustomization.yaml

Core Principles

• Base + Overlay Structure: Use base for common resources, overlays for environment-specific modifications (dev, staging, prod)
• Reusable Bases: Organize resources into logical bases that can be referenced across multiple overlays
• Components for CRDs: Use components to manage custom resource definitions and operators that must be installed before dependent resources
• Strategic Patching: Modify specific fields using patches rather than duplicating entire resource definitions

Creating New Overlays

When creating a new overlay, always:

1. Create the overlay directory under overlays/
2. Add a kustomization.yaml that references the base:

   apiVersion: kustomize.config.k8s.io/v1beta1
   kind: Kustomization
   resources:
     - ../../base
   

3. Add environment-specific patches in a patches/ subdirectory
4. Use namespace: field to set the target namespace
5. Use namePrefix: or nameSuffix: if needed for resource naming

Patch Strategies

Choose the appropriate patch type based on the modification:

Strategic Merge Patches (Preferred for simple modifications)

Use patchesStrategicMerge for straightforward field updates:

patchesStrategicMerge:
  - patches/deployment-replicas.yaml
  - patches/service-type.yaml

When to use: Modifying simple fields like replicas, image tags, resource limits, labels, annotations

JSON Patches (For complex or precise modifications)

Use patches with target for complex operations or when you need precise control:

patches:
  - path: patches/add-sidecar.yaml
    target:
      kind: Deployment
      name: myapp

When to use: Adding array elements, removing fields, conditional modifications, working with CRDs

Replacements (Kustomize v4+)

Use replacements instead of deprecated vars for cross-resource field substitution:

replacements:
  - source:
      kind: ConfigMap
      name: app-config
      fieldPath: data.version
    targets:
      - select:
          kind: Deployment
        fieldPaths:
          - spec.template.metadata.labels.version

Common Operations

Modifying Image Tags

Use images field for image transformations:

images:
  - name: myapp
    newTag: v1.2.3
  - name: nginx
    newName: registry.example.com/nginx
    newTag: 1.21

Managing ConfigMaps and Secrets

Use generators to create ConfigMaps and Secrets with automatic hash suffixes:

configMapGenerator:
  - name: app-config
    files:
      - config.properties
    literals:
      - LOG_LEVEL=info

secretGenerator:
  - name: db-credentials
    envs:
      - secrets.env

Applying Common Metadata

Use commonLabels and commonAnnotations to apply consistent metadata:

commonLabels:
  app: myapp
  environment: prod
  managed-by: kustomize

commonAnnotations:
  contact: team@example.com

Adding New Resources

When adding a new resource to an overlay:

1. First check if it should go in the base (shared across environments)
2. If environment-specific, add the resource file to the overlay directory
3. Reference it in the overlay's kustomization.yaml under resources:
4. Prefer patching base resources over creating entirely new ones

Validation and Testing

Always validate kustomizations before deploying:

# Build and view the output
kustomize build overlays/prod

# Validate with kubectl dry-run
kustomize build overlays/prod | kubectl apply --dry-run=client -f -

# Validate against Kubernetes schemas
kustomize build overlays/prod | kubeval

# Validate with kubeconform (faster alternative)
kustomize build overlays/prod | kubeconform -strict

Best Practices

• Keep kustomization.yaml clean: Add comments explaining the purpose of patches and transformations
• Avoid templating: Kustomize is not a templating tool. Use it with Flux substitutions or Helm if you need variable interpolation
• Limit complexity: Avoid excessive nesting (max 2-3 levels), complex patch chains, or resource ordering dependencies
• Component ordering: When using components, apply them in dependency order (CRDs first, then controllers, then workloads)
• No hardcoded secrets: Always use secretGenerator with external sources or integrate with secret management tools
• Namespace consistency: Set namespace once at the overlay level, not in individual resources
• Use kustomize CLI: Prefer standalone kustomize CLI over kubectl kustomize for latest features

When to Move to Helm

Consider migrating to Helm if you encounter:

• Need for extensive templating and conditionals
• Complex resource ordering requirements
• Dependency management across multiple charts
• Need for lifecycle hooks (pre-install, post-upgrade)
• Extensive value file hierarchies