Crossplane Composition Function Patch Not Applied

In this tutorial, you'll learn about Crossplane Composition Function Patch Not Applied. We cover key concepts, practical examples, and best practices.

A Crossplane composition function's patches don't apply, or the resource is created with wrong field values.

Wrong ❌

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
metadata:
  name: db-composition
spec:
  compositeTypeRef:
    apiVersion: example.org/v1alpha1
    kind: XMyDatabase
  resources:
    - name: database
      base:
        apiVersion: rds.aws.upbound.io/v1beta1
        kind: Instance
        spec:
          forProvider:
            region: us-east-1
      patches:
        - type: FromCompositeFieldPath
          fromFieldPath: spec.parameters.size
          toFieldPath: spec.forProvider.instanceClass
        # Wrong: missing transform for size mapping

Wrong Output

Resource 'database' created but with wrong instance class:
Expected: db.t3.small for size=small
Actual: spec.forProvider.instanceClass is empty or uses default

Right ✅

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
metadata:
  name: db-composition
spec:
  compositeTypeRef:
    apiVersion: example.org/v1alpha1
    kind: XMyDatabase
  resources:
    - name: database
      base:
        apiVersion: rds.aws.upbound.io/v1beta1
        kind: Instance
        spec:
          forProvider:
            region: us-east-1
      patches:
        - type: FromCompositeFieldPath
          fromFieldPath: spec.parameters.size
          toFieldPath: spec.forProvider.instanceClass
          transforms:
            - type: map
              map:
                small: db.t3.small
                medium: db.t3.medium
                large: db.r5.large
        - type: FromCompositeFieldPath
          fromFieldPath: spec.parameters.storageGB
          toFieldPath: spec.forProvider.allocatedStorage

Right Output

Resource 'database' patched successfully:
spec.forProvider.instanceClass: db.t3.small
spec.forProvider.allocatedStorage: 100

Prevention

• Always use transforms when mapping generic parameter values to provider-specific field values.
• Test patch transforms with a dry-run composition: crossplane beta render.
• Use FromCompositeFieldPath to read from the XR and ToCompositeFieldPath to write back outputs.
• Keep the base template minimal and use patches to fill in dynamic values.
• Validate patches with crossplane beta trace to see the resolved resource.

DodaTech applies similar defensive patterns across Doda Browser, DodaZIP, and Durga Antivirus Pro infrastructure for production reliability.

Common Mistakes with function patch

1. Forgetting that lazy evaluation defers computation until the value is forced, causing space leaks with unevaluated thunks
2. Using return to exit a function early instead of wrapping a pure value in the monad
3. Mixing let bindings with <- bindings in do notation, producing type errors

These mistakes appear frequently in real-world CROSSPLANE code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

This quick fix is part of the DodaTech infrastructure engineering series. Learn more at DodaTech tutorials.