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Version: 0.8

Crossplane KCL Function

Introduction​

Crossplane and Crossplane Composite Functions are used to decouple XR and Composite resource definitions. XRs allow developers to create higher-level abstractions that can encapsulate and compose multiple types of cloud resources across different providers and services. Using Crossplane Composite Functions to render these abstractions can effectively enhance template capabilities for various provider resources while reducing the amount of YAML code needed.

Combining KCL with Crossplane composite functions offers several benefits:

  • Simplification of Complex Configurations: KCL provides a more concise syntax and structure as a DSL, reducing the complexity of configurations. When combined with Crossplane’s composite resources, you can create more intuitive and easy-to-understand configuration templates with loop and condition features, simplifying the definition and maintenance of resources instead of duplicate YAML and Go code snippets.
  • Reusability and Modularity: KCL supports modularity and code reuse through OCI Registry, which means you can create reusable configuration components. Combined with Crossplane, this promotes the modularity of composite resources, increases the reuse of configurations, and reduces errors.
  • Automation and Policy-Driven: You can use KCL’s powerful features to write policies and constraints that, combined with Crossplane’s declarative resource management, can be automatically enforced, ensuring compliance within the cloud environment.

Prerequisites​

Quick Start​

Let’s write a KCL function abstraction which generates managed resources VPC and InternetGateway with an input resource Network.

1. Install the Crossplane KCL Function​

Installing a Function creates a function pod. Crossplane sends requests to this pod to ask it what resources to create when you create a composite resource.

Install a Function with a Crossplane Function object setting the spec.package value to the location of the function package.

kubectl apply -f- << EOF
apiVersion: pkg.crossplane.io/v1beta1
kind: Function
metadata:
  name: kcl-function
spec:
  package: xpkg.upbound.io/crossplane-contrib/function-kcl:latest
EOF

2. Apply the Composition Resource​

Just like a render function, you can apply the composition resource using KCL into cluster.

kubectl apply -f- << EOF
apiVersion: apiextensions.crossplane.io/v1
kind: Composition
metadata:
  name: xlabels.fn-demo.crossplane.io
  labels:
    provider: aws
spec:
  writeConnectionSecretsToNamespace: crossplane-system
  compositeTypeRef:
    apiVersion: fn-demo.crossplane.io/v1alpha1
    kind: XNetwork
  mode: Pipeline
  pipeline:
  - step: normal
    functionRef:
      name: kcl-function
    input:
      apiVersion: krm.kcl.dev/v1alpha1
      kind: KCLRun
      metadata:
        name: basic
      spec:
        # Generate new resources
        target: Resources
        # OCI, Git or inline source
        # source: oci://ghcr.io/kcl-lang/crossplane-xnetwork-kcl-function
        # source: github.com/kcl-lang/modules/crossplane-xnetwork-kcl-function
        source: |
          # Get the XR spec fields
          id = option("params")?.oxr?.spec.id or ""
          # Render XR to crossplane managed resources
          network_id_labels = {"networks.meta.fn.crossplane.io/network-id" = id} if id else {}
          vpc = {
              apiVersion = "ec2.aws.upbound.io/v1beta1"
              kind = "VPC"
              metadata.name = "vpc"
              metadata.labels = network_id_labels
              spec.forProvider = {
                  region = "eu-west-1"
                  cidrBlock = "192.168.0.0/16"
                  enableDnsSupport = True
                  enableDnsHostnames = True
              }
          }
          gateway = {
              apiVersion = "ec2.aws.upbound.io/v1beta1"
              kind = "InternetGateway"
              metadata.name = "gateway"
              metadata.labels = network_id_labels
              spec.forProvider = {
                  region = "eu-west-1"
                  vpcIdSelector.matchControllerRef = True
              }
          }
          items = [vpc, gateway]
EOF

3. Create Crossplane XRD​

We define a schema using the crossplane XRD for the input resource Network, it has a field named id which denotes the network id.

kubectl apply -f- << EOF
apiVersion: apiextensions.crossplane.io/v1
kind: CompositeResourceDefinition
metadata:
  name: xnetworks.fn-demo.crossplane.io
spec:
  group: fn-demo.crossplane.io
  names:
    kind: XNetwork
    plural: xnetworks
  claimNames:
    kind: Network
    plural: networks
  versions:
    - name: v1alpha1
      served: true
      referenceable: true
      schema:
        openAPIV3Schema:
          type: object
          properties:
            spec:
              type: object
              properties:
                id:
                  type: string
                  description: ID of this Network that other objects will use to refer to it.
              required:
                - id
EOF

4. Apply the Crossplane XR​

kubectl apply -f- << EOF
apiVersion: fn-demo.crossplane.io/v1alpha1
kind: Network
metadata:
  name: network-test-functions
  namespace: default
spec:
  id: network-test-functions
EOF

5. Verify the Generated Managed Resources​

  • VPC
kubectl get VPC -o yaml | grep network-id
      networks.meta.fn.crossplane.io/network-id: network-test-functions
  • InternetGateway
kubectl get InternetGateway -o yaml | grep network-id
      networks.meta.fn.crossplane.io/network-id: network-test-functions

It can be seen that we have indeed successfully generated VPC and InternetGateway resources, and their fields meet expectations.

6. Debugging KCL Functions Locally​

See here for more information.

Client Enhancements​

It can be seen that the above abstract code often requires a crossplane as a control plane intermediary, and you can still complete the abstraction in a fully client-side manner and directly generate crossplane managed resources to reduce the burden on the cluster.

For example

kcl run oci://ghcr.io/kcl-lang/crossplane-xnetwork-kcl-function -S items -D params='{"oxr": {"spec": {"id": "network-test-functions"}}}'

The output is

apiVersion: ec2.aws.upbound.io/v1beta1
kind: VPC
metadata:
  name: vpc
  labels:
    networks.meta.fn.crossplane.io/network-id: network-test-functions
spec:
  forProvider:
    region: eu-west-1
    cidrBlock: 192.168.0.0/16
    enableDnsSupport: true
    enableDnsHostnames: true
---
apiVersion: ec2.aws.upbound.io/v1beta1
kind: InternetGateway
metadata:
  name: gateway
  labels:
    networks.meta.fn.crossplane.io/network-id: network-test-functions
spec:
  forProvider:
    region: eu-west-1
    vpcIdSelector:
      matchControllerRef: true

Guides for Developing KCL​

Here's what you can do in the KCL script:

  • Return an error using assert {condition}, {error_message}.
  • Read the ObservedCompositeResource from option("params").oxr.
  • Read the ObservedComposedResources from option("params").ocds.
  • Read the DesiredCompositeResource from option("params").dxr.
  • Read the DesiredComposedResources from option("params").dcds.
  • Read the environment variables. e.g. option("PATH") (Not yet implemented).

Library​

You can directly use KCL standard libraries such as regex.match, math.log.

More Documents and Examples​