Skip to main content
Version: 0.9

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:

  • 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 function pipeline's context from option("params").ctx.
  • Return an error using assert {condition}, {error_message}.
  • Read the PATH variables. e.g. option("PATH").
  • Read the environment variables. e.g. option("env").

Library

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

More Documents and Examples