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Crossplane Composition Functions using KCL

Introduction

Crossplane KCL function allows developers to use KCL (a DSL) to write composite logic without the need for repeated packaging of crossplane functions, and we support package management and the KRM KCL specification, which allows for OCI/Git source and the reuse of KCL's module ecosystem.

Check out these following blogs to learn more.

• Using KCL Programming Language to Write Crossplane Composition Functions
• KCL: The Game-Changer for Crossplane Composition Building

Here's a simple example:

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
metadata:
  name: example
spec:
  compositeTypeRef:
    apiVersion: example.crossplane.io/v1beta1
    kind: XR
  mode: Pipeline
  pipeline:
    - step: basic
      functionRef:
        name: function-kcl
      input:
        apiVersion: krm.kcl.dev/v1alpha1
        kind: KCLInput
        source: |
          # Read the XR
          oxr = option("params").oxr
          # Patch the XR with the status field
          dxr = {
              **option("params").dxr
              status.dummy = "cool-status"
          }
          # Construct a bucket
          bucket = {
              apiVersion = "s3.aws.upbound.io/v1beta1"
              kind = "Bucket"
              metadata.annotations: {
                  "krm.kcl.dev/composition-resource-name" = "bucket"
              }
              spec.forProvider.region = option("oxr").spec.region
          }
          # Return the bucket and patched XR
          items = [bucket, dxr]
    - step: automatically-detect-ready-composed-resources
      functionRef:
        name: function-auto-ready

Install the KCL Function to Cluster

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

Using this Function

Source Support

To use a KCLInput as the function config, the KCL source must be specified in the source field. Additional parameters can be specified in the params field. The params field supports any complex data structure as long as it can be represented in YAML. Besides, the function can load KCL codes from inline source, OCI source, Git source and FileSystem source.

• Inline source example

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
metadata:
  name: basic
spec:
  source: |
    {
        apiVersion = "s3.aws.upbound.io/v1beta1"
        kind = "Bucket"
        metadata.annotations: {
            "krm.kcl.dev/composition-resource-name" = "bucket"
        }
        spec.forProvider.region = option("oxr").spec.region
    }

• OCI source example

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
metadata:
  name: basic
spec:
  source: oci://ghcr.io/kcl-lang/crossplane-xnetwork-kcl-function

• Git source example

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
metadata:
  name: basic
spec:
  source: github.com/kcl-lang/modules/crossplane-xnetwork-kcl-function

• FileSystem source example

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
metadata:
  name: basic
spec:
  source: ./path/to/kcl/file.k

Note: You can't run the FileSystem example using crossplane render because it loads templates from a ConfigMap in the cluster. You can create a ConfigMap with the templates using the following command.

kubectl create configmap templates --from-file=templates.k -n crossplane-system

This ConfigMap will be mounted to the function pod and the templates will be available in the /templates directory. See the following function config for details.

apiVersion: pkg.crossplane.io/v1beta1
kind: Function
metadata:
  name: function-kcl
spec:
  package: xpkg.upbound.io/crossplane-contrib/function-kcl:latest
  runtimeConfigRef:
    name: mount-templates
---
apiVersion: pkg.crossplane.io/v1beta1
kind: DeploymentRuntimeConfig
metadata:
  name: mount-templates
spec:
  deploymentTemplate:
    spec:
      selector: {}
      template:
        spec:
          containers:
            - name: package-runtime
              volumeMounts:
                - mountPath: /templates
                  name: templates
                  readOnly: true
          volumes:
            - name: templates
              configMap:
                name: templates

Use as a Base Image

This function can also be used as a base image to build complex functions in KCL. To do this, add your KCL code to the image and set the FUNCTION_KCL_DEFAULT_SOURCE environment variable to the path where you put your code.

For example, if you have the following in main.k:

# Read the XR
oxr = option("params").oxr
# Patch the XR with the status field
dxr = {
    **option("params").dxr
    status.dummy = "cool-status"
}
# Construct a bucket
bucket = {
    apiVersion = "s3.aws.upbound.io/v1beta1"
    kind = "Bucket"
    metadata.annotations: {
        "krm.kcl.dev/composition-resource-name" = "bucket"
    }
    spec.forProvider.region = option("oxr").spec.region
}
# Return the bucket and patched XR
items = [bucket, dxr]

You can use the following Dockerfile to build a function that runs the code above and does not require any input:

FROM xpkg.upbound.io/crossplane-contrib/function-kcl:latest

ADD main.k /src/main.k
ENV FUNCTION_KCL_DEFAULT_SOURCE=/src/main.k

You may also wish to replace the /package.yaml metadata file to give your new function a unique name and remove or replace the input CRD.

Read the Function Requests and Values through the option Function

• 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}.
• Log variable values using the function print(variable) and it will be output to the stdout of the function pod.
• Read the PATH variables. e.g. option("PATH").
• Read the environment variables. e.g. option("env").

Custom Parameters

You can define your custom parameters in the params field and use option("params").custom_key to get the custom_value.

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
metadata:
  name: basic
spec:
  params:
    custom_key: custom_value
  source: oci://ghcr.io/kcl-lang/crossplane-xnetwork-kcl-function

Source Credentials

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
metadata:
  name: kcl-xnetwork
spec:
  params:
    annotations:
      krm.kcl.dev/allow-insecure-source: "true" # For localhost OCI registry
  source: oci://ghcr.io/kcl-lang/crossplane-xnetwork-kcl-function
  credentials: # If private OCI registry
    url: https://<oci-host-url> # or KCL_SRC_URL environment variable
    username: <username> # or KCL_SRC_USERNAME environment variable
    password: <password> # or KCL_SRC_PASSWORD environment variable

Run Config

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
spec:
  source: oci://ghcr.io/kcl-lang/crossplane-xnetwork-kcl-function
  config: # See [pkg/api/ConfigSpec]
    vendor: true
    sortKeys: true
    disableNone: true
    # omit other fields

Dependencies

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
spec:
  # Set the dependencies are the external dependencies for the KCL code.
  # The format of the `dependencies` field is same as the [dependencies]` in the `kcl.mod` file
  dependencies:
    k8s = "1.31"
  source: |
    import k8s.api.core.v1 as k8core

    k8core.Pod {
        spec: k8core.PodSpec{
            containers: [{
                name = "main"
            }]
        }
    }

Expect Output

A KRM YAML list means that each document must have an apiVersion, kind through the items field or a single YAML output.

• Using the items field

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
metadata:
  name: basic
spec:
  source: |
    items = [{
        apiVersion: "ec2.aws.upbound.io/v1beta1"
        kind: "Instance"
        metadata.name = "instance1"
        spec.forProvider.region: "us-east-2"
    }, {
        apiVersion: "ec2.aws.upbound.io/v1beta1"
        kind: "Instance"
        metadata.name = "instance2"
        spec.forProvider.region: "us-east-2"
    }]

• Single YAML output

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
metadata:
  name: basic
spec:
  source: |
    {
        apiVersion: "ec2.aws.upbound.io/v1beta1"
        kind: "Instance"
        metadata.name = "instance"
        spec.forProvider.ami: "ami-0d9858aa3c6322f73"
        spec.forProvider.instanceType: "t2.micro"
        spec.forProvider.region: "us-east-2"
    }

Note that when returning multiple resources, we need to set different metadata.name or metadata.annotations."krm.kcl.dev/composition-resource-name" to distinguish between different resources in the composition functions.

Target Support

The KCL function can target various types of objects:

• Default: create new resources and set fields on the XR.
• Resources: create new resources.
• PatchDesired: set fields on existing DesiredComposed Resources.
• PatchResources: set fields on existing resources fields. These resources will then be added to the desired resources map.
• XR: set fields on the XR.

This is controlled by fields on the KCInput

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
metadata:
  name: basic
spec:
  # default: Default
  target: Default | PatchDesired | PatchResources | Resources | XR
  source: |
    # Omit the source field
    ...

Extract Data from a Specific Composed Resource

To extract data from a specific composed resource by using the resource name, we can use the option("params").ocds variable, ocds is a mapping that its key is the resource name and its value is the observed composed resource like the example.

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
metadata:
  name: show-ocds
spec:
  source: |
    {
        metadata.name = "ocds"
        spec.ocds = option("params").ocds
        spec.user_kind = option("params").ocds["test-user"]?.Resource.Kind
        spec.user_metadata = option("params").ocds["test-user"]?.Resource.metadata
        spec.user_status = option("params").ocds["test-user"]?.Resource.status
    }

Composite Resource Connection Details

To return desired composite resource connection details, include a KCL config that produces the special CompositeConnectionDetails resource like the example:

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
metadata:
  name: basic
spec:
  source: |
    details = {
        apiVersion: "meta.krm.kcl.dev/v1alpha1"
        kind: "CompositeConnectionDetails"
        data: {
            "connection-secret-key": "connection-secret-value"
        }
    }
    # Omit other composite logics.
    # Input the details resource into the return resource list.
    items = [
        details
        # Omit other return resources.
    ]

Note: The value of the connection secret value must be base64 encoded. This is already the case if you are referencing a key from a managed resource's connectionDetails field. However, if you want to include a connection secret value from somewhere else, you will need to use the base64.encode function:

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
metadata:
  name: basic
spec:
  source: |
    import base64

    # Omit other logic
    ocds = option("params").ocds
    details = {
        apiVersion: "meta.krm.kcl.dev/v1alpha1"
        kind: "CompositeConnectionDetails"
        data: {
            "server-endpoint" = base64.encode(ocds["my-server"].Resource.status.atProvider.endpoint)
        }
    }

To mark a desired composed resource as ready, use the krm.kcl.dev/ready annotation:

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
metadata:
  name: basic
spec:
  source: |
    # Omit other logic
    user = {
        apiVersion: "iam.aws.upbound.io/v1beta1"
        kind: "User"
        metadata.name = "test-user"
        metadata.annotations: {
            "krm.kcl.dev/ready": "True"
        }
    }

Extra resources

By defining one or more special ExtraResources, you can ask Crossplane to retrieve additional resources from the local cluster and make them available to your templates. See the docs for more information.

With ExtraResources, you can fetch cluster-scoped resources, but not namespaced resources such as claims. If you need to get a composite resource via its claim name you can use matchLabels with crossplane.io/claim-name: <claimname>

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
metadata:
  name: basic
spec:
  source: |
    # Omit other logic
    details = {
        apiVersion: "meta.krm.kcl.dev/v1alpha1"
        kind: "ExtraResources"
        requirements = {
            foo = {
                apiVersion: "example.com/v1beta1",
                kind: "Foo",
                matchLabels: {
                    "foo": "bar"
                }
            },
            bar = {
                apiVersion: "example.com/v1beta1",
                kind: "Bar",
                matchName: "my-bar"
            }
        }
    }

    # Omit other composite logics.
    items = [
        details
        # Omit other return resources.
    ]

You can retrieve the extra resources either via labels with matchLabels or via name with matchName: somename.

This will result in Crossplane receiving the requested resources and make them available with the following format.

foo:
- Resource:
    apiVersion: example.com/v1beta1
    kind: Foo
    metadata:
      labels:
        foo: bar
    # Omitted for brevity
- Resource:
    apiVersion: example.com/v1beta1
    kind: Foo
    metadata:
      labels:
        foo: bar
    # Omit for brevity
bar:
- Resource:
    apiVersion: example.com/v1beta1
    kind: Bar
    metadata:
      name: my-bar
    # Omitted for brevity

You can access the retrieved resources in your code like this:

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
metadata:
  name: basic
spec:
  source: |
    er = option("params")?.extraResources
    
    name = er?.bar[0]?.Resource?.metadata?.name or ""
    # Omit other logic

Patching the XR status field

You can read the XR, patch it with the status field and return the new patched XR in the item result like this

apiVersion: krm.kcl.dev/v1alpha1
kind: KCLInput
metadata:
  name: basic
spec:
  source: |
    # Read the XR
    dxr = option("params").dxr
    # Patch the XR with the status field
    dxr.status.dummy = "cool-status"
    items = [dxr] # Omit other resources

Library

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

Tutorial

See here to study more features such as conditions and loops in KCL.

Examples

More examples can be found here

Debugging the KCL Function in Cluster

Logs are emitted to the Function's pod logs. Look for the Function pod in crossplane-system.

Levels

Info   # default
Debug  # run with --debug flag

Developing

# Run code generation - see input/generate.go
$ go generate ./...

# Run tests - see fn_test.go
$ go test ./...

# Build the function's runtime image - see Dockerfile
$ docker build . --tag=kcllang/crossplane-kcl

# Build a function package - see package/crossplane.yaml
$ crossplane xpkg build -f package --embed-runtime-image=kcllang/crossplane-kcl

# Push a function package to the registry
$ crossplane --verbose xpkg push -f package/*.xpkg xpkg.upbound.io/crossplane-contrib/function-kcl:latest