KCL is a constraint-based record & functional domain language. Full documents of KCL can be found here.

kpt is a package-centric toolchain that enables a WYSIWYG configuration authoring, automation, and delivery experience, which simplifies managing Kubernetes platforms and KRM-driven infrastructure.

The KPT KCL function SDK contains a KCL interpreter to run a KCL script to mutate or validate resources.

The KCL programming language can be used to:

• Add annotations on the basis of a condition.
• Inject a sidecar container in all KRM resources that contain a PodTemplate.
• Validate all KRM resources using KCL schema.

• Install kpt
• Install Docker
• Golang (at least version 1.23)

You need to put your KCL script source in the functionConfig of kind KCLRun and then the function will run the KCL script that you provide.

This function can be used both declaratively and imperatively.

kpt fn source ./testdata/resources.yaml --fn-config ./testdata/fn-config.yaml | go run main.go

The output is:

apiVersion: config.kubernetes.io/v1 kind: ResourceList items: - apiVersion: apps/v1 kind: Deployment metadata: annotations: config.kubernetes.io/index: '1' config.kubernetes.io/path: resources.yaml internal.config.kubernetes.io/index: '1' internal.config.kubernetes.io/path: resources.yaml internal.config.kubernetes.io/seqindent: compact labels: app: nginx name: nginx-deployment spec: replicas: '5' selector: matchLabels: app: nginx template: metadata: labels: app: nginx spec: containers: - image: nginx:1.14.2 name: nginx ports: - containerPort: 80 - apiVersion: v1 kind: Service metadata: annotations: config.kubernetes.io/index: '0' config.kubernetes.io/path: resources.yaml internal.config.kubernetes.io/index: '0' internal.config.kubernetes.io/path: resources.yaml internal.config.kubernetes.io/seqindent: wide name: test spec: ports: - port: 80 protocol: TCP targetPort: 9376 selector: app: MyApp functionConfig: apiVersion: krm.kcl.dev/v1alpha1 kind: KCLRun metadata: name: conditionally-add-annotations spec: params: replicas: "5" source: |  params = option("params")  replicas = params.replicas  setReplicas = lambda items, replicas {  [item | {if item.kind == "Deployment": spec.replicas = replicas} for item in items]  }  items = setReplicas(option("items"), replicas)

Thus, the spec.replicas of Deployment in the resource_list.yaml is changed to 5 from 2.

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.

apiVersion: krm.kcl.dev/v1alpha1 kind: KCLRun metadata: name: conditionally-add-annotations spec: params: toMatch: config.kubernetes.io/local-config: "true" toAdd: configmanagement.gke.io/managed: disabled source: |  resource = option("resource_list")  params = resource.functionConfig.spec.params  toMatch = params.toMatch  toAdd = params.toAdd  items = [item | {  # If all annotations are matched, patch more annotations  if all key, value in toMatch {  item.metadata.annotations[key] == value  }:  metadata.annotations: toAdd  } for item in resource.items]

In the example above, the script accesses the toMatch parameters using option("params").toMatch.

Firstly, build the container image from the source.

export FN_CONTAINER_REGISTRY=<Your GCR or docker hub> export TAG=<Your KRM function tag> docker build . -t ${FN_CONTAINER_REGISTRY}/${FUNCTION_NAME}:${TAG}

Have your Kptfile pointing to a functionConfig file that contains either a KCLRun.

After that, you can render it in the folder that contains KRM with:

kpt fn render

Or use the function config file.

kpt fn eval --image ${FN_CONTAINER_REGISTRY}/${FUNCTION_NAME}:${TAG} --fn-config fn-config.yaml

But for example, you can use the unstable kcl-kpt image docker.io/kcllang/kpt-kcl:v0.3.0 for testing.

kpt fn eval ./testdata/resources.yaml -i docker.io/kcllang/kpt-kcl:v0.3.0 --fn-config ./testdata/fn-config.yaml

Then the Kubernetes resource file resources.yaml will be modified in place.

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

• Read resources from option("resource_list"). The option("resource_list") complies with the KRM Functions Specification. You can read the input resources from option("items").
• Return a KPM list for output resources.
• Return an error using assert {condition}, {error_message}.
• Read the environment variables. e.g. option("PATH").

See here for more examples.

You can directly use KCL standard libraries without importing them, such as regex.match, math.log.