Kustomize KCL Function

This is an example of implementing a KCL function for Kustomize. KCL is a constraint-based record & functional domain language. Full documents of KCL can be found here.

KCL can be used to create functions to mutate and/or validate the YAML Kubernetes Resource Model (KRM) input/output format, and we provide Kustomize KCL functions to simplify the function authoring process.

Quick Start

Let’s write a KCL function that adds annotation managed-by=kustomize-kcl only to Deployment resources.

Prerequisites

Install kustomize

Test and Run

# Note: you need add sudo and --as-current-user flags to ensure KCL has permission to write temp files in the container filesystem.
sudo kustomize fn run examples/set-annotation/local-resource/ --as-current-user --dry-run

The output YAML is

# kcl-fn-config.yaml
apiVersion: krm.kcl.dev/v1alpha1
kind: KCLRun
metadata:
  annotations:
    config.kubernetes.io/function: |
      container:
        image: docker.io/peefyxpf/kustomize-kcl:v0.1.1
    config.kubernetes.io/path: example-use.yaml
    internal.config.kubernetes.io/path: example-use.yaml
# EDIT THE SOURCE!
# This should be your KCL code which preloads the `ResourceList` to `option("resource_list")
spec:
  source: |
    [resource | {if resource.kind == "Deployment": metadata.annotations: {"managed-by" = "kustomize-kcl"}} for resource in option("resource_list").items]
---
apiVersion: v1
kind: Service
metadata:
  name: test
  annotations:
    config.kubernetes.io/path: example-use.yaml
    internal.config.kubernetes.io/path: example-use.yaml
spec:
  selector:
    app: MyApp
  ports:
  - protocol: TCP
    port: 80
    targetPort: 9376
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: nginx
  annotations:
    config.kubernetes.io/path: example-use.yaml
    internal.config.kubernetes.io/path: example-use.yaml
    managed-by: kustomize-kcl
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.14.2
        ports:
        - containerPort: 80

This exists non-zero if the KCL code has no errors.

Function Implementation

The function is implemented as an image, and built using make image, and its function is implemented as a go program, which reads a collection of input Resource configuration, passing them to KCL.

Function Configuration

See the API struct definition in main.go for documentation.

source - the KCL function code.
params - top-level arguments for KCL

Function Invocation

The function is invoked by authoring a local Resource with metadata.annotations.[config.kubernetes.io/function].

Guides for Developing KCL

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("resource_list")["items"] and the functionConfig from option("resource_list")["functionConfig"].
Return a KPM list for output resources.
Return an error using assert {condition}, {error_message}.
Read the environment variables. e.g. option("PATH") (Not yet implemented).
Read the OpenAPI schema. e.g. option("open_api")["definitions"]["io.k8s.api.apps.v1.Deployment"] (Not yet implemented).

Examples

See here for more examples.

Library

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

KusionStack/kustomize-kcl