KRM Function Developer Guide on kpt documentation

Containerizing

Mon, 01 Jan 0001 00:00:00 +0000

KRM functions are distributed as container images. This guide covers building and running containerized functions.

Dockerfile #

The krm-functions-catalog provides a shared Dockerfile at build/docker/go/Dockerfile that all the catalog functions use. It accepts BUILDER_IMAGE and BASE_IMAGE as build args.

For standalone functions or local development, use a multi-stage build with a minimal base image. The function binary should be statically linked (no CGO), so it can run on scratch or distroless:

Interfaces

Mon, 01 Jan 0001 00:00:00 +0000

The SDK provides two interfaces for implementing KRM functions. Choose according to your function requirements.

The main functions below call fn.AsMain without fn.WithDocs to keep the interface examples focused. Production functions should embed documentation and pass fn.WithDocs(readme, metadata) — see the tutorial.

fn.Runner #

Use fn.Runner for transformers (mutators) and validators. This is the recommended interface for most functions.

type Runner interface {
 Run(context *Context, functionConfig *KubeObject, items KubeObjects, results *Results) bool
}

Characteristics:

Testing

Mon, 01 Jan 0001 00:00:00 +0000

The SDK provides a golden test framework in fn/testhelpers for snapshot-based testing of KRM functions.

Golden Test Pattern #

Golden tests compare the function output against the expected baseline files. This approach catches regressions and makes it easy to review output changes.

Directory Structure #

testdata/
├── test-case-1/
│ ├── _expected.yaml # Expected output (full ResourceList YAML)
│ ├── _fnconfig.yaml # FunctionConfig for this test case
│ └── resources.yaml # Input KRM resources
└── test-case-2/
 ├── _expected.yaml
 ├── _fnconfig.yaml
 └── resources.yaml

Conventions:

Tutorial

Mon, 01 Jan 0001 00:00:00 +0000

This tutorial walks through the end-to-end workflow for building a KRM function using the Go SDK. By the end, you will have a working function with embedded documentation, golden tests, and support for --help, --doc, and standalone file mode.

For a complete working example, see go/get-started/.

1. Create Your Function #

A KRM function implements the fn.Runner interface:

type Runner interface {
 Run(context *Context, functionConfig *KubeObject, items KubeObjects, results *Results) bool
}

Here is a minimal function that sets labels on all the resources: