function-kro

A Crossplane Composition Function for YAML+CEL resource composition that implements the same syntax and experience provided by KRO (Kubernetes Resource Orchestrator). Define complex, interdependent Kubernetes resources using CEL expressions — all inline in your Crossplane Composition's function pipeline.

Overview

function-kro is a Crossplane composition function for declarative resource orchestration. It adapts KRO's approach to run inside Crossplane, letting you combine CEL-based resource definitions with other Crossplane functions in a single pipeline.

There is no need to install KRO itself — function-kro natively embeds KRO's graph builder, CEL evaluator, and runtime engine with full feature parity. See the KRO documentation for details on all available capabilities.

Usage

Use function-kro as a step in a Crossplane Composition pipeline. The function takes a ResourceGraph input that defines your resources and their relationships using ${...} CEL expressions.

If you already have KRO resource definitions, the resources and status blocks are identical to what you'd write in a standalone KRO ResourceGraphDefinition — they drop into the pipeline input without changes.

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
spec:
  compositeTypeRef:
    apiVersion: example.crossplane.io/v1alpha1
    kind: MyResource
  mode: Pipeline
  pipeline:
  - step: kro
    functionRef:
      name: function-kro
    input:
      apiVersion: kro.fn.crossplane.io/v1alpha1
      kind: ResourceGraph
      status:
        vpcId: ${vpc.status.atProvider.id}
      resources:
      - id: vpc
        template:
          apiVersion: ec2.aws.upbound.io/v1beta1
          kind: VPC
          spec:
            forProvider:
              region: ${schema.spec.region}
              cidrBlock: "10.0.0.0/16"
      - id: subnet
        template:
          apiVersion: ec2.aws.upbound.io/v1beta1
          kind: Subnet
          spec:
            forProvider:
              region: ${schema.spec.region}
              vpcId: ${vpc.status.atProvider.id}
              cidrBlock: "10.0.1.0/24"

Why Crossplane?

Running resource definitions inside Crossplane's architecture adds capabilities that come with the platform:

• Pipeline composability. Add functions from a broad ecosystem of pipeline steps alongside your resource definitions. Enforce policy, inject tags, pull secrets, or call external APIs — each as an independent pipeline step that adds functionality while leaving your resource definitions untouched.
• Safe rollouts. CompositionRevisions let you pin existing instances to a known-good revision, roll forward incrementally, and roll back without a rewrite.
• Multi-implementation APIs. Define one API backed by multiple Compositions. The same API schema can have separate AWS, GCP, and Azure implementations behind it.
• Operational controls. Pause reconciliation, set management policies, and control resource lifecycle per-instance without touching the Composition.
• Developer experience. Test and validate locally with crossplane render, diff against a live environment, and run unit and integration tests before deploying.

CEL Expressions

Expressions use ${...} syntax within resource templates:

• Reference the XR spec: ${schema.spec.region}
• Reference other resources' observed state: ${vpc.status.atProvider.id}
• Execute logic inline: ${schema.spec.replicas * 2}, arn:aws:s3:::${bucket.status.atProvider.id}

You don't need to manually order your resources or wire up dependencies. function-kro analyzes the expressions in your templates, builds a dependency graph (DAG), and automatically determines the correct order to create and reconcile resources. If a resource references another resource's status, it will wait until that dependency is ready before proceeding.

Examples

See the example/ directory for complete working examples:

Example	Description
basic	Resource dependencies and status aggregation
conditionals	Conditional resource creation with includeWhen
readiness	Custom readiness conditions with readyWhen
externalref	Referencing existing cluster resources outside of the XR/composition
collections	Dynamic resource expansion with forEach
omit	Conditional field removal using the omit() CEL function
collection-limits	Enforcing maximum collection size limits

See the examples README for setup instructions and walkthroughs.

Development

# 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=runtime

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

Contributing

We welcome contributions of all kinds — opening issues, improving documentation, fixing bugs, or adding new features. If you don't know where to start or have any questions, please reach out to us on the #function-kro channel on Crossplane's Slack.

License

Apache 2.0. See LICENSE for details.