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title state alphaVersion betaVersion weight description aliases
Composition Functions beta 1.11 1.14 80 Composition Functions allow you to template resources using general-purpose programming languages
/knowledge-base/guides/composition-functions

Composition functions (or just functions, for short) are custom programs that template Crossplane resources. Crossplane calls composition functions to determine what resources it should create when you create a composite resource (XR). You can write a function to template resources using a general purpose programming language like Go or Python. Using a general purpose programming language allows a Function to use more advanced logic to template resources, like loops and conditionals.

You can build a function using general purpose programming languages such as Go or Python. The Crossplane community has also built functions that let you template Crossplane resources using CUE, Helm-like Go templates or [Patch and Transforms]({{<ref "./patch-and-transform">}}).

Install a composition function

Installing a Function creates a function pod. Crossplane sends requests to this pod to ask it what resources to create when you create a composite resource.

Install a Function with a Crossplane {{}}Function{{}} object setting the {{}}spec.package{{}} value to the location of the function package.

For example, to install Function Patch and Transform,

apiVersion: pkg.crossplane.io/v1beta1
kind: Function
metadata:
  name: function-patch-and-transform
spec:
  package: xpkg.upbound.io/crossplane-contrib/function-patch-and-transform:v0.1.4

{{< hint "tip" >}} Functions are Crossplane Packages. Read more about Packages in the [Packages documentation]({{<ref "packages" >}}). {{< /hint >}}

By default, the Function pod installs in the same namespace as Crossplane (crossplane-system).

Verify a composition function

View the status of a Function with kubectl get functions

During the install a Function reports INSTALLED as True and HEALTHY as Unknown.

kubectl get functions
NAME                              INSTALLED   HEALTHY   PACKAGE                                                                  AGE
function-patch-and-transform      True        Unknown   xpkg.upbound.io/crossplane-contrib/function-patch-and-transform:v0.1.4   10s

After the Function install completes and it's ready for use the HEALTHY status reports True.

Use a function in a composition

Crossplane calls a Function to determine what resources it should create when you create a composite resource. The Function also tells Crossplane what to do with these resources when a you update or delete a composite resource.

When Crossplane calls a Function it sends it the current state of the composite resource. It also sends it the current state of any managed resources the composite resource owns.

Crossplane knows what Function to call when a composite resource changes by looking at the Composition the composite resource uses.

{{<expand "Confused about Composite Resources and Compositions?" >}} Crossplane has four core components that users commonly mix up:

  • [Composition]({{<ref "./compositions">}}) - A template to define how to create resources.
  • [CompositeResourceDefinition]({{<ref "./composite-resource-definitions">}}) (XRD) - A custom API specification.
  • [Composite Resource]({{<ref "./composite-resources">}}) (XR) - Created by using the custom API defined in a CompositeResourceDefinition. XRs use the Composition template to create new managed resources.
  • [Claim]({{<ref "./claims" >}}) (XRC) - Like a Composite Resource, but with namespace scoping. {{}}

To use composition functions set the Composition {{}}mode{{}} to {{}}Pipeline{{}}.

Define a {{}}pipeline{{}} of {{}}steps{{}}. Each {{}}step{{}} calls a Function.

Each {{}}step{{}} uses a {{}}functionRef{{}} to reference the {{}}name{{}} of the Function to call.

{{<hint "important" >}} Compositions using {{}}mode: Pipeline{{}} can't specify resource templates with a resources field.

Use function "Patch and Transform" to create resource templates. {{< /hint >}}

Some Functions also allow you to specify an {{}}input{{}}.
The function defines the {{}}kind{{}} of input.

This example uses Function Patch and Transform.
Function Patch and Transform implements Crossplane resource templates.
The input kind is {{}}Resources{{}}, and it accepts [Patch and Transform]({{<ref "./patch-and-transform">}}) {{}}resources{{}} as input.

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
# Removed for Brevity
spec:
  # Removed for Brevity
  mode: Pipeline
  pipeline:
  - step: patch-and-transform
    functionRef:
      name: function-patch-and-transform
    input:
      apiVersion: pt.fn.crossplane.io/v1beta1
      kind: Resources
      resources:
      - name: storage-bucket
        base:
          apiVersion: s3.aws.upbound.io/v1beta1
          kind: Bucket
          spec:
            forProvider:
              region: "us-east-2"

Use a pipeline of functions in a composition

Crossplane can ask more than one Function what to do when a composite resource changes. When a Composition has a pipeline of two or more steps, Crossplane calls them all. It calls them in the order they appear in the pipeline.

Crossplane passes each Function in the pipeline the result of the previous Function. This enables powerful combinations of Functions. In this example, Crossplane calls {{}}function-cue{{}} to create an S3 bucket. Crossplane then passes the bucket to {{}}function-auto-ready{{}}, which marks the composite resource as ready when the bucket becomes ready.

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
# Removed for Brevity
spec:
  # Removed for Brevity
  mode: Pipeline
  pipeline:
  - step: cue-export-resources
    functionRef:
      name: function-cue
    input:
      apiVersion: cue.fn.crossplane.io/v1beta1
      kind: CUEInput
      name: storage-bucket
      export:
        target: Resources
        value: |
          apiVersion: "s3.aws.upbound.io/v1beta1"
          kind: "Bucket"
          spec: forProvider: region: "us-east-2"          
  - step: automatically-detect-readiness
    functionRef:
      name: function-auto-ready

Test a composition that uses functions

You can preview the output of any composition that uses composition functions using the Crossplane CLI. You don't need a Crossplane control plane to do this. The Crossplane CLI uses Docker Engine to run functions.

{{<hint "tip">}} See the [Crossplane CLI docs]({{<ref "../cli">}}) to learn how to install and use the Crossplane CLI. {{< /hint >}}

{{<hint "important">}} Running crossplane beta render requires Docker. {{< /hint >}}

Provide a composite resource, composition and composition functions to render the output locally.

crossplane beta render xr.yaml composition.yaml functions.yaml

crossplane beta render prints resources as YAML to stdout. It prints the composite resource first, followed by the resources the composition functions created.

---
apiVersion: example.crossplane.io/v1
kind: XBucket
metadata:
  name: example-render
---
apiVersion: s3.aws.upbound.io/v1beta1
kind: Bucket
metadata:
  annotations:
    crossplane.io/composition-resource-name: storage-bucket
  generateName: example-render-
  labels:
    crossplane.io/composite: example-render
  ownerReferences:
  - apiVersion: example.crossplane.io/v1
    blockOwnerDeletion: true
    controller: true
    kind: XBucket
    name: example-render
    uid: ""
spec:
  forProvider:
    region: us-east-2

{{<expand "The xr.yaml, composition.yaml and function.yaml files used in the example">}}

You can recreate the output below using by running crossplane beta render with these files.

The xr.yaml file contains the composite resource to render:

apiVersion: example.crossplane.io/v1
kind: XBucket
metadata:
  name: example-render
spec:
  bucketRegion: us-east-2

The composition.yaml file contains the Composition to use to render the composite resource:

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
metadata:
  name: example-render
spec:
  compositeTypeRef:
    apiVersion: example.crossplane.io/v1
    kind: XBucket
  mode: Pipeline
  pipeline:
  - step: patch-and-transform
    functionRef:
      name: function-patch-and-transform
    input:
      apiVersion: pt.fn.crossplane.io/v1beta1
      kind: Resources
      resources:
      - name: storage-bucket
        base:
          apiVersion: s3.aws.upbound.io/v1beta1
          kind: Bucket
        patches:
        - type: FromCompositeFieldPath
          fromFieldPath: spec.bucketRegion
          toFieldPath: spec.forProvider.region

The functions.yaml file contains the Functions the Composition references in its pipeline steps:

---
apiVersion: pkg.crossplane.io/v1beta1
kind: Function
metadata:
  name: function-patch-and-transform
spec:
  package: xpkg.upbound.io/crossplane-contrib/function-patch-and-transform:v0.1.4

{{}}

The Crossplane CLI uses Docker Engine to run functions. You can change how the Crossplane CLI run a function by adding an annotation in functions.yaml. Add the render.crossplane.io/runtime annotation to a Function to change how it's run.

crossplane beta render supports two render.crossplane.io/runtime values:

  • Docker (the default) connects to Docker Engine. It uses Docker to pull and run a function runtime.
  • Development connects to a function runtime you have run manually.

When you use the {{}}Development{{}} runtime the Crossplane CLI ignores the Function's {{}}package{{}}. Instead it expects you to make sure the function is listening on localhost port 9443. The function must be listening without gRPC transport security. Most function SDKs let you run a function with the --insecure flag to disable transport security. For example you can run a Go function locally using go run . --insecure.

apiVersion: pkg.crossplane.io/v1beta1
kind: Function
metadata:
  name: function-patch-and-transform
  annotation:
    render.crossplane.io/runtime: Development
spec:
  package: xpkg.upbound.io/crossplane-contrib/function-patch-and-transform:v0.1.4

{{<hint "tip">}} Use the Development runtime when you write a composition function to test your function end-to-end. {{}}

crossplane beta render also supports the following Function annotations. These annotations affect how it runs Functions:

  • render.crossplane.io/runtime-docker-cleanup - When using the Docker runtime this annotation specifies whether the CLI should stop the function container after it calls the function. It supports the values Stop, to stop the container, and Orphan, to leave it running.
  • render.crossplane.io/runtime-docker-pull-policy - When using the Docker runtime this annotation specifies when the CLI should pull the Function's package. It supports the values Always, Never, and IfNotPresent.
  • render.crossplane.io/runtime-development-target - When using the Development runtime this annotation tells the CLI to connect to a Function running at the specified target. It uses gRPC target syntax.

Write a composition function

Composition functions let you replace complicated Compositions with code written in your programming language of choice. Crossplane has tools, software development kits (SDKs) and templates to help you write a function.

Here's an example of a tiny, hello world function. This example is written in Go.

func (f *Function) RunFunction(_ context.Context, req *fnv1beta1.RunFunctionRequest) (*fnv1beta1.RunFunctionResponse, error) {
        rsp := response.To(req, response.DefaultTTL)
        response.Normal(rsp, "Hello world!")
        return rsp, nil
}

Some people design composition functions for you to use them with any kind of composite resource. Function Patch and Transform and Function Auto Ready work with any kind of composite resource.

Another common pattern is to write a composition function specific to one kind of composite resource. The function contains all the logic needed to tell Crossplane what resources to create when you create a composite resource. When you write a composition function like this, your Composition can be small. It just tells Crossplane what function to run when you create, update, or delete a composite resource.

This Composition tells Crossplane to call {{}}function-xr-xbucket{{}} whenever you create, update, or delete an {{}}XBucket{{}} composite resource. function-xr-xbucket is hard coded to handle XBucket composite resources.

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
metadata:
  name: example-bucket-function
spec:
  compositeTypeRef:
    apiVersion: example.crossplane.io/v1
    kind: XBucket
  mode: Pipeline
  pipeline:
  - step: handle-xbucket-xr
    functionRef:
      name: function-xr-xbucket

To write a composition function, you:

  1. Create the function from a template.
  2. Edit the template to add the function's logic.
  3. Test the function.
  4. Build the function, and push it to a package registry.

You use the [Crossplane CLI]({{<ref "../cli">}}) to create, test, build, and push a function. For example,

# Create the function from a template.
crossplane beta xpkg init function-example function-template-go
Initialized package "function-example" in directory "/home/negz/control/negz/function-example" from https://github.com/crossplane/function-template-go/tree/91a1a5eed21964ff98966d72cc6db6f089ad63f4 (main)

$ ls
Dockerfile  fn.go  fn_test.go  go.mod  go.sum  input  LICENSE  main.go  package  README.md  renovate.json

# Edit the template to add your function's logic
$ vim fn.go

# Build the function.
$ docker build . --quiet --tag runtime
sha256:2c31b0f7a34b34ba5b0b2dacc94c360d18aca1b99f56ca4f40a1f26535a7c1c4

# Package the function.
$ crossplane xpkg build -f package --embed-runtime-image=runtime

# Test the function.
$ go run . --insecure
$ crossplane beta render xr.yaml composition.yaml functions.yaml

# Push the function package to xpkg.upbound.io.
$ crossplane xpkg push -f package/*.xpkg crossplane-contrib/function-example:v0.1.0

{{<hint "tip">}} Crossplane has [language specific guides]({{<ref "../../knowledge-base/guides">}}) to writing a composition function. Refer to the guide for your preferred language for a more detailed guide to writing a function. {{}}

When you're writing a composition function it's useful to know how composition functions work. Read the next section to learn how composition functions work.

How composition functions work

Each composition function is actually a gRPC server. gRPC is a high performance, open source remote procedure call (RPC) framework. When you install a function Crossplane deploys the function as a gRPC server. Crossplane encrypts and authenticates all gRPC communication.

You don't have to be a gRPC expert to write a function. Crossplane's function SDKs setup gRPC for you. It's useful to understand how Crossplane calls your function though, and how your function should respond.

sequenceDiagram
    User->>+API Server: Create composite resource
    Crossplane Pod->>+API Server: Observe composite resource
    Crossplane Pod->>+Function Pod: gRPC RunFunctionRequest
    Function Pod->>+Crossplane Pod: gRPC RunFunctionResponse
    Crossplane Pod->>+API Server: Apply desired composed resources

When you create, update, or delete a composite resource that uses composition functions Crossplane calls each function in the order they appear in the Composition's pipeline. Crossplane calls each function by sending it a gRPC RunFunctionRequest. The function must respond with a gRPC RunFunctionResponse.

{{<hint "tip">}} You can find detailed schemas for the RunFunctionRequest and RunFunctionResponse RPCs in the Buf Schema Registry. {{}}

When Crossplane calls a function it includes four important things in the RunFunctionRequest.

  1. The observed state of the composite resource, and any composed resources.
  2. The desired state of the composite resource, and any composed resources.
  3. The function's input.
  4. The function pipeline's context.

A function's main job is to update the desired state and return it to Crossplane. It does this by returning a RunFunctionResponse.

Most composition functions read the observed state of the composite resource, and use it to add composed resources to the desired state. This tells Crossplane which composed resources it should create or update.

{{<hint "tip">}}

A composed resource is a resource created by a composite resource. Composed resources are usually Crossplane managed resources (MRs), but they can be any kind of Crossplane resource. For example a composite resource could also create a ProviderConfig, or another kind of composite resource.

{{}}

Observed state

When you create a composite resource like this one, Crossplane observes it and sends it to the composition function as part of the observed state.

apiVersion: example.crossplane.io/v1
kind: XBucket
metadata:
  name: example-render
spec:
  bucketRegion: us-east-2

If any composed resources already exist, Crossplane observes them and sends them to your function to as part of the observed state.

Crossplane also observes the connection details of your composite resource and any composed resources. It sends them to your function as part of the observed state.

Crossplane observes the composite resource and any composed resources once, right before it starts calling the functions in the pipeline. This means that Crossplane sends every function in the pipeline the same observed state.

Desired state

Desired state is the set of the changes the function pipeline wants to make to the composite resource and any composed resources. When a function adds composed resources to the desired state Crossplane creates them.

A function can change:

  • The status of the composite resource.
  • The metadata and spec of any composed resource.

A function can also change the connection details and readiness of the composite resource. A function indicates that the composite resource is ready by telling Crossplane whether its composed resources are ready. When the function pipeline tells Crossplane that all composed resources are ready, Crossplane marks the composite resource as ready.

A function can't change:

  • The metadata or spec of the composite resource.
  • The status of any composed resource.
  • The connection details of any composed resource.

A pipeline of functions accumulates desired state. This means that each function builds upon the desired state of previous functions in the pipeline. Crossplane sends a function the desired state accumulated by all previous functions in the pipeline. The function adds to or updates the desired state and then passes it on. When the last function in the pipeline has run, Crossplane applies the desired state it returns.

{{<hint "important">}} A function must copy all desired state from its RunFunctionRequest to its RunFunctionResponse. If a function adds a resource to its desired state the next function must copy it to its desired state. If it doesn't, Crossplane doesn't apply the resource. If the resource exists, Crossplane deletes it.

A function can intentionally choose not to copy parts of the desired state. For example a function may choose not to copy a desired resource to prevent that resource from existing.

Most function SDKs handle copying desired state automatically. {{}}

A function should only add the fields it cares about to the desired state. It should add these fields every time Crossplane calls it. If a function adds a field to the desired state once, but doesn't add it the next time it's called, Crossplane deletes the field. The same is true for composed resources. If a function adds a composed resource to the desired state, but doesn't add it the next time it's called, Crossplane deletes the composed resource.

{{<hint "tip">}} Crossplane uses server side apply to apply the desired state returned by a function pipeline. In server side apply terminology, the desired state is a fully specified intent. {{}}

For example, if all a function wants is to make sure an S3 bucket in region us-east-2 exists, it should add this resource to its desired composed resources.

apiVersion: s3.aws.upbound.io/v1beta1
kind: Bucket
spec:
  forProvider:
    region: us-east-2

Even if the Bucket already exists and has other spec fields, or a status, name, labels, etc the function should omit them. The function should only include the fields it has an opinion about. Crossplane takes care of applying the fields the function cares about, merging them with the existing Bucket.

{{<hint "tip">}} Composition functions don't actually use YAML for desired and observed resources. This example uses YAML for illustration purposes only. {{}}

Function input

If a Composition includes {{}}input{{}} Crossplane sends it to the function. Input is a useful way to provide extra configuration to a function. Supporting input is optional. Not all functions support input.

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
metadata:
  name: example-render
spec:
  compositeTypeRef:
    apiVersion: example.crossplane.io/v1
    kind: XBucket
  mode: Pipeline
  pipeline:
  - step: patch-and-transform
    functionRef:
      name: function-patch-and-transform
    input:
      apiVersion: pt.fn.crossplane.io/v1beta1
      kind: Resources
      resources:
      - name: storage-bucket
        base:
          apiVersion: s3.aws.upbound.io/v1beta1
          kind: Bucket
        patches:
        - type: FromCompositeFieldPath
          fromFieldPath: spec.bucketRegion
          toFieldPath: spec.forProvider.region

{{<hint "important">}} Crossplane doesn't validate function input. It's a good idea for a function to validate its own input. {{}}

Function pipeline context

Sometimes two functions in a pipeline want to share information with each other that isn't desired state. Functions can use context for this. Any function can write to the pipeline context. Crossplane passes the context to all following functions. When Crossplane has called all functions it discards the pipeline context.

Crossplane can write context too. If you enable the alpha [composition environment]({{<ref "environment-configs">}}) feature Crossplane writes the environment to the top-level context field apiextensions.crossplane.io/environment.

Disable composition functions

Crossplane enables composition functions by default. Disable support for composition functions by disabling the beta feature flag in Crossplane with helm install --args.

helm install crossplane --namespace crossplane-system crossplane-stable/crossplane \
    --create-namespace \
    --set "args='{--enable-composition-functions=false}'"

The preceding Helm command installs Crossplane with the composition functions feature flag disabled. Confirm you have disabled composition functions by looking for a log line:

 kubectl -n crossplane-system logs -l app=crossplane
{"level":"info","ts":1674535093.36186,"logger":"crossplane","msg":"Beta feature enabled","flag":"EnableBetaCompositionFunctions"}

If you don't see the log line emitted when Crossplane starts, you have disabled composition functions.