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.

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,

1apiVersion: pkg.crossplane.io/v1beta1
2kind: Function
3metadata:
4  name: function-patch-and-transform
5spec:
6  package: xpkg.upbound.io/crossplane-contrib/function-patch-and-transform:v0.1.4

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.

1kubectl get functions
2NAME                              INSTALLED   HEALTHY   PACKAGE                                                                  AGE
3function-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 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.

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.

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 resources as input.

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: Composition
 3# Removed for Brevity
 4spec:
 5  # Removed for Brevity
 6  mode: Pipeline
 7  pipeline:
 8  - step: patch-and-transform
 9    functionRef:
10      name: function-patch-and-transform
11    input:
12      apiVersion: pt.fn.crossplane.io/v1beta1
13      kind: Resources
14      resources:
15      - name: storage-bucket
16        base:
17          apiVersion: s3.aws.upbound.io/v1beta1
18          kind: Bucket
19          spec:
20            forProvider:
21              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.

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: Composition
 3# Removed for Brevity
 4spec:
 5  # Removed for Brevity
 6  mode: Pipeline
 7  pipeline:
 8  - step: cue-export-resources
 9    functionRef:
10      name: function-cue
11    input:
12      apiVersion: cue.fn.crossplane.io/v1beta1
13      kind: CUEInput
14      name: storage-bucket
15      export:
16        target: Resources
17        value: |
18          apiVersion: "s3.aws.upbound.io/v1beta1"
19          kind: "Bucket"
20          spec:
21            forProvider:
22              region: "us-east-2"          
23  - step: automatically-detect-readiness
24    functionRef:
25      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.

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

1crossplane 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.

 1---
 2apiVersion: example.crossplane.io/v1
 3kind: XBucket
 4metadata:
 5  name: example-render
 6---
 7apiVersion: s3.aws.upbound.io/v1beta1
 8kind: Bucket
 9metadata:
10  annotations:
11    crossplane.io/composition-resource-name: storage-bucket
12  generateName: example-render-
13  labels:
14    crossplane.io/composite: example-render
15  ownerReferences:
16  - apiVersion: example.crossplane.io/v1
17    blockOwnerDeletion: true
18    controller: true
19    kind: XBucket
20    name: example-render
21    uid: ""
22spec:
23  forProvider:
24    region: us-east-2

1apiVersion: example.crossplane.io/v1
2kind: XBucket
3metadata:
4  name: example-render
5spec:
6  bucketRegion: us-east-2

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: Composition
 3metadata:
 4  name: example-render
 5spec:
 6  compositeTypeRef:
 7    apiVersion: example.crossplane.io/v1
 8    kind: XBucket
 9  mode: Pipeline
10  pipeline:
11  - step: patch-and-transform
12    functionRef:
13      name: function-patch-and-transform
14    input:
15      apiVersion: pt.fn.crossplane.io/v1beta1
16      kind: Resources
17      resources:
18      - name: storage-bucket
19        base:
20          apiVersion: s3.aws.upbound.io/v1beta1
21          kind: Bucket
22        patches:
23        - type: FromCompositeFieldPath
24          fromFieldPath: spec.bucketRegion
25          toFieldPath: spec.forProvider.region

1---
2apiVersion: pkg.crossplane.io/v1beta1
3kind: Function
4metadata:
5  name: function-patch-and-transform
6spec:
7  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.

1apiVersion: pkg.crossplane.io/v1beta1
2kind: Function
3metadata:
4  name: function-patch-and-transform
5  annotations:
6    render.crossplane.io/runtime: Development
7spec:
8  package: xpkg.upbound.io/crossplane-contrib/function-patch-and-transform:v0.1.4

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.

1func (f *Function) RunFunction(_ context.Context, req *fnv1beta1.RunFunctionRequest) (*fnv1beta1.RunFunctionResponse, error) {
2        rsp := response.To(req, response.DefaultTTL)
3        response.Normal(rsp, "Hello world!")
4        return rsp, nil
5}

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.

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: Composition
 3metadata:
 4  name: example-bucket-function
 5spec:
 6  compositeTypeRef:
 7    apiVersion: example.crossplane.io/v1
 8    kind: XBucket
 9  mode: Pipeline
10  pipeline:
11  - step: handle-xbucket-xr
12    functionRef:
13      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 to create, test, build, and push a function. For example,

 1# Create the function from a template.
 2crossplane beta xpkg init function-example function-template-go
 3Initialized package "function-example" in directory "/home/negz/control/negz/function-example" from https://github.com/crossplane/function-template-go/tree/91a1a5eed21964ff98966d72cc6db6f089ad63f4 (main)
 4
 5$ ls
 6Dockerfile  fn.go  fn_test.go  go.mod  go.sum  input  LICENSE  main.go  package  README.md  renovate.json
 7
 8# Edit the template to add your function's logic
 9$ vim fn.go
10
11# Build the function.
12$ docker build . --quiet --tag runtime
13sha256:2c31b0f7a34b34ba5b0b2dacc94c360d18aca1b99f56ca4f40a1f26535a7c1c4
14
15# Package the function.
16$ crossplane xpkg build -f package --embed-runtime-image=runtime
17
18# Test the function.
19$ go run . --insecure
20$ crossplane beta render xr.yaml composition.yaml functions.yaml
21
22# Push the function package to xpkg.upbound.io.
23$ crossplane xpkg push -f package/*.xpkg crossplane-contrib/function-example:v0.1.0

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
    loop Extra resources needed?
      Crossplane Pod->>+API Server: Get Extra resources
      Crossplane Pod->>+Function Pod: gRPC RunFunctionRequest
      Function Pod->>+Crossplane Pod: gRPC RunFunctionResponse
    end
    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.

When Crossplane calls a function the first time 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.

If the function needs extra resources to determine the desired state it can request any cluster-scoped resource Crossplane already has access to, either by by name or labels through the returned RunFunctionResponse. Crossplane then calls the function again including the requested extra resources and the context returned by the Function itself alongside the same input, observed and desired state of the previous RunFunctionRequest. Functions can iteratively request extra resources if needed, but to avoid endlessly looping Crossplane limits the number of iterations to 5. Crossplane considers the function satisfied as soon as the extra resources requests become stable, so the Function returns the same exact request two times in a row. Crossplane errors if stability isn’t reached after 5 iterations.

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.

1apiVersion: example.crossplane.io/v1
2kind: XBucket
3metadata:
4  name: example-render
5spec:
6  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.

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.

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.

1apiVersion: s3.aws.upbound.io/v1beta1
2kind: Bucket
3spec:
4  forProvider:
5    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.

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.

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: Composition
 3metadata:
 4  name: example-render
 5spec:
 6  compositeTypeRef:
 7    apiVersion: example.crossplane.io/v1
 8    kind: XBucket
 9  mode: Pipeline
10  pipeline:
11  - step: patch-and-transform
12    functionRef:
13      name: function-patch-and-transform
14    input:
15      apiVersion: pt.fn.crossplane.io/v1beta1
16      kind: Resources
17      resources:
18      - name: storage-bucket
19        base:
20          apiVersion: s3.aws.upbound.io/v1beta1
21          kind: Bucket
22        patches:
23        - type: FromCompositeFieldPath
24          fromFieldPath: spec.bucketRegion
25          toFieldPath: spec.forProvider.region

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 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.

1helm install crossplane --namespace crossplane-system crossplane-stable/crossplane \
2    --create-namespace \
3    --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:

1 kubectl -n crossplane-system logs -l app=crossplane
2{"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.

Disable extra resources

Crossplane enables extra resources by default, allowing Functions to get access to any cluster-scoped resource Crossplane already has access to. Disable support for extra resources, while keeping composition functions enabled, by disabling the beta feature flag in Crossplane with helm install --args.

1helm install crossplane --namespace crossplane-system crossplane-stable/crossplane \
2    --create-namespace \
3    --set "args='{--enable-composition-functions-extra-resources=false}'"

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

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

If you don’t see the log line emitted when Crossplane starts, you have disabled extra resources for composition functions, which means requests by functions for extra resources are just ignored.