docs/content/master/concepts/compositions.md

title	weight	aliases	description
Compositions	30	composition composition-functions /knowledge-base/guides/composition-functions	Compositions are a template for creating Crossplane resources

Compositions are a template for creating multiple managed resources as a single object.

A Composition composes individual managed resources together into a larger, reusable, solution.

An example Composition may combine a virtual machine, storage resources and networking policies. A Composition template links all these individual resources together.

Here's an example Composition. When you create an {{}}AcmeBucket{{}} composite resource (XR) that uses this Composition, Crossplane uses the template to create the Amazon S3 {{}}Bucket{{}} managed resource.

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
metadata:
  name: example
spec:
  compositeTypeRef:
    apiVersion: custom-api.example.org/v1alpha1
    kind: AcmeBucket
  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"

{{<expand "Confused about Compositions, XRDs, XRs and Claims?" >}} Crossplane has four core components that users commonly mix up:

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

Create a Composition

Creating a Composition consists of:

• Using composition functions to define the resources to create.
• Enabling composite resources to use the Composition template.

A Composition is a pipeline of composition functions.

Composition functions (or just functions, for short) are Crossplane extensions that template Crossplane resources. Crossplane calls the composition functions to determine what resources it should create when you create a composite resource (XR).

{{<hint "tip" >}} The Crossplane community has built lots of functions that let you template Crossplane resources using CUE, KCL, Helm-like Go templates or legacy Crossplane [Patch and Transforms]({{<ref "../guides/function-patch-and-transform">}}).

You can also write your own function using Go or Python. {{< /hint >}}

{{<hint "important" >}} Crossplane has two modes of composition:

• mode: Pipeline
• mode: Resources

Use the Pipeline mode to use composition functions.

The Resources mode is deprecated, and you shouldn't use it. Crossplane supports Compositions that use the Resources mode for backward compatibility, but the feature is no longer maintained. Crossplane doesn't accept new Resources features, and only accepts security bug fixes.

See the [CLI documentation]({{<ref "../cli/command-reference#beta-convert">}}) to learn how to use the crossplane beta convert command to convert a legacy Resources Composition to the Pipeline mode. {{< /hint >}}

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]({{<ref "../guides/function-patch-and-transform">}}),

apiVersion: pkg.crossplane.io/v1
kind: Function
metadata:
  name: function-patch-and-transform
spec:
  package: xpkg.crossplane.io/crossplane-contrib/function-patch-and-transform:v0.8.2

{{< 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.crossplane.io/crossplane-contrib/function-patch-and-transform:v0.8.2   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.

{{<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]({{<ref "../guides/function-patch-and-transform">}}). Function Patch and Transform implements Crossplane resource templates. The input kind is {{}}Resources{{}}, and it accepts {{}}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

Enable composite resources

A Composition is only a template defining how to create managed resources. A Composition limits which Composite Resources can use this template.

A Composition's {{}}compositeTypeRef{{}} defines which Composite Resource type can use this Composition.

{{<hint "note" >}} Read more about Composite Resources in the [Composite Resources page]({{<ref "./composite-resources" >}}). {{< /hint >}}

Inside a Composition's {{}}spec{{}} define the Composite Resource {{}}apiVersion{{}} and {{}}kind{{}} that the Composition allows to use this template.

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
metadata:
  name: dynamodb-with-bucket
spec:
  compositeTypeRef:
    apiVersion: custom-api.example.org/v1alpha1
    kind: database
  # Removed for brevity

Store connection details

Some managed resources generate unique details like usernames, passwords, IP addresses, ports or other connection details.

When resources inside a Composition create connection details Crossplane creates a Kubernetes secret object for each managed resource generating connection details.

{{<hint "note">}} This section discusses creating Kubernetes secrets. Crossplane also supports using external secret stores like HashiCorp Vault.

Read the [external secrets store guide]({{<ref "../guides/vault-as-secret-store">}}) for more information on using Crossplane with an external secret store. {{}}

Composite resource combined secret

Crossplane can combine all the secrets generated by the resources inside a Composition into a single Kubernetes secret and optionally copy the secret object for claims.

Set the value of writeConnectionSecretsToNamespace to the namespace where Crossplane should store the combined secret object.

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
# Removed for Brevity
spec:
  writeConnectionSecretsToNamespace: my-namespace
  resources:
  # Removed for brevity

Composed resource secrets

Inside the spec of each resource producing connection details, define the writeConnectionSecretToRef, with a namespace and name of the secret object for the resource.

If a writeConnectionSecretToRef isn't defined, Crossplane doesn't write any keys to the secret.

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
spec:
  writeConnectionSecretsToNamespace: other-namespace
  mode: Pipeline
  pipeline:
  - step: patch-and-transform
    functionRef:
      name: function-patch-and-transform
    input:
      apiVersion: pt.fn.crossplane.io/v1beta1
      kind: Resources
      resources:
      - name: key
        base:
          apiVersion: iam.aws.upbound.io/v1beta1
          kind: AccessKey
          spec:
            forProvider:
            # Removed for brevity
            writeConnectionSecretToRef:
              namespace: docs
              name: key1

Crossplane saves a secret with the name in the namespace provided.

kubectl get secrets -n docs
NAME   TYPE                                DATA   AGE
key1   connection.crossplane.io/v1alpha1   4      4m30s

{{<hint "tip" >}} Remember to create a unique name for each secret. {{< /hint >}}

External secret stores

Crossplane [External Secret Stores]({{<ref "../guides/vault-as-secret-store" >}}) write secrets and connection details to external secret stores like HashiCorp Vault.

{{<hint "important" >}} External Secret Stores are an alpha feature.

They're not recommended for production use. Crossplane disables External Secret Stores by default. {{< /hint >}}

Use publishConnectionDetailsWithStoreConfigRef in place of writeConnectionSecretsToNamespace to define the StoreConfig to save connection details to.

For example, using a StoreConfig with the name "vault," use publishConnectionDetailsWithStoreConfigRef.name matching the StoreConfig.name, in this example, "vault."

apiVersion: gcp.crossplane.io/v1alpha1
kind: StoreConfig
metadata:
  name: vault
# Removed for brevity.
---
apiVersion: apiextensions.crossplane.io/v1
kind: Composition
# Removed for Brevity
spec:
  publishConnectionDetailsWithStoreConfigRef:
    name: vault
  # Removed for brevity

For more details read the [External Secret Stores]({{<ref "../guides/vault-as-secret-store" >}}) integration guide.

Test a composition

You can preview the output of any composition 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 "important">}} The crossplane render command only supports composition functions. It doesn't support mode: Resources Compositions. {{< /hint >}}

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

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

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

crossplane render xr.yaml composition.yaml functions.yaml

crossplane 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 by running crossplane 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/v1
kind: Function
metadata:
  name: function-patch-and-transform
spec:
  package: xpkg.crossplane.io/crossplane-contrib/function-patch-and-transform:v0.8.2

{{}}

The Crossplane CLI uses Docker Engine to run functions. You can change how the Crossplane CLI runs 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 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/v1
kind: Function
metadata:
  name: function-patch-and-transform
  annotations:
    render.crossplane.io/runtime: Development
spec:
  package: xpkg.crossplane.io/crossplane-contrib/function-patch-and-transform:v0.8.2

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

crossplane 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.

Verify a Composition

View all available Compositions with kubectl get composition.

kubectl get composition
NAME                                       XR-KIND        XR-APIVERSION                         AGE
xapps.aws.platformref.upbound.io           XApp           aws.platformref.upbound.io/v1alpha1   123m
xclusters.aws.platformref.upbound.io       XCluster       aws.platformref.upbound.io/v1alpha1   123m
xeks.aws.platformref.upbound.io            XEKS           aws.platformref.upbound.io/v1alpha1   123m
xnetworks.aws.platformref.upbound.io       XNetwork       aws.platformref.upbound.io/v1alpha1   123m
xservices.aws.platformref.upbound.io       XServices      aws.platformref.upbound.io/v1alpha1   123m
xsqlinstances.aws.platformref.upbound.io   XSQLInstance   aws.platformref.upbound.io/v1alpha1   123m

The XR-KIND lists the Composite Resource kind that's allowed to use the Composition template. The XR-APIVERSION lists the Composite Resource API versions allowed to use the Composition template.

{{<hint "note" >}} The output of kubectl get composition is different than kubectl get composite.

kubectl get composition lists all available Compositions.

kubectl get composite lists all created Composite Resources and their related Composition. {{< /hint >}}

Composition validation

When creating a Composition, Crossplane automatically validates its integrity, checking that the Composition is well formed, for example:

If using mode: Resources:

• The resources field isn't empty.
• All resources either use a name or don't. Compositions can't use both named and unnamed resources.
• No duplicate resource names.
• Patch sets must have names.
• Patches that require a fromFieldPath value provide it.
• Patches that require a toFieldPath value provide it.
• Patches that require a combine field provide it.
• Readiness checks using matchString aren't empty.
• Readiness checks using matchInteger isn't 0.
• Readiness checks requiring a fieldPath value provide it.

If using mode: Pipeline (Composition Functions):

• The pipeline field isn't empty.
• No duplicate step names.

Composition schema aware validation

Crossplane also performs schema aware validation of Compositions. Schema validation checks that patches, readinessChecks and connectionDetails are valid according to the resource schemas. For example, checking that the source and destination fields of a patch are valid according to the source and destination resource schema.

{{<hint "note" >}} Composition schema aware validation is a beta feature. Crossplane enables beta features by default.

Disable schema aware validation by setting the --enable-composition-webhook-schema-validation=false flag on the Crossplane pod.

The [Crossplane Pods]({{<ref "./pods#edit-the-deployment">}}) page has more information on enabling Crossplane flags. {{< /hint >}}

Schema aware validation modes

Crossplane always rejects Compositions in case of integrity errors.

Set the schema aware validation mode to configure how Crossplane handles both missing resource schemas and schema aware validation errors.

{{<hint "note" >}} If a resource schema is missing, Crossplane skips schema aware validation but still returns an error for integrity errors and a warning or an error for the missing schemas. {{< /hint >}}

The following modes are available:

{{< table "table table-sm table-striped" >}}

Mode	Missing Schema	Schema Aware Error	Integrity Error
warn	Warning	Warning	Error
loose	Warning	Error	Error
strict	Error	Error	Error
{{< /table >}}

Change the validation mode for a Composition with the {{}}crossplane.io/composition-schema-aware-validation-mode{{}} annotation.

If not specified, the default mode is warn.

For example, to enable loose mode checking set the annotation value to {{}}loose{{}}.

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
metadata:
  annotations:
    crossplane.io/composition-schema-aware-validation-mode: loose
  # Removed for brevity
spec:
  # Removed for brevity

{{<hint "important" >}} Validation modes also apply to Compositions defined by Configuration packages.

Depending on the mode configured in the Composition, schema aware validation issues may result in warnings or the rejection of the Composition.

View the Crossplane logs for validation warnings.

Crossplane sets a Configuration as unhealthy if there are validation errors. View the Configuration details with kubectl describe configuration to see the specific errors. {{< /hint >}}

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 *fnv1.RunFunctionRequest) (*fnv1.RunFunctionResponse, error) {
        rsp := response.To(req, response.DefaultTTL)
        response.Normal(rsp, "Hello world!")
        return rsp, nil
}

Crossplane has [language specific guides]({{<ref "../guides">}}) to writing a composition function. Refer to the guide for your preferred language to learn how to write a composition 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
    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.

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

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.

{{<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 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.