Composite Resources

This document is for an older version of Crossplane.

This document applies to Crossplane version v1.11 and not to the latest release v1.13.

Crossplane Composite Resources are opinionated Kubernetes Custom Resources that are composed of Managed Resources. We often call them XRs for short.

Diagram of claims, XRs, and Managed Resources

Composite Resources are designed to let you build your own platform with your own opinionated concepts and APIs without needing to write a Kubernetes controller from scratch. Instead, you define the schema of your XR and teach Crossplane which Managed Resources it should compose (i.e. create) when someone creates the XR you defined.

If you’re already familiar with Composite Resources and looking for a detailed configuration reference or some tips, tricks, and troubleshooting information, try the Composition Reference.

Below is an example of a Composite Resource:

 1apiVersion: database.example.org/v1alpha1
 2kind: XPostgreSQLInstance
 3metadata:
 4  name: my-db
 5spec:
 6  parameters:
 7    storageGB: 20
 8  compositionRef:
 9    name: production
10  writeConnectionSecretToRef:
11    namespace: crossplane-system
12    name: my-db-connection-details

You define your own XRs, so they can be of whatever API version and kind you like, and contain whatever spec and status fields you need.

How It Works

The first step towards using Composite Resources is configuring Crossplane so that it knows what XRs you’d like to exist, and what to do when someone creates one of those XRs. This is done using a CompositeResourceDefinition (XRD) resource and one or more Composition resources.

Once you’ve configured Crossplane with the details of your new XR you can either create one directly, or use a claim. Typically only the folks responsible for configuring Crossplane (often a platform or SRE team) have permission to create XRs directly. Everyone else manages XRs via a lightweight proxy resource called a Composite Resource Claim (or claim for short). More on that later.

Diagram combining all Composition concepts

If you’re coming from the Terraform world you can think of an XRD as similar to the variable blocks of a Terraform module, while the Composition is the rest of the module’s HCL code that describes how to use those variables to create a bunch of resources. In this analogy the XR or claim is a little like a tfvars file providing inputs to the module.

Defining Composite Resources

A CompositeResourceDefinition (or XRD) defines the type and schema of your XR. It lets Crossplane know that you want a particular kind of XR to exist, and what fields that XR should have. An XRD is a little like a CustomResourceDefinition (CRD), but slightly more opinionated. Writing an XRD is mostly a matter of specifying an OpenAPI “structural schema”.

The XRD that defines the XPostgreSQLInstance XR above would look like this:

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: CompositeResourceDefinition
 3metadata:
 4  name: xpostgresqlinstances.database.example.org
 5spec:
 6  group: database.example.org
 7  names:
 8    kind: XPostgreSQLInstance
 9    plural: xpostgresqlinstances
10  claimNames:
11    kind: PostgreSQLInstance
12    plural: postgresqlinstances
13  versions:
14  - name: v1alpha1
15    served: true
16    referenceable: true
17    schema:
18      openAPIV3Schema:
19        type: object
20        properties:
21          spec:
22            type: object
23            properties:
24              parameters:
25                type: object
26                properties:
27                  storageGB:
28                    type: integer
29                required:
30                - storageGB
31            required:
32            - parameters

You might notice that the XPostgreSQLInstance example above has some fields that don’t appear in the XRD, like the writeConnectionSecretToRef and compositionRef fields. This is because Crossplane automatically injects some standard Crossplane Resource Model (XRM) fields into all XRs.

Configuring Composition

A Composition lets Crossplane know what to do when someone creates a Composite Resource. Each Composition creates a link between an XR and a set of one or more Managed Resources - when the XR is created, updated, or deleted the set of Managed Resources are created, updated or deleted accordingly.

You can add multiple Compositions for each XRD, and choose which should be used when XRs are created. This allows a Composition to act like a class of service - for example you could configure one Composition for each environment you support, such as production, staging, and development.

A basic Composition for the above XPostgreSQLInstance might look like this:

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: Composition
 3metadata:
 4  name: example
 5  labels:
 6    crossplane.io/xrd: xpostgresqlinstances.database.example.org
 7    provider: gcp
 8spec:
 9  writeConnectionSecretsToNamespace: crossplane-system
10  compositeTypeRef:
11    apiVersion: database.example.org/v1alpha1
12    kind: XPostgreSQLInstance
13  resources:
14  - name: cloudsqlinstance
15    base:
16      apiVersion: database.gcp.crossplane.io/v1beta1
17      kind: CloudSQLInstance
18      spec:
19        forProvider:
20          databaseVersion: POSTGRES_12
21          region: us-central1
22          settings:
23            tier: db-custom-1-3840
24            dataDiskType: PD_SSD
25            ipConfiguration:
26              ipv4Enabled: true
27              authorizedNetworks:
28                - value: "0.0.0.0/0"
29    patches:
30    - type: FromCompositeFieldPath
31      fromFieldPath: spec.parameters.storageGB
32      toFieldPath: spec.forProvider.settings.dataDiskSizeGb

The above Composition tells Crossplane that when someone creates an XPostgreSQLInstance XR Crossplane should create a CloudSQLInstance in response. The storageGB field of the XPostgreSQLInstance should be used to configure the dataDiskSizeGb field of the CloudSQLInstance. This is only a small subset of the functionality a Composition enables - take a look at the reference page to learn more.

We almost always talk about XRs composing Managed Resources, but actually an XR can also compose other XRs to allow nested layers of abstraction. XRs don’t support composing arbitrary Kubernetes resources (e.g. Deployments, operators, etc) directly but you can do so using our Kubernetes and Helm providers.

Claiming Composite Resources

Crossplane uses Composite Resource Claims (or just claims, for short) to allow application operators to provision and manage XRs. When we talk about using XRs it’s typically implied that the XR is being used via a claim. Claims are almost identical to their corresponding XRs. It helps to think of a claim as an application team’s interface to an XR. You could also think of claims as the public (app team) facing part of the opinionated platform API, while XRs are the private (platform team) facing part.

A claim for the XPostgreSQLInstance XR above would look like this:

 1apiVersion: database.example.org/v1alpha1
 2kind: PostgreSQLInstance
 3metadata:
 4  namespace: default
 5  name: my-db
 6spec:
 7  parameters:
 8    storageGB: 20
 9  compositionRef:
10    name: production
11  writeConnectionSecretToRef:
12    name: my-db-connection-details

There are three key differences between an XR and a claim:

Claims are namespaced, while XRs (and Managed Resources) are cluster scoped.
Claims are of a different kind than the XR - by convention the XR’s kind without the proceeding X. For example a PostgreSQLInstance claims an XPostgreSQLInstance.
An active claim contains a reference to its corresponding XR, while an XR contains both a reference to the claim an array of references to the managed resources it composes.

Not all XRs offer a claim - doing so is optional. See the XRD section of the Composition reference to learn how to offer a claim.

Diagram showing the relationship between claims and XRs

Claims may seem a little superfluous at first, but they enable some handy scenarios, including:

Private XRs. Sometimes a platform team might not want a type of XR to be directly consumed by their application teams. For example because the XR represents ‘supporting’ infrastructure - consider the above VPC XNetwork XR. App teams might create PostgreSQLInstance claims that reference (i.e. consume) an XNetwork, but they shouldn’t be creating their own. Similarly, some kinds of XR might be intended only for ’nested’ use - intended only to be composed by other XRs.
Global XRs. Not all infrastructure is conceptually namespaced. Say your organisation uses team scoped namespaces. A PostgreSQLInstance that belongs to Team A should probably be part of the team-a namespace - you’d represent this by creating a PostgreSQLInstance claim in that namespace. On the other hand the XNetwork XR we mentioned previously could be referenced (i.e. used) by XRs from many different namespaces - it doesn’t exist to serve a particular team.
Pre-provisioned XRs. Finally, separating claims from XRs allows a platform team to pre-provision certain kinds of XR. Typically an XR is created on-demand in response to the creation of a claim, but it’s also possible for a claim to instead request an existing XR. This can allow application teams to instantly claim infrastructure like database instances that would otherwise take minutes to provision on-demand.

This reference provides detailed examples of defining, configuring, and using Composite Resources in Crossplane. You can also refer to Crossplane’s [API documentation][api-docs] for more details. If you’re looking for a more general overview of Composite Resources and Composition in Crossplane, try the [Composite Resources][xr-concepts] page under Concepts.

Composite Resources and Claims

The type and most of the schema of Composite Resources and claims are largely of your own choosing, but there is some common ‘machinery’ injected into them. Here’s a hypothetical XR that doesn’t have any user-defined fields and thus only includes the automatically injected Crossplane machinery:

 1apiVersion: database.example.org/v1alpha1
 2kind: XPostgreSQLInstance
 3metadata:
 4  # This XR was created automatically by a claim, so its name is derived from
 5  # the claim's name.
 6  name: my-db-mfd1b
 7  annotations:
 8    # The external name annotation has special meaning in Crossplane. When a
 9    # claim creates an XR its external name will automatically be propagated to
10    # the XR. Whether and how the external name is propagated to the resources
11    # the XR composes is up to its Composition.
12    crossplane.io/external-name: production-db-0
13spec:
14  # XRs have a reference to the claim that created them (or, if the XR was
15  # pre-provisioned, to the claim that later claimed them).
16  claimRef:
17    apiVersion: database.example.org/v1alpha1
18    kind: PostgreSQLInstance
19    name: my-db
20  # The compositeDeletePolicy specifies the propagation policy that will be used by Crossplane
21  # when deleting the Composite Resource that is associated with the Claim.  The default
22  # value is Background, which causes the Composite resource to be deleted using
23  # the kubernetes default propagation policy of Background, and all associated
24  # resources will be deleted simultaneously.  The other value for this field is Foreground,
25  # which will cause the Composite resource to be deleted using Foreground Cascading Deletion.
26  # Kubernetes will add a foregroundDeletion finalizer to all of the resources in the
27  # dependency graph, and they will be deleted starting with the edge or leaf nodes and
28  # working back towards the root Composite.  See https://kubernetes.io/docs/concepts/architecture/garbage-collection/#cascading-deletion
29  # for more information on cascading deletion.
30  compositeDeletePolicy: Background
31  # The compositionRef specifies which Composition this XR will use to compose
32  # resources when it is created, updated, or deleted. This can be omitted and
33  # will be set automatically if the XRD has a default or enforced composition
34  # reference, or if the below composition selector is set.
35  compositionRef:
36    name: production-us-east
37  # The compositionSelector allows you to match a Composition by labels rather
38  # than naming one explicitly. It is used to set the compositionRef if none is
39  # specified explicitly.
40  compositionSelector:
41    matchLabels:
42      environment: production
43      region: us-east
44      provider: gcp
45  # The environment is an in-memory object that can be patched from / to during
46  # rendering.
47  # The environment is composed by merging the 'data' of all EnvironmentConfigs
48  # referenced below. It is disposed after every reconcile.
49  # NOTE: EnvironmentConfigs are an alpha feature and need to be enabled with
50  #       the '--enable-environment-configs' flag on startup.
51  environment:
52    # EnvironmentConfigs is a list of object references that is made up of
53    # name references and label selectors
54    environmentConfigs:
55      - type: Reference # this is the default
56        ref:
57          name: example-environment
58      - type: Selector
59        selector:
60          matchLabels:
61          - key: stage
62            type: FromCompositeFieldPath # this is the default
63            valueFromFieldPath: spec.parameters.stage
64          - key: provider
65            type: Value
66            value: "gcp"
67  # The resourceRefs array contains references to all of the resources of which
68  # this XR is composed. Despite being in spec this field isn't intended to be
69  # configured by humans - Crossplane will take care of keeping it updated.
70  resourceRefs:
71  - apiVersion: database.gcp.crossplane.io/v1beta1
72    kind: CloudSQLInstance
73    name: my-db-mfd1b-md9ab
74  # The writeConnectionSecretToRef field specifies a Kubernetes Secret that this
75  # XR should write its connection details (if any) to.
76  writeConnectionSecretToRef:
77    namespace: crossplane-system
78    name: my-db-connection-details
79status:
80  # An XR's 'Ready' condition will become True when all of the resources it
81  # composes are deemed ready. Refer to the Composition 'readinessChecks' field
82  # for more information.
83  conditions:
84  - type: Ready
85    statue: "True"
86    reason: Available
87    lastTransitionTime: 2021-10-02T07:20:50.52Z
88  # The last time the XR published its connection details to a Secret.
89  connectionDetails:
90    lastPublishedTime: 2021-10-02T07:20:51.24Z

Similarly, here’s an example of the claim that corresponds to the above XR:

 1apiVersion: database.example.org/v1alpha1
 2kind: PostgreSQLInstance
 3metadata:
 4  # Claims are namespaced, unlike XRs.
 5  namespace: default
 6  name: my-db
 7  annotations:
 8    # The external name annotation has special meaning in Crossplane. When a
 9    # claim creates an XR its external name will automatically be propagated to
10    # the XR. Whether and how the external name is propagated to the resources
11    # the XR composes is up to its Composition.
12    crossplane.io/external-name: production-db-0
13spec:
14  # The resourceRef field references the XR this claim corresponds to. You can
15  # either set it to an existing (compatible) XR that you'd like to claim or
16  # (the more common approach) leave it blank and let Crossplane automatically
17  # create and reference an XR for you.
18  resourceRef:
19    apiVersion: database.example.org/v1alpha1
20    kind: XPostgreSQLInstance
21    name: my-db-mfd1b
22  # A claim's compositionRef and compositionSelector work the same way as an XR.
23  compositionRef:
24    name: production-us-east
25  compositionSelector:
26    matchLabels:
27      environment: production
28      region: us-east
29      provider: gcp
30  # A claim's writeConnectionSecretToRef mostly works the same way as an XR's.
31  # The one difference is that the Secret is always written to the namespace of
32  # the claim.
33  writeConnectionSecretToRef:
34    name: my-db-connection-details
35status:
36  # A claim's 'Ready' condition will become True when its XR's 'Ready' condition
37  # becomes True.
38  conditions:
39  - type: Ready
40    statue: "True"
41    reason: Available
42    lastTransitionTime: 2021-10-02T07:20:50.52Z
43  # The last time the claim published its connection details to a Secret.
44  connectionDetails:
45    lastPublishedTime: 2021-10-02T07:20:51.24Z

If your XR or claim isn’t working as you’d expect you can try running kubectl describe against it for details - pay particular attention to any events and status conditions. You may need to follow the references from claim to XR to composed resources to find out what’s happening.

CompositeResourceDefinitions

Below is an example CompositeResourceDefinition that includes all configurable fields.

 1apiVersion: apiextensions.crossplane.io/v1
 2kind: CompositeResourceDefinition
 3metadata:
 4  # XRDs must be named '<plural>.<group>', per the plural and group names below.
 5  name: xpostgresqlinstances.example.org
 6spec:
 7  # This XRD defines an XR in the 'example.org' API group.
 8  group: example.org
 9  # The kind of this XR will be 'XPostgreSQLInstance`. You may also optionally
10  # specify a singular name and a listKind.
11  names:
12    kind: XPostgreSQLInstance
13    plural: xpostgresqlinstances
14  # This type of XR offers a claim. Omit claimNames if you don't want to do so.
15  # The claimNames must be different from the names above - a common convention
16  # is that names are prefixed with 'X' while claim names are not. This lets app
17  # team members think of creating a claim as (e.g.) 'creating a
18  # PostgreSQLInstance'.
19  claimNames:
20    kind: PostgreSQLInstance
21    plural: postgresqlinstances
22  # Each type of XR can declare any keys they write to their connection secret
23  # which will act as a filter during aggregation of the connection secret from
24  # composed resources. It's recommended to provide the set of keys here so that
25  # consumers of claims and XRs can see what to expect in the connection secret.
26  # If no key is given, then all keys in the aggregated connection secret will
27  # be written to the connection secret of the XR.
28  connectionSecretKeys:
29  - hostname
30  # Each type of XR may specify a default Composition to be used when none is
31  # specified (e.g. when the XR has no compositionRef or selector). A similar
32  # enforceCompositionRef field also exists to allow XRs to enforce a specific
33  # Composition that should always be used.
34  defaultCompositionRef:
35    name: example
36  # Each type of XR may be served at different versions - e.g. v1alpha1, v1beta1
37  # and v1 - simultaneously. Currently Crossplane requires that all versions
38  # have an identical schema, so this is mostly useful to 'promote' a type of XR
39  # from alpha to beta to production ready.
40  versions:
41  - name: v1alpha1
42    # Served specifies that XRs should be served at this version. It can be set
43    # to false to temporarily disable a version, for example to test whether
44    # doing so breaks anything before a version is removed wholesale.
45    served: true
46    # Referenceable denotes the version of a type of XR that Compositions may
47    # use. Only one version may be referenceable.
48    referenceable: true
49    # Schema is an OpenAPI schema just like the one used by Kubernetes CRDs. It
50    # determines what fields your XR and claim will have. Note that Crossplane
51    # will automatically extend with some additional Crossplane machinery.
52    schema:
53      openAPIV3Schema:
54        type: object
55        properties:
56          spec:
57            type: object
58            properties:
59              parameters:
60                type: object
61                properties:
62                  storageGB:
63                    type: integer
64                required:
65                - storageGB
66            required:
67            - parameters
68          status:
69            type: object
70            properties:
71              address:
72                description: Address of this MySQL server.
73                type: string

Take a look at the Kubernetes CRD documentation for a more detailed guide to writing OpenAPI schemas. Note that the following fields are reserved for Crossplane machinery, and will be ignored if your schema includes them:

spec.resourceRef
spec.resourceRefs
spec.claimRef
spec.writeConnectionSecretToRef
status.conditions
status.connectionDetails

If your CompositeResourceDefinition isn’t working as you’d expect you can try running kubectl describe xrd for details - pay particular attention to any events and status conditions.

Compositions

You’ll encounter a lot of ‘field paths’ when reading or writing a Composition. Field paths reference a field within a Kubernetes object via a simple string ‘path’. API conventions describe the syntax as:

Standard JavaScript syntax for accessing that field, assuming the JSON object was transformed into a JavaScript object, without the leading dot, such as metadata.name.

Valid field paths include:

metadata.name - The name field of the metadata object.
spec.containers[0].name - The name field of the 0th containers element.
data[.config.yml] - The .config.yml field of the data object.
apiVersion - The apiVersion field of the root object.

While the following are invalid:

.metadata.name - Leading period.
metadata..name - Double period.
metadata.name. - Trailing period.
spec.containers[] - Empty brackets.
spec.containers.[0].name - Period before open bracket.

Below is a detailed example of a Composition. While detailed, this example doesn’t include every patch, transform, connection detail, and readiness check type. Keep reading below to discover those.

  1apiVersion: apiextensions.crossplane.io/v1
  2kind: Composition
  3metadata:
  4  name: example
  5  labels:
  6    # An optional convention is to include a label of the XRD. This allows
  7    # easy discovery of compatible Compositions.
  8    crossplane.io/xrd: xpostgresqlinstances.database.example.org
  9    # The following label marks this Composition for GCP. This label can 
 10    # be used in 'compositionSelector' in an XR or Claim.
 11    provider: gcp
 12spec:
 13
 14  # Each Composition must declare that it is compatible with a particular type
 15  # of Composite Resource using its 'compositeTypeRef' field. The referenced
 16  # version must be marked 'referenceable' in the XRD that defines the XR.
 17  compositeTypeRef:
 18    apiVersion: database.example.org/v1alpha1
 19    kind: XPostgreSQLInstance
 20
 21  # When an XR is created in response to a claim Crossplane needs to know where
 22  # it should create the XR's connection secret. This is configured using the
 23  # 'writeConnectionSecretsToNamespace' field.
 24  writeConnectionSecretsToNamespace: crossplane-system
 25
 26  # Each Composition must specify at least one composed resource template. In
 27  # this case the Composition tells Crossplane that it should create, update, or
 28  # delete a CloudSQLInstance whenever someone creates, updates, or deletes an
 29  # XPostgresSQLInstance.
 30  resources:
 31
 32    # It's good practice to provide a unique name for each entry. Note that
 33    # this identifies the resources entry within the Composition - it's not
 34    # the name the CloudSQLInstance. The 'name' field will be required in a
 35    # future version of this API.
 36  - name: cloudsqlinstance
 37
 38    # The 'base' template for the CloudSQLInstance Crossplane will create.
 39    # You can use the base template to specify fields that never change, or
 40    # default values for fields that may optionally be patched over. Bases must
 41    # be a valid Crossplane resource - a Managed Resource, Composite Resource,
 42    # or a ProviderConfig.
 43    base:
 44      apiVersion: database.gcp.crossplane.io/v1beta1
 45      kind: CloudSQLInstance
 46      spec:
 47        forProvider:
 48          databaseVersion: POSTGRES_12
 49          region: us-central1
 50          settings:
 51            dataDiskType: PD_SSD
 52            ipConfiguration:
 53              ipv4Enabled: true
 54              authorizedNetworks:
 55                - value: "0.0.0.0/0"
 56      
 57    # Each resource can optionally specify a set of 'patches' that copy fields
 58    # from (or to) the XR.
 59    patches:
 60      # FromCompositeFieldPath is the default when 'type' is omitted, but it's
 61      # good practice to always include the type for readability.
 62    - type: FromCompositeFieldPath
 63      fromFieldPath: spec.parameters.size
 64      toFieldPath: spec.forProvider.settings.tier
 65
 66      # Each patch can optionally specify one or more 'transforms', which
 67      # transform the 'from' field's value before applying it to the 'to' field.
 68      # Transforms are applied in the order they are specified; each transform's
 69      # output is passed to the following transform's input.
 70      transforms:
 71      - type: map
 72        map:
 73          medium: db-custom-1-3840
 74
 75      policy:
 76        # By default a patch from a field path that does not exist is simply
 77        # skipped until it does. Use the 'Required' policy to instead block and
 78        # return an error when the field path does not exist.
 79        fromFieldPath: Required
 80
 81        # You can patch entire objects or arrays from one resource to another.
 82        # By default the 'to' object or array will be overwritten, not merged.
 83        # Use the 'mergeOptions' field to override this behaviour. Note that
 84        # these fields accidentally leak Go terminology - 'slice' means 'array'.
 85        # 'map' means 'map' in YAML or 'object' in JSON.
 86        mergeOptions:
 87          appendSlice: true
 88          keepMapValues: true
 89    
 90    # You can include connection details to propagate from this CloudSQLInstance
 91    # up to the XPostgreSQLInstance XR (and then on to the PostgreSQLInstance
 92    # claim). Remember that your XRD must declare which connection secret keys
 93    # it supports.
 94    connectionDetails:
 95    - name: hostname
 96      fromConnectionSecretKey: hostname
 97    
 98    # By default an XR's 'Ready' status condition will become True when the
 99    # 'Ready' status conditions of all of its composed resources become true.
100    # You can optionally specify custom readiness checks to override this.
101    readinessChecks:
102    - type: None
103
104    
105  # If you find yourself repeating patches a lot you can group them as a named
106  # 'patch set' then use a PatchSet type patch to reference them.
107  patchSets:
108  - name: metadata
109    patches:
110    - type: FromCompositeFieldPath
111      # When both field paths are the same you can omit the 'toFieldPath' and it
112      # will default to the 'fromFieldPath'.
113      fromFieldPath: metadata.labels[some-important-label]

Pause Annotation

There is an annotation named crossplane.io/paused that you can use on Composite Resources and Composite Resource Claims to temporarily pause reconciliations of their respective controllers on them. An example for a Composite Resource Claim is as follows:

 1apiVersion: test.com/v1alpha1
 2kind: MyResource
 3metadata:
 4  annotations:
 5     crossplane.io/paused: "true"
 6  namespace: upbound-system
 7  name: my-resource
 8spec:
 9  parameters:
10    tagValue: demo-test
11  compositionRef:
12    name: example

where MyResource is a Composite Resource Claim kind. When a Composite Resource or a Claim has the crossplane.io/paused annotation with its value set to true, the Composite Resource controller or the Claim controller pauses reconciliations on the resource until the annotation is removed or its value set to something other than true. Before temporarily pausing reconciliations, an event with the type Synced, the status False, and the reason ReconcilePaused is emitted on the resource. Please also note that annotations on a Composite Resource Claim are propagated to the associated Composite Resource but when the crossplane.io/paused: "true" annotation is added to a Claim, because reconciliations on the Claim are now paused, this newly added annotation will not be propagated. However, whenever the annotation’s value is set to a non-true value, reconciliations on the Claim will now resume, and thus the annotation will now be propagated to the associated Composite Resource with a non-true value. An implication of the described behavior is that pausing reconciliations on the Claim will not inherently pause reconciliations on the associated Composite Resource.

Patch Types

You can use the following types of patch in a Composition:

FromCompositeFieldPath. The default if the type is omitted. This type patches from a field within the XR to a field within the composed resource. It’s commonly used to expose a composed resource spec field as an XR spec field.

1# Patch from the XR's spec.parameters.size field to the composed resource's
2# spec.forProvider.settings.tier field.
3- type: FromCompositeFieldPath
4  fromFieldPath: spec.parameters.size
5  toFieldPath: spec.forProvider.settings.tier

ToCompositeFieldPath. The inverse of FromCompositeFieldPath. This type patches from a field within the composed resource to a field within the XR. It’s commonly used to derive an XR status field from a composed resource status field.

1# Patch from the composed resource's status.atProvider.zone field to the XR's
2# status.zone field.
3- type: ToCompositeFieldPath
4  fromFieldPath: status.atProvider.zone
5  toFieldPath: status.zone

FromCompositeFieldPath and ToCompositeFieldPath patches can also take a wildcarded field path in the toFieldPath parameter and patch each array element in the toFieldPath with the singular value provided in the fromFieldPath.

 1# Patch from the XR's spec.parameters.allowedIPs to the CIDRBlock elements
 2# inside the array spec.forProvider.firewallRules on the composed resource.
 3resources:
 4- name: exampleFirewall
 5  base:
 6    apiVersion: firewall.example.crossplane.io/v1beta1
 7    kind: Firewall
 8    spec:
 9      forProvider:
10        firewallRules:
11        - Action: "Allow"
12          Destination: "example1"
13          CIDRBlock: ""
14        - Action: "Allow"
15          Destination: "example2"
16          CIDRBlock: ""
17- type: FromCompositeFieldPath
18  fromFieldPath: spec.parameters.allowedIP
19  toFieldPath: spec.forProvider.firewallRules[*].CIDRBlock

FromEnvironmentFieldPath. This type patches from a field within the in-memory environment to a field within the composed resource. It’s commonly used to expose a composed resource spec field as an XR spec field. Note that EnvironmentConfigs are an alpha feature and need to be enabled with the --enable-environment-configs flag on startup.

1# Patch from the environment's tier.name field to the composed resource's
2# spec.forProvider.settings.tier field.
3- type: FromEnvironmentFieldPath
4  fromFieldPath: tier.name
5  toFieldPath: spec.forProvider.settings.tier

ToEnvironmentFieldPath. This type patches from a composed field to the in-memory environment. Note that, unlike ToCompositeFieldPath patches, this is executed before the composed resource is applied on the cluster which means that the status is not available. Note that EnvironmentConfigs are an alpha feature and need to be enabled with the --enable-environment-configs flag on startup.

1# Patch from the environment's tier.name field to the composed resource's
2# spec.forProvider.settings.tier field.
3- type: ToEnvironmentFieldPath
4  fromFieldPath: spec.forProvider.settings.tier
5  toFieldPath: tier.name

Note that the field to be patched requires some initial value to be set.

CombineFromComposite. Combines multiple fields from the XR to produce one composed resource field.

 1# Patch from the XR's spec.parameters.location field and the
 2# metadata.labels[crossplane.io/claim-name] label to the composed
 3# resource's spec.forProvider.administratorLogin field.
 4- type: CombineFromComposite
 5  combine:
 6    # The patch will only be applied when all variables have non-zero values.
 7    variables:
 8    - fromFieldPath: spec.parameters.location
 9    - fromFieldPath: metadata.labels[crossplane.io/claim-name]
10    strategy: string
11    string:
12      fmt: "%s-%s"
13  toFieldPath: spec.forProvider.administratorLogin
14  # By default Crossplane will skip the patch until all of the variables to be
15  # combined have values. Set the fromFieldPath policy to 'Required' to instead
16  # abort composition and return an error if a variable has no value.
17  policy:
18    fromFieldPath: Required

CombineFromEnvironment. Combines multiple fields from the in-memory environment to produce one composed resource field. Note that EnvironmentConfigs are an alpha feature and need to be enabled with the --enable-environment-configs flag on startup.

 1# Patch from the environments's location field and region to the composed
 2# resource's spec.forProvider.administratorLogin field.
 3- type: CombineFromEnvironment
 4  combine:
 5    # The patch will only be applied when all variables have non-zero values.
 6    variables:
 7    - fromFieldPath: location
 8    - fromFieldPath: region
 9    strategy: string
10    string:
11      fmt: "%s-%s"
12  toFieldPath: spec.forProvider.administratorLogin

At the time of writing only the string combine strategy is supported. It uses Go string formatting to combine values, so if the XR’s location was us-west and its claim name was db the composed resource’s administratorLogin would be set to us-west-db.

CombineToComposite is the inverse of CombineFromComposite.

 1# Patch from the composed resource's spec.parameters.administratorLogin and
 2# status.atProvider.fullyQualifiedDomainName fields back to the XR's
 3# status.adminDSN field.
 4- type: CombineToComposite
 5  combine:
 6    variables:
 7      - fromFieldPath: spec.parameters.administratorLogin
 8      - fromFieldPath: status.atProvider.fullyQualifiedDomainName
 9    strategy: string
10    # Here, our administratorLogin parameter and fullyQualifiedDomainName
11    # status are formatted to a single output string representing a DSN.
12    string:
13      fmt: "mysql://%s@%s:3306/my-database-name"
14  toFieldPath: status.adminDSN

CombineToEnvironment is the inverse of CombineFromEnvironment. Note that EnvironmentConfigs are an alpha feature and need to be enabled with the --enable-environment-configs flag on startup.

 1# Patch from the composed resource's spec.parameters.administratorLogin and
 2# spec.forProvider.domainName fields back to the environment's adminDSN field.
 3- type: CombineToEnvironment
 4  combine:
 5    variables:
 6      - fromFieldPath: spec.parameters.administratorLogin
 7      - fromFieldPath: spec.forProvider.domainName
 8    strategy: string
 9    # Here, our administratorLogin parameter and fullyQualifiedDomainName
10    # status are formatted to a single output string representing a DSN.
11    string:
12      fmt: "mysql://%s@%s:3306/my-database-name"
13  toFieldPath: adminDSN

PatchSet. References a named set of patches defined in the spec.patchSets array of a Composition.

1# This is equivalent to specifying all of the patches included in the 'metadata'
2# PatchSet.
3- type: PatchSet
4  patchSetName: metadata

The patchSets array may not contain patches of type: PatchSet. The transforms and patchPolicy fields are ignored by type: PatchSet.

Transform Types

You can use the following types of transform on a value being patched:

map. Transforms values using a map.

1# If the value of the 'from' field is 'us-west', the value of the 'to' field
2# will be set to 'West US'.
3- type: map
4  map:
5    us-west: West US
6    us-east: East US
7    au-east: Australia East

match. A more complex version of map that can match different kinds of patterns. It should be used if more advanced pattern matchings than a simple string equality check are required. The result of the first matching pattern is used as the output of this transform.

 1- type: match
 2  match:
 3    patterns:
 4      - type: literal # Not needed. This is the default.
 5        literal: us-west
 6        result: West US
 7      - type: regexp
 8        regexp: '^af-.*'
 9        result: Somewhere in Africa
10    fallbackValue: Unknown

math. Transforms values using math. The input value must be an integer. Currently only multiply is supported.

1# If the value of the 'from' field is 2, the value of the 'to' field will be set
2# to 4.
3- type: math
4  math:
5    multiply: 2

string. Transforms string values.

string transform type Format, Currently only Go style fmt is supported. Go style fmt is supported.
string transform type Convert, accepts one of ToUpper, ToLower, ToBase64, FromBase64, ToJson, ToSha1, ToSha256, ToSha512.
string transform type TrimPrefix, accepts a string to be trimmed from the beginning of the input.
string transform type TrimSuffix, accepts a string to be trimmed from the end of the input.
string transform type Regexp, accepts a string for regexp to be applied to.

 1# If you omit the field type, by default type is set to `Format` 
 2# If the value of the 'from' field is 'hello', the value of the 'to' field will
 3# be set to 'hello-world'.
 4- type: string
 5  string:
 6    fmt: "%s-world"
 7
 8# This is the same as above
 9# the value of the 'to' field will be set to 'hello-world'.
10- type: string
11  string:
12    type: Format
13    fmt: "%s-world"
14
15# If the value of the 'from' field is 'hello', the value of the 'to' field will
16# be set to 'HELLO'.
17- type: string
18  string:
19    type: Convert
20    convert: ToUpper
21
22# If the value of the 'from' field is 'Hello', the value of the 'to' field will
23# be set to 'hello'.
24- type: string
25  string:
26    type: Convert
27    convert: ToLower
28
29# If the value of the 'from' field is 'Hello', the value of the 'to' field will
30# be set to 'SGVsbG8='.
31- type: string
32  string:
33     type: Convert
34     convert: ToBase64
35
36# If the value of the 'from' field is 'SGVsbG8=', the value of the 'to' field will
37# be set to 'Hello'.
38- type: string
39  string:
40     type: Convert
41     convert: FromBase64
42
43# If the value of the 'from' field is not nil, the value of the 'to' field will be
44# set to raw JSON representation of the 'from' field.
45- type: string
46  string:
47    type: Convert
48    convert: ToJson
49
50# The output will be the hash of the JSON representation of the 'from' field.
51- type: string
52  string:
53    type: Convert
54    convert: ToSha1 # alternatives: 'ToSha256' or 'ToSha512'
55
56# If the value of the 'from' field is https://crossplane.io, the value of the 'to' field will
57# be set to crossplane.io
58- type: string
59  string:
60    type: TrimPrefix
61    trim: 'https://'
62
63# If the value of the 'from' field is my-string-test, the value of the 'to' field will
64# be set to my-string
65- type: string
66  string:
67     type: TrimSuffix
68     trim: '-test'
69
70# If the value of the 'from' field is 'arn:aws:iam::42:example, the value of the
71# 'to' field will be set to "42". Note that the 'to' field is always a string. 
72- type: string
73  string:
74     type: Regexp
75     regexp:
76      match: 'arn:aws:iam::(\d+):.*'
77      group: 1  # Optional capture group. Omit to match the entire regexp.

convert. Transforms values of one type to another, for example from a string to an integer. The following values are supported by the from and to fields:

string
bool
int
int64
float64

The strings 1, t, T, TRUE, true, and True are considered ’true’, while 0, f, F, FALSE, false, and False are considered ‘false’. The integer 1 and float 1.0 are considered true, while all other values are considered false. Similarly, boolean true converts to integer 1 and float 1.0, while false converts to 0 and 0.0.

1# If the value to be converted is "1" (a string), the value of the 'toType'
2# field will be set to 1 (an integer).
3- type: convert
4  convert:
5   toType: int

Converting string to float64 additionally supports parsing string in K8s quantity format, such as 1000m or 500 Mi:

1- type: convert
2  convert:
3   toType: float64
4   format: quantity

Connection Details

Connection details secret of XR is an aggregated sum of the connection details of the composed resources. It’s recommended that the author of XRD specify exactly which keys will be allowed in the XR connection secret by listing them in spec.connectionSecretKeys so that consumers of claims and XRs can see what they can expect in the connection details secret.

If spec.connectionSecretKeys is empty, then all keys of the aggregated connection details secret will be propagated.

You can derive the following types of connection details from a composed resource to be aggregated:

FromConnectionSecretKey. Derives an XR connection detail from a connection secret key of a composed resource.

1# Derive the XR's 'user' connection detail from the 'username' key of the
2# composed resource's connection secret.
3- type: FromConnectionSecretKey
4  name: user
5  fromConnectionSecretKey: username

FromFieldPath. Derives an XR connection detail from a field path within the composed resource.

1# Derive the XR's 'user' connection detail from the 'adminUser' status field of
2# the composed resource.
3- type: FromFieldPath
4  name: user
5  fromFieldPath: status.atProvider.adminUser

FromValue. Derives an XR connection detail from a fixed value.

1# Always sets the XR's 'user' connection detail to 'admin'.
2- type: FromValue
3  name: user
4  value: admin

Readiness Checks

Crossplane can use the following types of readiness check to determine whether a composed resource is ready (and therefore whether the XR and claim should be considered ready). Specify multiple readiness checks if multiple conditions must be met for a composed resource to be considered ready.

Note that if you don’t specify any readiness checks Crossplane will consider the composed resource to be ready when its ‘Ready’ status condition becomes ‘True’.

MatchString. Considers the composed resource to be ready when the value of a field within that resource matches a specified string.

1# The composed resource will be considered ready when the 'state' status field
2# matches the string 'Online'.
3- type: MatchString
4  fieldPath: status.atProvider.state
5  matchString: "Online"

MatchInteger. Considers the composed resource to be ready when the value of a field within that resource matches a specified integer.

1# The composed resource will be considered ready when the 'state' status field
2# matches the integer 4.
3- type: MatchInteger
4  fieldPath: status.atProvider.state
5  matchInteger: 4

NonEmpty. Considers the composed resource to be ready when a field exists in the composed resource. The name of this check can be a little confusing in that a field that exists with a zero value (e.g. an empty string or zero integer) is not considered to be ’empty’, and thus will pass the readiness check.

1# The composed resource will be considered ready if and when 'online' status
2# field  exists.
3- type: NonEmpty
4  fieldPath: status.atProvider.online

None. Considers the composed resource to be ready as soon as it exists.

Missing Functionality

You might find while reading through this reference that Crossplane is missing some functionality you need to compose resources. If that’s the case, please raise an issue with as much detail about your use case as possible. Please understand that the Crossplane maintainers are growing the feature set of the Composition type conservatively. We highly value the input of our users and community, but we also feel it’s critical to avoid bloat and complexity. We therefore wish to carefully consider each new addition. We feel some features may be better suited for a real, expressive programming language and intend to build an alternative to the Composition type as it is documented here per this proposal.

Tips, Tricks, and Troubleshooting

In this section we’ll cover some common tips, tricks, and troubleshooting steps for working with Composite Resources. If you’re trying to track down why your Composite Resources aren’t working the [Troubleshooting][trouble-ref] page also has some useful information.

Troubleshooting Claims and XRs

Crossplane relies heavily on status conditions and events for troubleshooting. You can see both using kubectl describe - for example:

1# Describe the PostgreSQLInstance claim named my-db
2kubectl describe postgresqlinstance.database.example.org my-db

Per Kubernetes convention, Crossplane keeps errors close to the place they happen. This means that if your claim is not becoming ready due to an issue with your Composition or with a composed resource you’ll need to “follow the references” to find out why. Your claim will only tell you that the XR is not yet ready.

To follow the references:

Find your XR by running kubectl describe on your claim and looking for its “Resource Ref” (aka spec.resourceRef).
Run kubectl describe on your XR. This is where you’ll find out about issues with the Composition you’re using, if any.
If there are no issues but your XR doesn’t seem to be becoming ready, take a look for the “Resource Refs” (or spec.resourceRefs) to find your composed resources.
Run kubectl describe on each referenced composed resource to determine whether it is ready and what issues, if any, it is encountering.

Composite Resource Connection Secrets

Claim and Composite Resource connection secrets are often derived from the connection secrets of the managed resources they compose. This is a common source of confusion because several things need to align for it to work:

The claim must specify the secret where the aggregated connection details should be written
- This is the spec.writeConnectionSecretToRef field in a claim
- If creating a composite resource directly (without a claim) then this same field must be set on your composite resource instead
The composite resource definition must state which connection details to aggregate from its children to publish to the claim
- This is the spec.connectionSecretKeys field in a CompositeResourceDefinition
The composition must define where to write its aggregated connection details
- This is the spec.writeConnectionSecretsToNamespace field in the Composition
Each child composed resource must define the connection details it publishes and where to write them
- These are the connectionDetails and base.spec.writeConnectionSecretToRef fields of the composed resources

Finally, you can’t currently edit a XRD’s supported connection details. The XRD’s spec.connectionSecretKeys is effectively immutable. This may change in future per this issue

Claiming an Existing Composite Resource

Most people create Composite Resources using a claim, but you can actually claim an existing Composite Resource as long as its a type of XR that offers a claim and no one else has already claimed it. To do so:

Set the spec.resourceRef of your claim to reference the existing XR.
Make sure the rest of your claim’s spec fields match the XR’s.

If your claim’s spec fields don’t match the XR’s Crossplane will still claim it but will then try to update the XR’s spec fields to match the claim’s.

Influencing External Names

The crossplane.io/external-name annotation has special meaning to Crossplane managed resources - it specifies the name (or identifier) of the resource in the external system, for example the actual name of a CloudSQLInstance in the GCP API. Some managed resources don’t let you specify an external name - in those cases Crossplane will set it for you to whatever the external system requires.

If you add the crossplane.io/external-name annotation to a claim Crossplane will automatically propagate it when it creates an XR. It’s good practice to have your Composition further propagate the annotation to one or more composed resources, but it’s not required.

Mixing and Matching Providers

Crossplane has providers for many things in addition to the big clouds. Take a look at the Upbound Marketplace to find many of them. Keep in mind that you can mix and match managed resources from different providers within a Composition to create Composite Resources. For example you might use provider-aws and provider-sql to create an XR that provisions an RDSInstance then creates an SQL Database and User, or provider-gcp and provider-helm to create a GKECluster and deploy a Helm Chart Release to it.

Often when mixing and matching providers you’ll need to compose a ProviderConfig for one provider that loads credentials from the connection secret of a managed resource from another provider. Sometimes you may need to use an intermediary XR to mutate the connection details to suit your needs. This example from provider-helm demonstrates using a GKE cluster connection secret as Helm ProviderConfig credentials.

Patching From One Composed Resource to Another or Itself

It’s not possible to patch directly from one composed resource to another - i.e. from one entry in the spec.resources array of a Composition to another. It is however possible to achieve this by using the XR as an intermediary. To do so:

Use a ToCompositeFieldPath patch to patch from your source composed resource to the XR. Typically you’ll want to patch to a status field or an annotation.
Use a FromCompositeFieldPath patch to patch from the ‘intermediary’ field you patched to in step 1 to a field on the destination composed resource.

Note that the source and the target composed resource can be the same.