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Crossplane Patching Basics

Component relationships

flowchart LR 

XRD(Composite Resource Definition)
C(Composition)
CR(Composite Resource)
MR(Managed Resource)
MR2(Managed Resource)
Claim
Bucket(S3 Bucket)
Table(DynamoDB Table)

C --satisfies--> XRD
XRD --define schema \n create CRDs--> CRDs
C --defines--> CR --> MR --managed--> Bucket
CR --> MR2 --manage--> Table
Claim --trigger instantiation--> CR
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From Composite Resource to Managed Resource

Crossplane compositions allow you to modify sub resources based on arbitrary fields from their composite resource. This type of patches are referred as FromCompositeFieldPath. Take for an example:

type: FromCompositeFieldPath
fromFieldPath: spec.region
toFieldPath: spec.forProvider.region

This tells Crossplane to:

  1. Look at the spec.region field in the Composite Resource.
  2. Then copy that value into the spec.forProvider.region field in this instance of managed resource.
flowchart LR

subgraph Composite Resource
    cs(spec: \n&nbsp region: <font color=red>us-west-2</font>)
end

subgraph Managed Resource
    ms(spec: \n&nbsp forProvider: \n&nbsp&nbsp region: <font color=red>us-west-2</font>)
end

style cs text-align:left
style ms text-align:left 

cs --> ms
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From Managed Resource to Composite Resource

Compositions also allow you to modify the composite resource from its sub resources. For example:

type: ToCompositeFieldPath
fromFieldPath: status.atProvider.arn
toFieldPath: status.bucketArn
policy:
  fromFieldPath: Optional # This can be omitted since it defaults to Optional.

This tells Crossplane to:

  1. Look at the status.atProvider.arn field on the managed resource.
  2. If the status.atProvider.arn field is empty, skip this patch.
  3. Copy the value into the status.bucketArn field on the composite resource.
flowchart LR

subgraph Managed Resource
    ms(status: \n&nbsp atProvider: \n&nbsp&nbsp arn: <font color=red>arn&#58aws&#58s3&#58&#58&#58my-bucket</font>)
end

subgraph Composite Resource
    cs(status: \n&nbsp bucketArn: <font color=red>arn&#58aws&#58s3&#58&#58&#58my-bucket</font>)
end


style cs text-align:left
style ms text-align:left

ms --> cs
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Putting them together

With these patching methods together, you can pass values between managed resources.

type: FromCompositeFieldPath
fromFieldPath: status.bucketArn
toFieldPath: spec.forProvider.bucketArn
policy:
  fromFieldPath: Required

This tells Crossplane to:

  1. Look at the status.bucketArn field in the Composite Resource.
  2. If the status.bucketArn field is empty, do not skip. Stop composing this managed resource.
  3. Once the status.bucketArn field is filled with a value, copy that value into the spec.forProvider.bucketArn in the managed resource.

With the use of Required policy, you can create a soft dependency. This is useful when you do not want to create a resource before another resource is ready.

flowchart LR

subgraph Managed Resource 1
    ms(status: \n&nbsp atProvider: \n&nbsp&nbsp arn: <font color=red>arn&#58aws&#58s3&#58&#58&#58my-bucket</font>)
end

subgraph Managed Resource 2
    ms2(spec: \n&nbsp forProvider: \n&nbsp&nbsp bucketArn: <font color=red>arn&#58aws&#58s3&#58&#58&#58my-bucket</font>)
end

subgraph Composite Resource
    cs(status: \n&nbsp bucketArn: <font color=red>arn&#58aws&#58s3&#58&#58&#58my-bucket</font>)
end

style cs text-align:left
style ms text-align:left
style ms2 text-align:left

ms --> cs
cs --> ms2
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Transform

You can also perform modifications to values when patching. For example, you can use the following transformation to extract the accountId of this managed policy.

type: ToCompositeFieldPath
fromFieldPath: status.policyArn
toFieldPath: status.accountId
transforms:
  - type: string
    string:
      type: Regexp
      regexp:
        match: 'arn:aws:iam::(\d+):.*'
        group: 1

This tells Crossplane to:

  1. Look at the status.policyArn field in the Managed Resource.
  2. If the field has a value, take that value and run a regular expression match against it.
  3. When there is a match, take the first capture group and store it in the status.accountId field in the Composite Resource.
flowchart LR

subgraph Managed Resource
    ms(status: \n&nbsp policyArn: arn:aws:iam::<font color=red>12345</font>:policy/my-policy)
end

subgraph Composite Resource
    cs(status: \n&nbsp accountId: <font color=red>12345</font>)
end

style cs text-align:left
style ms text-align:left

ms --regular expression match--> cs
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Reference

See the official documentation for more information. https://docs.crossplane.io/master/concepts/composition/#patch-types