Crossplane Composition Functions IO

fnio.yaml

apiVersion: apiextensions.crossplane.io/v1alpha1
kind: FunctionIO
# Optional arbitrary KRM-like resource. Each function in a pipeline will be
# called with its own function config (i.e. config is not pipelined).
config:
  apiVersion: database.example.org/v1alpha1
  kind: Config
  metadata:
    name: cloudsql
  spec:
    version: POSTGRES_9_6
# The composite resource (including metadata, spec, and status)
composite:
  apiVersion: database.example.org/v1alpha1
  kind: XPostgreSQLInstance
  metadata:
    name: my-db
  spec:
    parameters:
      storageGB: 20
    compositionSelector:
      matchLabels:
        provider: gcp
    writeConnectionSecretToRef:
      name: db-conn
# TODO(negz): What if we just took the resources array of a Composition,
# including patches etc? Is there value in that? Code could just ignore patches
# and return fully formed bases if it wanted to. I like that this would make
# the data structure identical to a Composition resources array, but is there
# any other value? Would it be limiting to try to keep them identical? (Probably yes.)
resources:
- name: cloudsqlinstance
  base:
    apiVersion: database.gcp.crossplane.io/v1beta1
    kind: CloudSQLInstance
    metadata:
      name: cloudsqlpostgresql
    spec:
      forProvider:
        databaseVersion: POSTGRES_9_6
        region: us-central1
        settings:
          tier: db-custom-1-3840
          dataDiskType: PD_SSD
          dataDiskSizeGb: 20
      writeConnectionSecretToRef:
        namespace: crossplane-system
        name: cloudsqlpostgresql-conn
  connectionDetails:
  - name: hostname
    fromConnectionSecretKey: hostname
  # TODO(negz): Do we want readiness checks too? Should the function be able to
  # write to the status of the XR directly to set a status?
results:
- severity: Error
  message: "Could not render Database.postgresql.crossplane.io/v1alpha1`

Would it be an option to require that the Helm charts used for Composition render (i.e. "return") a single FunctionIO manifest, rather than a stream of manifests?

I would be worried that this might result in more complicated helm charts, either long FunctionIO yamls or more advanced templating. Also by not being as intuitive, this might have a negative affect on adoption.

Another way to think about this - is our goal only to allow people to use Helm to specify their Composition logic, or do we want to go further and allow them to adapt their existing Helm charts into an XR? My feeling is the former is more important.

I agree that the former is more important and not sure whether the latter should be in scope at all since existing helm charts does not produce managed resources which would mean crossplane to deploy arbitrary resources on control plane. My concern is more around UX as I mentioned above.

I'm generally against structured data in annotations, and more broadly a little worried about how much we're stuffing into annotations in the ResourceList design (required name annotation, required type annotation, optional structured connection details annotation, etc).

Yes, it is definitely not great.
Actually I am wondering if we need to pass this connectionDetails field at all? If we want to pass existing connection secrets as input and expect XR connection secret as output (as we discussed in a separate thread), I believe outputting the final XR secret should be enough.

@turkenh I ended up pushing a commit to crossplane/crossplane#2886 that adopts a stripped down version of the FunctionIO API. I'd like to roll this conversation into that PR - I'll respond to your latest comment there. Thanks!