Crisscross is a special Crossplane provider that allows you to write "Providers as a Function" (PaaF).

Traditional Crossplane providers utilize crossplane-runtime to defer much of the logic for interacting with Kubernetes objects to the generic managed resource reconciler it implements. You can find more documentation on this model here, or check out one of the many providers in the Crossplane community. This is a powerful model that has allowed many community members to create new providers in a few hours. However, though much of the code is boilerplate from a template such as provider-template, there is still quite a lot of machinery required to add support for a simple API. This can be especially challenging if you are not familiar with Kubernetes controllers.

Crisscross shifts more of the burden that currently resides on provider authors to a single system. Instead of provider authors implementing many managed reconcilers for their different API types, they deploy a single service and a corresponding Registration object in their cluster for each API type. When the Registration is created, Crisscross spins up a new controller watching for the referenced API type that will call out to the service at the supplied endpoint.

For example, a Registration for a Bucket on GCP could look as follows:

apiVersion: crisscross.crossplane.io/v1alpha1
kind: Registration
metadata:
  name: bucket-paaf
spec:
  typeRef:
    apiVersion: storage.gcp.crossplane.io/v1alpha3
    kind: Bucket
  endpoint: http://172.18.0.2:32062

The spec.endpoint field can point to any service, whether it is a traditional Pod, a Knative Service, or a public API on the internet. The only requirement for this service is that it serves the following methods:

• /observe
• /create
• /update
• /delete

Crisscross will send the managed resource to these endpoints, and will take appropriate action in the cluster based on the response. For an example of how simple a "PaaF" can be, take a look at nop-paaf, which just reports that a resource exists and all other operations are no-ops.

Crisscross is under the Apache 2.0 license.