Filter and Transform CRDs

The Filter and Transform CRDs are prototypes that build on the Knative extensibility model to implement filter / transformation functions for use with knative/eventing at a higher level!

Problem Statement

As is, the low-level primitives that knative/eventing provides are extremely powerful, supporting: delivery, fanout, and composition. Composition is extremely cool because the first function can transform or filter the message that's queued into the result channel.

However, to express even simple filters and transformations, you currently have to write a Service that performs the filtering (by returning an empty response), and preserves the appropriate cloud event headers. You then stitch it in via:

apiVersion: eventing.knative.dev/v1alpha1
kind: Subscription
metadata:
  name: filter-subscription
spec:
  channel:
    apiVersion: eventing.knative.dev/v1alpha1
    kind: Channel
    name: githubchannel
  subscriber:
    ref:
      apiVersion: serving.knative.dev/v1alpha1
      kind: Service
      name: im-a-filter
  reply:
    channel:
      apiVersion: eventing.knative.dev/v1alpha1
      kind: Channel
      name: filtered-githubchannel

The purpose of this repository is to illustrate that the Knative extensibility model is designed to enable folks to build even higher-level abstractions than functions.

The Filter CRD

The Filter CRD is an abstraction that builds on knative/serving to provide a scale-to-zero "filter" that can compose with knative/eventing and be entirely described via a CRD (vs. code that needs to be containerized).

You would describe your Filter via a resource like:

apiVersion: kfilter.mattmoor.io/v1alpha1
kind: Filter
metadata:
  name: im-a-filter
spec:
  # Keep pull request events only.
  eventType: dev.knative.source.github.issues

This can then be linked as a filter via:

apiVersion: eventing.knative.dev/v1alpha1
kind: Subscription
metadata:
  name: filter-subscription
spec:
  channel:
    apiVersion: eventing.knative.dev/v1alpha1
    kind: Channel
    name: githubchannel
  subscriber:
    ref:
      apiVersion: kfilter.mattmoor.io/v1alpha1
      kind: Filter
      name: im-a-filter
  reply:
    channel:
      apiVersion: eventing.knative.dev/v1alpha1
      kind: Channel
      name: filtered-githubchannel

Supported Filters

Event Type

As shown in the above example, you can filter on event types via:

apiVersion: kfilter.mattmoor.io/v1alpha1
kind: Filter
metadata:
  name: im-a-filter
spec:
  # Keep pull request events only.
  eventType: dev.knative.source.github.issues

Body Patterns

You can match the body of the event via pattern matches. The default mode of matching is partial matches, so for example the following pattern:

apiVersion: kfilter.mattmoor.io/v1alpha1
kind: Filter
metadata:
  name: im-a-filter
spec:
  body: {
    "foo": "bar"
  }

Will match any body that has a key "foo" with value "bar". For arrays the partial match will accept a matching array prefix.

To turn a match from a partial match into an exact match you can wrap it as follows:

apiVersion: kfilter.mattmoor.io/v1alpha1
kind: Filter
metadata:
  name: im-a-filter
spec:
  body: {
    "[exact]": {
      "foo": "bar"
    }
  }

To match an arbitrary element, you can use the special string [anything], so for example if we want to match messages that have the key "foo" with a slice containing at least three elements we can write:

apiVersion: kfilter.mattmoor.io/v1alpha1
kind: Filter
metadata:
  name: im-a-filter
spec:
  body: {
    "foo": [
      "[anything]",
      "[anything]",
      "[anything]"
    ]
  }

To match one of several values, you can use the [oneof] keyword:

apiVersion: kfilter.mattmoor.io/v1alpha1
kind: Filter
metadata:
  name: im-a-filter
spec:
  body: {
    "foo": {"[oneof]": ["bar", "baz"]}
  }

This will match any message with a top level key "foo" iff it has one of the values: "bar" or "baz".

The Transform CRD

The Transform CRD is an abstraction that builds on knative/serving to provide a scale-to-zero "transform" that can compose with knative/eventing and be entirely described via a CRD (vs. code that needs to be containerized).

You would describe your Transform via a resource like:

apiVersion: kfilter.mattmoor.io/v1alpha1
kind: Transform
metadata:
  name: im-a-transform
spec:
  template: |
    # Any Go template acting on the expected body.
    # If the result is empty, this will filter the result.
    foo: {{ .bar }}

This can then be linked as a transformation via:

apiVersion: eventing.knative.dev/v1alpha1
kind: Subscription
metadata:
  name: transform-subscription
spec:
  channel:
    apiVersion: eventing.knative.dev/v1alpha1
    kind: Channel
    name: githubchannel
  subscriber:
    ref:
      apiVersion: kfilter.mattmoor.io/v1alpha1
      kind: Transform
      name: im-a-transform
  reply:
    channel:
      apiVersion: eventing.knative.dev/v1alpha1
      kind: Channel
      name: transformed-githubchannel

Try it.

The filters this supports today are not particularly exciting, but if you'd like to try it out release.yaml contains all of the components needed to install it on your cluster.