pkg/apis/duck/README.md

Duck Types

Knative leverages duck-typing to interact with resources inside of Kubernetes without explicit knowledge of the full resource shape. knative/pkg defines three duck types that are used throughout Knative: Addressable, Binding, and Source.

For APIs leveraging ObjectReference, the context of the resource in question identifies the duck-type. To enable the case where no ObjectReference is used, we have labeled the Custom Resource Definition with the duck-type. Those labels are as follows:

Label	Duck-Type
duck.knative.dev/addressable=true	Addressable
duck.knative.dev/binding=true	Binding
duck.knative.dev/source=true	Source

Addressable Shape

Addressable is expected to be the following shape:

apiVersion: group/version
kind: Kind
status:
  address:
    url: http://host/path?query

Binding Shape

Binding is expected to be in the following shape:

(with direct subject)

apiVersion: group/version
kind: Kind
spec:
  subject:
    apiVersion: group/version
    kind: SomeKind
    namespace: the-namespace
    name: a-name

(with indirect subject)

apiVersion: group/version
kind: Kind
spec:
  subject:
    apiVersion: group/version
    kind: SomeKind
    namespace: the-namespace
    selector:
      matchLabels:
        key: value

Source Shape

Source is expected to be in the following shape:

(with ref sink)

apiVersion: group/version
kind: Kind
spec:
  sink:
    ref:
      apiVersion: group/version
      kind: AnAddressableKind
      name: a-name
  ceOverrides:
    extensions:
      key: value
status:
  observedGeneration: 1
  conditions:
    - type: Ready
      status: "True"
  sinkUri: http://host

(with uri sink)

apiVersion: group/version
kind: Kind
spec:
  sink:
    uri: http://host/path?query
  ceOverrides:
    extensions:
      key: value
status:
  observedGeneration: 1
  conditions:
    - type: Ready
      status: "True"
  sinkUri: http://host/path?query

(with ref and uri sink)

apiVersion: group/version
kind: Kind
spec:
  sink:
    ref:
      apiVersion: group/version
      kind: AnAddressableKind
      name: a-name
    uri: /path?query
  ceOverrides:
    extensions:
      key: value
status:
  observedGeneration: 1
  conditions:
    - type: Ready
      status: "True"
  sinkUri: http://host/path?query