Duck typing¶

Knative enables loose coupling of its components by using duck typing.

Duck typing means that the compatibility of a resource for use in a Knative system is determined by certain properties that are used to identify the resource control plane shape and behaviors. These properties are based on a set of common definitions for different types of resources, called duck types.

Knative can use a resource as if it is the generic duck type, without specific knowledge about the resource type, if:

The resource has the same fields in the same schema locations as the common definition specifies
The same control or data plane behaviors as the common definition specifies

Some resources can opt in to multiple duck types.

A fundamental use of duck typing in Knative is using object references in resource specs to point to other resources. The definition of the object containing the reference prescribes the expected duck type of the resource being referenced.

Example¶

In the following example, a Knative Example resource named pointer references a Dog resource named pointee in its spec:

apiVersion: sample.knative.dev/v1
kind: Example
metadata:
  name: pointer
spec:
  size:
    apiVersion: extension.example.com/v1
    kind: Dog
    name: pointee

If the expected shape of a Sizable duck type is that, in the status, the schema shape is the following:

status:
  height: <in centimetres>
  weight: <in kilograms>

Now the instance of pointee could look like this:

apiVersion: extension.example.com/v1
kind: Dog
metadata:
  name: pointee
spec:
  owner: Smith Family
  etc: more here
status:
  lastFeeding: 2 hours ago
  hungry: true
  age: 2
  height: 60
  weight: 20

When the Example resource functions, it only acts on the information in the Sizable duck type shape, and the Dog implementation is free to have the information that makes the most sense for that resource. The power of duck typing is apparent when we extend the system with a new type, for example, Human, if the new resource adheres to the contract set by Sizable.

apiVersion: sample.knative.dev/v1
kind: Example
metadata:
  name: pointer
spec:
  size:
    apiVersion: people.example.com/v1
    kind: human
    name: pointee
---
apiVersion: people.example.com/v1
kind: Human
metadata:
  name: pointee
spec:
  etc: even more here
status:
  college: true
  hungry: true
  age: 22
  height: 170
  weight: 50

The Example resource is able to apply the logic configured for it, without explicit knowledge of Human or Dog.

Knative Duck Types¶

Knative defines several duck type contracts that are in use across the project:

Addressable
Binding
Channelable
Podspecable
Source

Addressable¶

Addressable is expected to be the following shape:

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

Binding¶

With a direct subject, Binding is expected to be in the following shape:

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

With an indirect subject, Binding is expected to be in the following shape:

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

Source¶

With a ref Sink, Source is expected to be in the following shape:

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 a uri Sink, Source is expected to be in the following shape:

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 Sinks, Source is expected to be in the following shape:

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