Sequence should support delivery specification

[lionelvillard]

Problem
Sequence does not support deadLetterSink

Persona:
Which persona is this feature for?

Exit Criteria
A measurable (binary) test that would indicate that the problem has been resolved.

A sequence with dead letter sink.

Time Estimate (optional):
How many developer-days do you think this may take to resolve?
a day

Additional context (optional)
Add any other context about the feature request here.

It should be possible to specify different delivery options per step. Either we do it this way:

apiVersion: flows.knative.dev/v1alpha1
kind: Sequence
metadata:
  name: sequence
spec:
  steps:
  - ref:
      apiVersion: serving.knative.dev/v1alpha1
      kind: Service
      name: step1
    delivery:
      deadLetterSink:
         uri: dls1.example.com
 - ref:
      apiVersion: serving.knative.dev/v1alpha1
      kind: Service
      name: step2
    delivery:
      deadLetterSink:
        uri: dls2.example.com

or to be more aligned with parallel, we could do it this way (not backward compatible!):

apiVersion: flows.knative.dev/v1alpha1
kind: Sequence
metadata:
  name: sequence
spec:
  steps:
  - subscriber:
      ref:
        apiVersion: serving.knative.dev/v1alpha1
        kind: Service
        name: step1
   delivery:
     deadLetterSink:
       uri: dls1.example.com
 - subscriber:
      ref:
        apiVersion: serving.knative.dev/v1alpha1
        kind: Service
        name: step2
    delivery:
      deadLetterSink:
        uri: dls2.example.com

It should also be possible to specify delivery spec for the whole sequence

apiVersion: flows.knative.dev/v1alpha1
kind: Sequence
metadata:
  name: sequence
spec:
  delivery:
    deadLetterSink:
      uri: dls-sequence.example.com

@vaikas @n3wscott what do you guys think?

[lionelvillard]

/area api

[lionelvillard]

Currently Sequence is seen as being an atomic unit and for the usecase above it is recommended to use channel/subscription instead.

I personally think that not allowing delivery on higher-level constructs like Sequence and Parallel diminishes their usefulness. It might even scared people off when they start to realize it's not possible to fine-control event delivery. Potentially each subscriber has its own fault-tolerance characteristics and many times those are not known in advance. If one service behaves badly then the whole thing has to rewritten, eg to something like this:

apiVersion: messaging.knative.dev/v1alpha1
kind: KafkaChannel
metadata:
  name: step1
spec:
---
apiVersion: messaging.knative.dev/v1alpha1
kind: Subscription
metadata:
  name: step1
spec:
  channel:
    apiVersion: messaging.knative.dev/v1alpha1
    kind: KafkaChannel
    name: step1
  subscriber:
    ref:
      apiVersion: serving.knative.dev/v1
      kind: Service
      name: step1
  delivery:
      deadLetterSink:
         uri: dls1.example.com
  reply:
    ref:
      apiVersion: messaging.knative.dev/v1alpha1
      kind: KafkaChannel
      name: step2
---
apiVersion: messaging.knative.dev/v1alpha1
kind: KafkaChannel
metadata:
  name: step2
spec:
---
apiVersion: messaging.knative.dev/v1alpha1
kind: Subscription
metadata:
  name: step2
spec:
  channel:
    apiVersion: messaging.knative.dev/v1alpha1
    kind: Channel
    name: step2
  subscriber:
    ref:
      apiVersion: serving.knative.dev/v1
      kind: Service
      name: step2
  delivery:
    deadLetterSink:
       uri: dls2.example.com   

[lionelvillard]

/label proposal/0.13