Sequences

The unix philosophy states:

Write programs that do one thing and do it well. Write programs to work together.

In an event driven architecture too, it is useful to split up functionality into smaller chunks. These chunks can then be implemented as either microservices or functions. We then need to pass the events from the producer through a series of consumers. This time rather than creating several channels and subscriptions, we will create a Sequence instead. A Sequence lets us define an in-order list of functions that will be invoked. Sequence creates Channels and Subscriptions under the hood.

Let’s see this in action now. We will create the following topology where the event is passed from Producer to Consumer 1, who transforms the event, and then passes it along to Consumer 2, which displays the event.

First, let’s create the final consumer:

cat <<EOF | kubectl create -f -
apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: event-display-chain
spec:
  template:
    spec:
      containers:
        - image: gcr.io/knative-releases/knative.dev/eventing/cmd/event_display
EOF
```{{execute}}

Then create the first consumer which will modify the message:

cat «EOF | kubectl create -f - apiVersion: serving.knative.dev/v1 kind: Service metadata: name: first spec: template: spec: containers: - image: gcr.io/knative-releases/knative.dev/eventing/cmd/appender env: - name: MESSAGE value: " - Updating this message a little…" EOF

Now, let's create the Sequence to execute the service "first" and then pass its result to the service "event-display-chain"

cat «EOF | kubectl create -f - apiVersion: flows.knative.dev/v1 kind: Sequence metadata: name: sequence spec: channelTemplate: apiVersion: messaging.knative.dev/v1beta1 kind: InMemoryChannel steps: - ref: apiVersion: serving.knative.dev/v1 kind: Service name: first reply: ref: kind: Service apiVersion: serving.knative.dev/v1 name: event-display-chain EOF

Finally, create the producer to create the event:

cat «EOF | kubectl create -f - apiVersion: sources.knative.dev/v1beta2 kind: PingSource metadata: name: ping-source-sequence spec: schedule: “*/1 * * * *” data: ‘{“message”: “Hello Sequence!"}’ sink: ref: apiVersion: flows.knative.dev/v1beta1 kind: Sequence name: sequence EOF

Verify that the sequence was executed and the message was updated by running the below command
(You will not see an event there for a minute after creating the producer):

it is likely that is pod is still being created after scaling down to zero

kubectl wait –for=condition=ready pod -l serving.knative.dev/service=event-display-chain –timeout=90s

see the logs

kubectl -n default logs -l serving.knative.dev/service=event-display-chain -c user-container –since=10m –tail=50