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8.AddingChannelsAndSubscriptions.md

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Adding Channels and Subscriptions

We can fan-out delivery from an Event source to multiple endpoints using Channels and Subscriptions. In this case, the Channel implementation ensures that messages are delivered to the requested destinations and should buffer the events if the destination service is unavailable.

Create a Channel

Different types of channels are possible such as Apache Kafka, GCPPubSub, InMemoryChannel etc. In this example we will create an InMemoryChannel with the following yaml. This is a best effort channel for development usage.

apiVersion: messaging.knative.dev/v1alpha1
kind: InMemoryChannel
metadata:
  name: imc-msgtxr

Let us create the channel.

$ oc create -f eventing/in-memory-channel.yaml 

inmemorychannel.messaging.knative.dev/imc-msgtxr created

Verify

$ oc get inmemorychannel

NAME         READY   REASON   URL                                                      AGE
imc-msgtxr   True             http://imc-msgtxr-kn-channel.kn-demo.svc.cluster.local   24s

Note the URL from the output as we will use it while adding the event source in the next step.

Add an Event Source with Channel as the Sink

This time we will again add an event source. But unlike last direct delivery example, the event source in this case will use the above channel as the sink.

kn source ping create msgtxr-pingsource \
--schedule="* * * * *" \
--data="This message is from PingSource" \
--sink=http://imc-msgtxr-kn-channel.kn-demo.svc.cluster.local
  • We are using PingSource as the event source in this example as well
  • Note the sink here points to the channel we created above. Look at the URL.

Verify

% kn source ping list
NAME                SCHEDULE    SINK   AGE   CONDITIONS   READY   REASON
msgtxr-pingsource   * * * * *          13m   6 OK / 6     True

At this point the events are being generated but no data should be flowing into the application yet. Run

% oc get po | grep Running
pingsource-msgtxr-pingsour-e574f4cc-1d71-4d6f-b1b9-47f18c6kl9pq   1/1     Running     0          41s

You should see the event source pod running. It is pushing workloads to a sink which is the InMemory channel. But, since the channel is not subscribed by anyone the events are not getting anywhere.

Add an Event Subscriber

Event subscription allows an event destination to subscribe to a channel. In our case we will configure our service as the destination.

apiVersion: messaging.knative.dev/v1alpha1
kind: Subscription
metadata:
  name: msgtxr-subscriber 
spec:
  channel:
    apiVersion: messaging.knative.dev/v1alpha1
    kind: InMemoryChannel
    name: imc-msgtxr
  subscriber:
    ref:
      apiVersion: serving.knative.dev/v1alpha1
      kind: Service
      name: msgtxr-sl

Let us create the subscription

$ oc create -f eventing/event-subscription.yaml


subscription.messaging.knative.dev/msgtxr-subscriber created

Verify

$ oc get subscription.messaging.knative.dev
NAME                                                         READY   REASON                      AGE
msgtxr-subscriber                                            True                                42s

Wait a min or so and you should see the serverless pod coming up

% oc get po | grep Running 
msgtxr-sl-fzpfp-1-deployment-d55996b47-chxhh                      2/2     Running     0          3m3s
pingsource-msgtxr-pingsour-e574f4cc-1d71-4d6f-b1b9-47f18c6kl9pq   1/1     Running     0          10m

Check the serverless pod logs for events received from the event source via channel

__  ____  __  _____   ___  __ ____  ______ 
 --/ __ \/ / / / _ | / _ \/ //_/ / / / __/ 
 -/ /_/ / /_/ / __ |/ , _/ ,< / /_/ /\ \   
--\___\_\____/_/ |_/_/|_/_/|_|\____/___/   
2020-05-14 04:56:02,354 INFO  [io.quarkus] (main) getting-started 1.0-SNAPSHOT (powered by Quarkus 1.3.2.Final) started in 0.013s. Listening on: http://0.0.0.0:8080
2020-05-14 04:56:02,354 INFO  [io.quarkus] (main) Profile prod activated. 
2020-05-14 04:56:02,354 INFO  [io.quarkus] (main) Installed features: [cdi, resteasy]
04:56:03.350 IN {"body":"This message is from PingSource"} OUT {"body":"This message is from PingSource"}
04:57:00.011 IN {"body":"This message is from PingSource"} OUT {"body":"This message is from PingSource"}

Summary

In this lab, we have learnt to add a Channel that listens to an Event Source and then we added our Knative service as the destination that subscribes to the Channel

Cleanup

Let us delete the event source.

$ kn source ping delete msgtxr-pingsource

Ping source 'msgtxr-pingsource' deleted in namespace 'kn-demo'.

You should the event source pod go away immediately. If you wait a minute,our serverless service also scales down to zero.

Delete the channel

$ oc delete -f eventing/in-memory-channel.yaml 

inmemorychannel.messaging.knative.dev "imc-msgtxr" deleted

Delete subscription

$ oc delete -f eventing/event-subscription.yaml