1. Introduction

Several ER-Demo services are serverless enabled using the capabilities of Knative on OpenShift.

Knative extends facilities in Openshift and Kubernetes to provide a scalable environment for deploying services that scale up and down based on demand. It also provides an eventing mechanism to provide decoupled messaging to applications running inside an Openshift or Kubernetes cluster.

Knative consists of the following components:

  • Serving - Request-driven compute that can scale to zero Knative Serving is ideal for running application services inside OpenShift by providing a more simplified deployment syntax with automated scale-to-zero and scale-out based on HTTP load . The Knative platform will manage your service’s deployments, revisions, networking and scaling. Knative Serving exposes your service via an HTTP URL. As the Knative Serving Service has the built in ability to automatically scale down to zero when not in use, it is apt to call it as “serverless service”. Knative Serving is mostly adapted for stateless applications with minimal or no dependencies on other systems.

  • Eventing - Management and delivery of events to and from Knative services Knative Eventing is designed to decouple event sources and event consumers. Knative Eventing services are loosely coupled and can be developed and deployed independently. Event producers and event consumers are independent. Any producer (or source), can generate events before there are active event consumers that are listening. Any event consumer can express interest in an event or class of events, before there are producers that are creating those events. Cross-service interoperability is provided by adhering to the CloudEvents specification that is developed by the Cloud Native Computing Foundation Serverless working Group.

In the Emergency Response Demo application, both Knative Serving and Knative Eventing are installed by default. The list of Knative enabled services can be seen as per the following:


$ oc get kservice -n $OCP_USERNAME-er-demo

NAME            URL                                                    LATESTCREATED         LATESTREADY          READY   REASON
datawarehouse   http://datawarehouse-user1-er-demo.apps.ratwater.xyz   datawarehouse-8sfmk                        False   RevisionMissing
find-service    http://find-service-user1-er-demo.apps.ratwater.xyz    find-service-xff7s    find-service-xff7s   True    
kafdrop         http://kafdrop-user1-er-demo.apps.ratwater.xyz         kafdrop-wn479         kafdrop-wn479        True

Details pertaining to each type of Knative capability are found in the following sections:

2. Knative Serving:

This section provides details how to verify the installation of Knative Serving functionality in ER-Demo.

  1. Verify the successful installation of the knative operator in the openshift-operators namespace:

     $oc get subscription serverless-operator -n openshift-operators 
     NAMESPACE              NAME                          PACKAGE                       SOURCE                CHANNEL
     openshift-operators    serverless-operator           serverless-operator           redhat-operators      4.4
  2. Verify the successful installation of Knative Serving:

     $ oc get knativeserving.operator.knative.dev/knative-serving \
          -n knative-serving \
  3. View scale-out capability of knative enabled kafdrop service in ER-Demo

    3.1 Tail container orchestration events in the er-demo namespace $oc get events -w -n $OCP_USERNAME-er-demo Keep this terminal window open.

    3.2 Navigate a browser tab to the output of the following command:

       echo -en "\n\n$(oc get kservice kafdrop --template='' -n $OCP_USERNAME-er-demo)\n\n"

    3.3 Notice container orchestration events such as the following as a kafkdrop service is spun up from zero:

       0s          Normal    ScalingReplicaSet   deployment/kafdrop-wn479-deployment                 Scaled up replica set kafdrop-wn479-deployment-687449d4b6 to 1
       0s          Normal    SuccessfulCreate    replicaset/kafdrop-wn479-deployment-687449d4b6      Created pod: kafdrop-wn479-deployment-687449d4b6-p7btj
       0s          Normal    Scheduled           pod/kafdrop-wn479-deployment-687449d4b6-p7btj       Successfully assigned user1-er-demo/kafdrop-wn479-deployment-687449d4b6-p7btj to worker-1.ratwater.xyz
       0s          Normal    AddedInterface      pod/kafdrop-wn479-deployment-687449d4b6-p7btj       Add eth0 []
       0s          Normal    Pulled              pod/kafdrop-wn479-deployment-687449d4b6-p7btj       Container image "index.docker.io/obsidiandynamics/kafdrop@sha256:b7ba8577ce395b1975b0ed98bb53cb6b13e7d32d5442188da1ce41c0838d1ce9" already present on machine
       0s          Normal    Created             pod/kafdrop-wn479-deployment-687449d4b6-p7btj       Created container user-container
       0s          Normal    Started             pod/kafdrop-wn479-deployment-687449d4b6-p7btj       Started container user-container
       0s          Normal    Pulled              pod/kafdrop-wn479-deployment-687449d4b6-p7btj       Container image "registry.redhat.io/openshift-serverless-1/serving-queue-rhel8@sha256:8d45d901a58ec5da58e97b5e0d38ce22a9318837a13926ef6c21bb5b81f0bcb0" already present on machine

    If interested, the following write-up and webinar details the internals of the Knative Serving auto-scaling implementation.

3. Knative Eventing:

The datawarehouse service of the ER-Demo is Knative Eventing enabled and implemented using a Source to Sink approach. The Knative tutorial mentions the following about the Source to Sink usage pattern:

Source to Sink provides the simplest getting started experience with Knative Eventing. It provides single Sink — that is, event receiving service –, with no queuing, backpressure, and filtering. The Source to Service does not support replies, which means the response from the Sink service is ignored. The responsibility of the Event Source is just to deliver the message without waiting for the response from the Sink, hence it is comparable to the fire and forget messaging pattern.

Source To Sink

This section provides details how to verify the installation of Knative Eventing functionality in ER-Demo.

  1. Verify the successful installation of Knative Eventing:
    $ oc get knativeeventing.operator.knative.dev/knative-eventing \
         -n knative-eventing \
  2. Verify the various types of Knative eventing sources:
    $ oc api-resources --api-group='sources.knative.dev'
    NAME               APIGROUP              NAMESPACED   KIND
    apiserversources   sources.knative.dev   true         ApiServerSource
    kafkasources       sources.knative.dev   true         KafkaSource
    pingsources        sources.knative.dev   true         PingSource
    sinkbindings       sources.knative.dev   true         SinkBinding
  3. Verify the datawarehouse-source (used by the Knative Eventing enabled datawarehouse service):
    $ oc get kafkasources -n $OCP_USERNAME-er-demo
    NAME                   TOPICS                     BOOTSTRAPSERVERS                                         READY   REASON     AGE
    datawarehouse-source   [topic-incident-command]   [kafka-cluster-kafka-bootstrap.user1-er-demo.svc:9092]   False   NotFound   4d3h
  4. Verify that the datawarehouse-service is configured to scale-to-1:
    $ oc get kservice datawarehouse -o template --template 

4. Appendix

4.1. Reference

  1. Knative Tutorial
  2. Advanced Cloud Native Development: 07_1_Serverless_Lab