Sources, Brokers, Triggers, Sinks, oh my!

For the purposes of this tutorial, let’s keep it simple. You will focus on four powerful Eventing components: Source, Trigger, Broker, and Sink.

Let’s take a look at how these components interact:

Source 1 and Source 2 are transmitting some data -- ones and twos -- to the Broker, which then gets filtered by Triggers to the desired Sink.

ComponentBasic Definition
Sources, Brokers, Triggers, Sinks - 图2 SourceA Kubernetes Custom Resource which emits events to the Broker.
Sources, Brokers, Triggers, Sinks - 图3 BrokerA “hub” for events in your infrastructure; a central location to send events for delivery.
Sources, Brokers, Triggers, Sinks - 图4 TriggerActs as a filter for events entering the broker, can be configured with desired event attributes.
Sources, Brokers, Triggers, Sinks - 图5 SinkA destination for events.

A note on Sources and Sinks

A Knative Service can act as both a Source and a Sink for events, and for good reason. You may want to consume events from the Broker and send modified events back to the Broker, as you would in any pipeline use-case.

CloudEvents

Knative Eventing uses CloudEvents to send information back and forth between your Services and these components.

What are CloudEvents?

For our purposes, the only thing you need to know about CloudEvents are:

  1. CloudEvents can carry some attributes (like id, Source, type, etc) as well as data payloads (JSON, plaintext, reference to data that lives elsewhere, etc).
  2. CloudEvents can be “emitted” by almost anything and can be transported to anywhere in your deployment.
  3. CloudEvents follow the CloudEvents 1.0 Specification, with required and optional attributes.

To find out more about CloudEvents, check out the CloudEvents website!

Examining the Broker

As part of the kn quickstart install, an In-Memory Broker should already be installed in your Cluster. Check to see that it is installed by running the command:

  1. kn broker list

Expected output

  1. NAME             URL                                                                                AGE   CONDITIONS   READY   REASON
  2. example-broker   http://broker-ingress.knative-eventing.svc.cluster.local/default/example-broker     5m    5 OK / 5    True

Warning

In-Memory Brokers are for development use only and must not be used in a production deployment.

Are there any other components of Knative Eventing?

Though it is out of scope for this tutorial, Knative Eventing has many components which can be used in many ways to suit your needs.

If you want to find out more about the different components of Knative Eventing, such as Channels, Sequences and Parallel flows, check out the Knative Eventing documentation.

Next, you’ll take a look at a simple implementation of Sources, Brokers, Triggers and Sinks using an app called the CloudEvents Player.