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Sources, Brokers, Triggers, Sinks

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 (1's and 2's) to the Broker, which then gets filtered by Triggers to the desired Sink.
Component Basic Definition
Source ℹ A Kubernetes Custom Resource which emits events to the Broker.
Broker A "hub" for events in your infrastructure; a central location to send events for delivery.
Trigger Acts as a filter for events entering the broker, can be configured with desired event attributes.
Sink A 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 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 follow the CloudEvents 1.0 Specification, with required and optional attributes.
  2. CloudEvents can be "emitted" by almost anything and can be transported to anywhere in your deployment.
  3. CloudEvents can carry some attributes (things like id, source, type, etc) as well as data payloads (JSON, plaintext, reference to data that lives elsewhere, etc).

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

Examining the Broker

As part of the KonK install, you should have an in-memory Broker already installed. 

kn broker list

Expected Output 

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

Warning

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

What other components exist in Knative Eventing?

If you want to find out more about the different components of Knative Eventing, like Channels, Sequences, Parallels, etc. check out "Eventing Components."