Create ApplicationSource Controller

[devguyio]

Description
We need to create a Knative Event Source for receiving CloudEvents 1.0 from external applications and ingesting them into the event mesh.

For this issue, a controller is needed for ApplicationSource CRD which provisions an adapter that will act similar to existing EventSource but with a new API endpoint and receives only CloudEvents 1.0.

Note It is assumed that resulting adapter is a K8s Service & Deployment until we validate how ksvc behaves in Istio mTLS Service Mesh.

AC

1. There is a controller which reconciles ApplicationSource CRD.
2. Documentations are updated.

[antoineco]

The code for the source controller is currently in https://github.com/antoineco/kyma-event-sources/. We should find a better home for it before starting working on this issue.

[antoineco]

Proposal

The CRD for this story will be called HTTPSource. We decided that the name ApplicationSource was too broad because the notion of Application is already the responsibility of the Application Operator. By differentiating the protocol, we open the door to future event sources (e.g. MQTT, which is planned for next sprint) with potentially different specs.