Clarify usage of source for CDEvents

[afrittoli]

CDEvents include a source field that matches the source in CloudEvents (type: URI-reference). source + id should be globally unique, which means that source should identify the producer of events using a URI-reference.

I'm using Tekton events as an example of the possible options.
In Tekton today the source looks like:

"Ce-Id": "77f78ae7-ff6d-4e39-9d05-b9a0b7850527",
"Ce-Source": "/apis/tekton.dev/v1beta1/namespaces/default/taskruns/curl-run-6gplk",
"Ce-Specversion": "1.0",
"Ce-Subject": "curl-run-6gplk",
"Ce-Time": "2021-01-29T14:47:58.157819Z",
"Ce-Type": "dev.tekton.event.taskrun.unknown.v1",

However this is problematic because the source is different for every subject, which makes filtering events harder.
An alternative could be to use in the source the website of the platform that produces events, e.g.

"Ce-Id": "77f78ae7-ff6d-4e39-9d05-b9a0b7850527",
"Ce-Source": "https://tekton.dev/",
"Ce-Specversion": "1.0",
"Ce-Subject": "/apis/tekton.dev/v1beta1/namespaces/default/taskruns/curl-run-6gplk",
"Ce-Time": "2021-01-29T14:47:58.157819Z",
"Ce-Type": "dev.tekton.event.taskrun.unknown.v1",

That means that all events generated by any instance of Tekton would have the same source though.
I'm not aware of any standard way of identifying an instance of an application via an URI.
We could define something like <org-namespace>/tekton/<instance-descriptor>:

"Ce-Source": "tekton.dev/tekton/dogfooding",

This could identify the dogfooding Tekton instance used by the Tekton community.
Thoughts?

[m-linner-ericsson]

In Eiffel we have an object to describe the source, see the meta.source in Activity Triggered.

I would like to state something like

Source: Describes the sender of the event. By using the source the receiver should have the possibility to pinpoint the instance that sent the event.

[afrittoli]

A diagram would help clarifying what source means for different event types.

• Create an issue to track the work on the diagram
• Collect source from current implementation (Tekton, Jenkins, Spinnaker, Testkube)

Subject source adds to the complexity / confusion.