I wish they went more in-depth into the particular events. They seem to imply the events are user actions, which I took to mean user played this song, user viewed this playlist, and other analytics.

However do they, or would you, use a system like Kafka as an event source instead of the db. So you would also capture events like user added this song to their playlist that would get persisted to the event source db and then eventually a view model instead of directly to a relational db. It doesn't sound like they do that due to the delays, but I can't find a lot of real usage blog posts about how much to put into something like kafka.