Hi Ali,
One starting point would be the low level Processor API, where you get each
event and process it. You can also use a persistent state store to keep track
of the events seen so far, it can probably be an in-memory store. An an entry
can probably be deleted once both start and stop
I have a tricky use case where a user initiates an event (by clicking a
button) and then stops it (by clicking it again, losing connection, closing
the browser, etc).
Each time the event starts or stops, a notification is sent to a kafka
topic, with the user's id as the message key and the
