Data engineering·Time and correctness in streams
the window has no fixed length; it just stays open until the user does nothing for thirty minutes, and then it closes.
Session window
Also calledgap-based window, activity session
A window defined by inactivity rather than by the clock: events for a key are grouped until a gap longer than the timeout appears. It is the natural shape for anything that is about a visit, a conversation or a driving trip, and it is the most expensive of the three because window boundaries are data-dependent and two sessions merge when a late event lands between them. Restarts and late data are correspondingly harder, because a session's identity can change after the fact.