Jungtaek Lim, et al, announce support for session windows in Spark Structured Streaming:
Tumbling windows are a series of fixed-sized, non-overlapping and contiguous time intervals. An input can only be bound to a single window.
Sliding windows are similar to the tumbling windows from the point of being “fixed-sized”, but windows can overlap if the duration of the slide is smaller than the duration of the window, and in this case, an input can be bound to the multiple windows.
Session windows have a different characteristic compared to the previous two types. Session window has a dynamic size of the window length, depending on the inputs. A session window starts with an input and expands itself if the following input has been received within the gap duration. A session window closes when there’s no input received within the gap duration after receiving the latest input. This enables you to group events until there are no new events for a specified time duration (inactivity).
Click through for more details. You could implement session windows when querying existing data using a gaps and islands approach (where you increment the island count when you have a lagged difference greater than the cutoff point), but for streaming scenarios, it’s very handy to have this as a native window type.