Hopping and sliding window

Hopping and sliding window - two names for one construct, which settles the hopping window vs sliding window question up front: a window of fixed size that opens again every slide, before the last one has closed. Kafka Streams says hopping, Flink says sliding (Kafka Streams keeps "sliding" for a separate, data-driven variant). Same picture either way: windows that overlap.

The overlap is where the surprise lives. Each event belongs to every window that covers it, size divided by slide of them, so the engine copies it into each one. A one-minute window sliding every 30 seconds puts each event in two windows; slide every 5 seconds and it is in twelve. Ubik's own docs show five input rows becoming nine output rows with a 30 second slide over a one minute size. The multiplication is the semantics working, not a bug, but it does show up on your sink's disk.

Do you need it? Sometimes. A smoothed rate ("errors over the last minute, refreshed every ten seconds") is exactly this shape. In ubik it is HOP(event_time, INTERVAL '30 SECOND', INTERVAL '1 MINUTE'). If you wanted each event counted once, you wanted a tumbling window.