Incremental snapshot
An incremental snapshot is a session replay event that records only what changed since the previous event, such as DOM mutations, mouse movement, scrolling, or input values. Replay reconstructs the page by applying incremental snapshots in order on top of the last full snapshot, which keeps recordings small.
How it works
In the rrweb format, incremental snapshots are event type 3, and a source field says what kind of change the event carries (see incremental source); the events documentation shows the typed payloads. A one-line DOM change produces an event of a few hundred bytes instead of a new copy of the page. The trade-off is dependency: incremental events only make sense as a chain, so a corrupted or missing event can break everything after it until the next full snapshot.
Where you encounter it
Incremental snapshots are the bulk of any recording by count, and mousemove events are usually the bulk by volume, which is why rrweb throttles them via the sampling option by default. See the storage optimization recipe for configuration details.