segmentObserver

Where documents are reported as they are written into segments. See SegmentObserver.

This is the seam the modelling and indexing layers attach through, and it belongs in the options rather than in a setter because flush and compaction may begin the moment the store is open: an observer installed afterwards would silently miss whatever was written in between, and a model with a hole in it is worse than no model. Use DocumentStore.backfill to cover data that was written before the observer existed.