With 4.0 all updates to indexes are made asynchronously. Each index has an event listener that uses the normal asynchronous ModeShape event mechanism to update indexes.

Some applications may want indexes to be updated during the save operation. Therefore, we should allow an index definition to specify whether the corresponding indexes (one in each applicable workspace) are to be updated synchronously or asynchronously.

Specifying the behavior on the index definition gives app developers a lot of power to get the behavior they need/want. This is better than in 3.x of always doing it synchronously, and better than with 4.0 of always doing it asynchronously (which some applications understandably cannot use).