Biggie implements fastcount similar to how it is implemented on WT. The fastcount get updated immediately on inserts and deletes before the write unit of work is committed. In case of a rollback it is adjusted back.

However write conflicts on the biggie engine happens at commit time leading to the fastcount will be wrong until the commit where the write conflict is thrown.

Running jstests/concurrency/fsm_workloads/upsert_unique_index.js over biggie triggers the following assert: https://github.com/mongodb/mongo/blob/e57156eff2a2ff5dbb1b6a75fee246241206453f/src/mongo/db/storage/biggie/biggie_record_store.cpp#L576

The fastcount will go negative during the test above