collection commit_queue, what's the purpose?

Hi All, I have a question of collection commit_queue. What’s the purpose of this queue? void BlueStore::_txc_committed_kv(TransContext *txc) { if (txc->ch->commit_queue){ txc->ch->commit_queue->queue(txc->oncommits); }else{ finisher.queue(txc->oncommits); } } I didn’t see clear places that process the ch->commit_queue(did I miss something here?). Why don’t we just use finisher_queue to notify the client the I/O is finished? Please advice. Thanks, Yiming