Sync vs semisync vs async writes Sync vs semisync vs async writes Synchronous Client write Leader wait ack Follower OK after ack slow node blocks safe on 2 nodes · one slow node blocks writes Semisync (1+1) Client write Leader wait 1 ack Sync one OK after 1 ack swap in a new sync always 2 fresh copies · rest async Asynchronous Client write Leader no waiting Async rest OK at once may lose acked writes never blocks · unreplicated acks can be lost rule: all-sync halts on one failure · all-async can lose acked writes