Ack Now or Ack Safe

Synchronous replication lets the leader keep accepting writes while it is partitioned from its follower, whereas asynchronous replication blocks new writes until the partition heals and the follower is reachable again

Synchronous acknowledges the client only after the follower has the write, so a stalled follower blocks writes; asynchronous is fast but can lose acknowledged writes when the leader dies

Synchronous replication removes replication lag from reads entirely, because the follower applies each change on a faster schedule than the leader commits it

Asynchronous replication is fully durable for every acknowledged write and only affects how soon the read replicas reflect it, so the choice is purely a freshness question