Database availability ultimately reduces to one physical law: light is not infinitely fast. Between Seoul and Virginia, the round trip via fiber-optic is roughly 180ms—not something AWS can shorten with money, but a hard limit set by light speed. If a database "waits for replication to replicas before completing writes" (synchronous), every transaction slows by this round-trip time. If it "doesn't wait" (asynchronous), it's fast but replicas lag. All difference between RDS Multi-AZ and Read Replica stems from this synchronous/asynchronous choice.