When you look seriously at database availability, you always arrive at the same question. "I just committed data. To not lose it even if one server dies, someone else must hold a copy. When, how many, and where do we place that copy?" The answers branch at the juncture of synchronous vs. asynchronous replication, single-AZ vs. multi-AZ, 2-copy vs. 6-copy quorum. AWS's RDS Multi-AZ, Aurora, and ElastiCache appear as simple checkboxes for "HA option," but underneath lie decades of distributed-systems theory — replication, consensus, quorum, CAP — from the 1970s onward