High Availability for MariaDB and Postgres : replication or active-passive (SAN)
(Relating to MySQL only...)

In my opinion, a Galera-based solution is the best HA available today.  MariaDB includes Galera in the latest releases; PXC also has it; or you can tack Galera onto Oracle's MySQL.


"Replication" is an integral part of Galera, but there are low-level differences
Automatically recovers from single-point-of-failure
3 node (server) minimum
Uses quorum voting to discover what has failed, and avoid "split-brain"
If the 3 nodes are in 3 different datacenters, no single natural disaster can take out the 'system'
Read scaling (multiple nodes providing such)


Load balancing, proxying requests, etc, are related topics.