Galera Cluster
A synchronous multi-master cluster solution for MySQL/MariaDB (Codership). All nodes can take writes, and a transaction is replicated to all before commit is acknowledged — via "certification-based replication". A popular alternative to MySQL Group Replication.
Mechanic: on commit, the transaction's write-set is broadcast to all nodes, which each "certify" that it doesn't conflict with concurrent transactions (deterministic conflict detection at the row level). If all pass, it commits everywhere; otherwise it's rolled back. Called "virtually synchronous" — replication is synchronous but application can happen slightly after. Win: read/write on any node, no data loss, automatic node recovery (SST/IST). Trade-off: write conflicts across nodes cause rollbacks (best with a "write to one node" pattern), and the whole cluster runs at the pace of the slowest node. Implemented via the wsrep API (Percona XtraDB Cluster, MariaDB Galera). Related to group replication and multi-leader.