Serializability
The strictest isolation level — execution of parallel transactions yields the same result as if they were run one at a time in some order.
Implemented via: 2-phase locking (classic), serializable snapshot isolation (SSI, Postgres), strict-2PL + index locks. Eliminates all anomalies (dirty/phantom/lost-update/write-skew). Price: contention bottlenecks, deadlocks have to be detected + rolled back. Default in many DBs is Read Committed — faster but allows write-skew. Postgres SSI is the only mainstream OSS that has true serializable without locks.