Explain Serializable Snapshot Isolation (SSI).

SSI provides serializable isolation without the blocking of traditional locking. Based on snapshot isolation, it detects and aborts transactions that could cause serialization anomalies. Tracks read-write dependencies between concurrent transactions; if dangerous pattern detected (write skew potential), one transaction aborts. Used in PostgreSQL SERIALIZABLE mode and CockroachDB. Offers better performance than S2PL while maintaining serializability. Some false positives lead to unnecessary aborts.