Feb 28, 2025
Snapshot isolation rightly enjoys a reputation as a comfortable compromise: it prevents dirty reads, non-repeatable reads, and most phantom reads, without the locking overhead of a full serializable level. Even so, it still allows a subtle but dangerous anomaly known as write skew, where two transactions independently make decisions that each look consistent on their own but together violate a business rule. Serializable snapshot isolation, or SSI, closes exactly that last gap. This article explains both isolation levels in detail and walks through a classic write skew example step by step.