Storage & Databases

Snapshot isolation

Each transaction sees a consistent snapshot taken at start.

In plain terms

Postgres, Oracle default. Prevents dirty reads, non-repeatable reads, phantoms — but allows write-skew anomalies.

Origin

Berenson et al., "A Critique of ANSI SQL Isolation Levels," 1995 — the same paper that introduced the concept by name. Predates SSI (Serializable Snapshot Isolation) by Cahill, Röhm, Fekete in 2008.

Where it shows up in production

PostgreSQL REPEATABLE READ is actually snapshot isolation (admits write-skew). SERIALIZABLE adds SSI on top.
Oracle SERIALIZABLE in Oracle is actually SI by default — a common source of cross-database surprise.
CockroachDB / FoundationDB Use snapshot semantics underneath their stronger commercial guarantees.

On Semicolony

Simulator isolation levels

Sources & further reading

Paper Berenson et al. — A Critique of ANSI SQL Isolation Levels (1995)
Paper Cahill et al. — SSI (2008)

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