Serializability

Serializable Snapshot Isolation (SSI)

SSI is the first serializable isolation level that scales well. It is Optimistic: it assumes things will be fine but checks for trouble at commit time.

How it works

The database tracks two things:

1. Detecting reads of stale MVCC versions: When a transaction commits, it checks if any of the values it read have since been updated.
2. Detecting writes that affect prior reads: When a transaction writes, it checks if any other active transaction has read that row.

If the database finds a potential "Write Skew" cycle, it aborts one of the transactions.

• Pros: No locking! Readers don't block writers. High concurrency.
• Cons: Abort rate can be high if there is heavy contention.