Designing Data-Intensive Applications: The Big Ideas Behind Reliable, Scalable, and Maintainable Systems

by Martin Kleppmann

Read skew A client sees different parts of the database at different points in time. Some cases of read skew are also known as nonrepeatable reads. This issue is most commonly prevented with snapshot isolation, which allows a transaction to read from a consistent snapshot corresponding to one particular point in time. It is usually implemented with multi-version concurrency control (MVCC). Lost updates Two clients concurrently perform a read-modify-write cycle. One overwrites the other’s write without incorporating its changes, so data is lost. Some implementations of snapshot isolation prevent this anomaly automatically, while others require a manual lock (SELECT FOR UPDATE). Write skew A transaction reads something, makes a decision based on the value it saw, and writes the decision to the database. However, by the time the write is made, the premise of the decision is no longer true. Only serializable isolation prevents this anomaly. Phantom reads A transaction reads objects that match some search condition. Another client makes a write that affects the results of that search. Snapshot isolation prevents straightforward phantom reads, but phantoms in the context of write skew require special treatment, such as index-range locks.