README.md

A Locking mode example (Optimistic Locking)

This example demonstrates a locking mode example with optimistic locking.

In JPA "Optimistic Read Lock" provides Repeatable Read transaction isolation level that prevents the so-called non-repeatable read anomaly.

Transaction Anomalies

The ANSI/ISO SQL standard defines four levels of transaction isolation in terms of three phenomena that must be prevented between concurrent transactions. These undesirable phenomena are:

dirty reads

A transaction reads data written by concurrent uncommitted transaction.

non-repeatable reads

A transaction re-reads data it has previously read and finds that data has been modified by another transaction (that committed since the initial read).

phantom read

A transaction re-executes a query returning a set of rows that satisfy a search condition and finds that the set of rows satisfying the condition has changed due to another recently-committed transaction.

The four transaction isolation levels and the corresponding behaviors are described in following table:

Isolation Level	Dirty Read	Non-Repeatable Read	Phantom Read
Read uncommitted	Possible	Possible	Possible
Read committed	Not possible	Possible	Possible
Repeatable read	Not possible	Not possible	Possible
Serializable	Not possible	Not possible	Not possible

We should remember that for versioned entities optimistic locking is available by default. (source: baeldung.com/jpa-optimistic-locking)

JPA supports Read Committed isolation level by default.