Optimistic Concurrency

Optimistic locking lets concurrent transactions process simultaneously, but detects and prevent collisions, this works best for applications where most concurrent transactions do not conflict. JPA Optimistic locking allows anyone to read and update an entity, however a version check is made upon commit and an exception is thrown if the version was updated in the database since the entity was read.  In JPA for Optimistic locking you annotate an attribute with @Version as shown below:

public class Employee {     @ID int id;     @Version int version;

The Version attribute will be incremented with a successful commit. The Version attribute can be an int, short, long, or timestamp.  This results in SQL like the following:

“UPDATE Employee SET ..., version = version + 1      WHERE id = ? AND version = readVersion”

The advantages of optimistic locking are that no database locks are held which can give better scalability. The disadvantages are that the user or application must refresh and retry failed updates.

Optimistic Locking Example

In the optimistic locking example below, 2 concurrent transactions are updating employee e1. The transaction on the left commits first causing the e1 version attribute to be incremented with the update. The transaction on the right throws an OptimisticLockException because the e1 version attribute is higher than when e1 was read, causing the transaction to roll back.

Additional Locking with JPA Entity Locking APIs

With JPA it is possible to lock an entity, this allows you to control when, where and which kind of locking to use. JPA 1.0 only supported Optimistic read or Optimistic write locking.  JPA 2.0 supports Optimistic and Pessimistic locking, this is layered on top of @Version checking described above.

JPA 2.0 LockMode values :

• OPTIMISTIC (JPA 1.0 READ):
  • perform a version check on locked Entity before commit, throw an OptimisticLockException if Entity version mismatch.
• OPTIMISTIC_FORCE_INCREMENT (JPA 1.0 WRITE)
  • perform a version check on locked Entity before commit, throw an OptimisticLockException if Entity version mismatch, force an increment to the version at the end of the transaction, even if the entity is not modified.

• PESSIMISTIC:
  • lock the database row when reading
    
• PESSIMISTIC_FORCE_INCREMENT
  • lock the database row when reading, force an increment to the version at the end of the transaction, even if the entity is not modified.

There are multiple APIs to specify locking an Entity:

• EntityManager methods: lock, find, refresh
• Query methods: setLockMode 
• NamedQuery annotation: lockMode element
  

OPTIMISTIC (READ) LockMode Example

In the optimistic locking example below,  transaction1 on the left updates the department name for dep , which causes dep's version attribute to be incremented. Transaction2 on the right gives an employee a raise if he's in the "Eng" department. Version checking on the employee attribute would not throw an exception in this example since it was the dep Version attribute that was updated in transaction1. In this example the employee change should not commit if the department was changed after reading, so an OPTIMISTIC lock is used : em.lock(dep, OPTIMISTIC).  This will cause a version check on the  dep Entity before committing transaction2  which will throw an OptimisticLockException because the dep version attribute is higher than when dep was read, causing the transaction to roll back.

OPTIMISTIC_FORCE_INCREMENT (write) LockMode Example

In the OPTIMISTIC_FORCE_INCREMENT locking example below,  transaction2 on the right wants to be sure that the dep name does not change during the transaction, so transaction2 locks the dep Entity em.lock(dep, OPTIMISTIC_FORCE_INCREMENT) and then calls em.flush() which causes dep's version attribute to be incremented in the database. This will cause any parallel updates to dep  to throw an OptimisticLockException and roll back. In transaction1 on the left at commit time when the dep version attribute is checked and found to be stale, an OptimisticLockException is thrown

Pessimistic Concurrency

Pessimistic concurrency locks the database row when data is read, this is the equivalent of a (SELECT . . . FOR UPDATE [NOWAIT]) .  Pessimistic locking ensures that transactions do not update the same entity at the same time, which can simplify application code, but it limits concurrent access to the data which can cause bad scalability and may cause deadlocks. Pessimistic locking is better for applications with a higher risk of contention among concurrent transactions.
The examples below show:

1. reading an entity and then locking it later
2. reading an entity with a lock
3. reading an entity, then later refreshing it with a lock

The Trade-offs are the longer you hold the lock the greater the risks of bad scalability and deadlocks. The later you lock the greater the risk of stale data, which can then cause an optimistic lock exception, if the entity was updated after reading but before locking.


The right locking approach depends on your application:

• what is the risk of risk of contention among concurrent transactions?
• What are the requirements for scalability?
• What are the requirements for user re-trying on failure?

References and More Information:

Preventing Non-Repeatable Reads in JPA Using EclipseLink
Java Persistence API 2.0: What's New ?
What's New and Exciting in JPA 2.0
Beginning Java™ EE 6 Platform with GlassFish™ 3
Pro EJB 3: Java Persistence API (JPA 1.0)
Java Persistence API: Best Practices and Tips