I am confused about the working of LockModeTypes in JPA:
LockModeType.Optimistic
LockModeType.OPTIMISTIC_FORCE_INCREMENT
LockModeType
.LockModeType.PESSIMISTIC_READ
select for update nowait
(if no hint timeout specified)..Read
lock?LockModeType.PESSIMISTIC_WRITE
select for update nowait
(if no hint timeout specified).LockModeType.PESSIMISTIC_READ
as I see both fires same queries?LockModeType.PESSIMISTIC_FORCE_INCREMENT
select for update nowait
(if no hint timeout specified) and also increments the version number.for update no wait
is there?A lock with LockModeType. PESSIMISTIC_WRITE can be used when querying data and there is a high likelihood of deadlock or update failure among concurrent updating transactions. The persistence implementation must support use of locks of type LockModeType.
There are two models for locking data in a database: Optimistic locking , where a record is locked only when changes are committed to the database. Pessimistic locking , where a record is locked while it is edited.
Pessimistic Locking: When a user accesses an object to update it, the database locks the object until the update is completed. No other user can read or update the object until the first user releases the lock. The database offers this locking type.
1.2 Optimistic Locking in Hibernate Version checks the version numbers or the timestamps to detect conflicting updates and to prevent lost updates. In here, A record is only locked while updating and when it is updated, hibernate increments the version count by one.
I would first differentiate between optimistic and pessimistic locks, because they are different in their underlying mechanism.
Optimistic locking is fully controlled by JPA and only requires additional version column in DB tables. It is completely independent of underlying DB engine used to store relational data.
On the other hand, pessimistic locking uses locking mechanism provided by underlying database to lock existing records in tables. JPA needs to know how to trigger these locks and some databases do not support them or only partially.
Now to the list of lock types:
LockModeType.Optimistic
Example:
`LockModeType lockMode = resolveLockMode(); A a = em.find(A.class, 1, lockMode);`
LockModeType.OPTIMISTIC_FORCE_INCREMENT
LockModeType.PESSIMISTIC_READ
LockModeType.PESSIMISTIC_WRITE
, but different in one thing: until write lock is in place on the same entity by some transaction, it should not block reading the entity. It also allows other transactions to lock using LockModeType.PESSIMISTIC_READ
. The differences between WRITE and READ locks are well explained here (ObjectDB) and here (OpenJPA). If an entity is already locked by another transaction, any attempt to lock it will throw an exception. This behavior can be modified to waiting for some time for the lock to be released before throwing an exception and roll back the transaction. In order to do that, specify the javax.persistence.lock.timeout
hint with the number of milliseconds to wait before throwing the exception. There are multiple ways to do this on multiple levels, as described in the Java EE tutorial.LockModeType.PESSIMISTIC_WRITE
LockModeType.PESSIMISTIC_READ
. When WRITE
lock is in place, JPA with the help of the database will prevent any other transaction to read the entity, not only to write as with READ
lock.READ
lock. SELECT...FOR UPDATE
is really rather a WRITE
lock. It may be a bug in hibernate or just a decision that, instead of implementing custom "softer" READ
lock, the "harder" WRITE
lock is used instead. This mostly does not break consistency, but does not hold all rules with READ
locks. You could run some simple tests with READ
locks and long running transactions to find out if more transactions are able to acquire READ
locks on the same entity. This should be possible, whereas not with WRITE
locks.PESSIMISTIC
and OPTIMISTIC
mechanisms. Using plain PESSIMISTIC_WRITE
would fail in following scenario: LockModeType.PESSIMISTIC_FORCE_INCREMENT
will force transaction B to update version number and causing transaction A to fail with OptimisticLockException
, even though B was using pessimistic locking.If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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