What's the difference between PESSIMISTIC_READ and PESSIMISTIC_WRITE in JPA?
I have read the article Locking and Concurrency in Java Persistence 2.0, and run the sample application. But I still can't realize the difference between PESSIMISTIC_READ and PESSIMISTIC_WRITE. I tried to modify the code, and where the code using PESSIMISTIC_READ and PESSIMISTIC_WRITE will have the same result that the SQL will invoked with for update.
The difference lies in locking mechanism.
PESSIMISTIC_READ lock means that dirty reads and non-repeatable reads are impossible when you have such a lock. If data should be changed it's required to obtain PESSIMISTIC_WRITE lock
PESSIMISTIC_WRITE lock guarantees that besides dirty and non-repeatable reads are impossible you can update data without obtaining additional locks(and possible deadlocks while waiting for exclusive lock).
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║ LockModeType ║ PESSIMISTIC_READ ║ PESSIMISTIC_WRITE ║
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║ type ║ SHARED LOCK ║ EXCLUSIVE LOCK ║
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║ isReadOnly without ║ ║ ║
║ additional locks ║ YES ║ NO ║
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║ dirty reads ║ NO ║ NO ║
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║ non-repeatable reads ║ NO ║ NO ║
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║ how to update data ║ obtain PESSIMISTIC_WRITE ║ ALLOWED ║
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║ ║ no one holds ║ no one holds ║
║ how to obtain lock ║ PESSIMISTIC_WRITE ║ PESSIMISTIC_READ or ║
║ ║ ║ PESSIMISTIC_WRITE ║
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║ ║ ║ when there is a high ║
║ ║ you want to ensure no ║ likelihood of deadlock or║
║ when to use ║ dirty or non-repeatable ║ update failure among ║
║ ║ reads are possible ║ concurrent updating ║
║ ║ ║ transactions ║
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Resources:
JPA 2.1
One is a read lock and the other is a write lock, or during a read or an update, respectively.
FTA:
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PESSIMISTIC_READ. The entity manager locks the entity as soon as a transaction reads it. The lock is held until the transaction completes. This lock mode is used when you want to query data using repeatable-read semantics. In other words, you want to ensure that the data is not updated between successive reads. This lock mode does not block other transactions from reading the data.
PESSIMISTIC_WRITE. The entity manager locks the entity as soon as a transaction updates it. This lock mode forces serialization among transactions attempting to update the entity data. This lock mode is often used when there is a high likelihood of update failure among concurrent updating transactions.