Chapter 19. Set Up Isolation Levels

19.1. About Isolation Levels

Isolation levels determine when readers can view a concurrent write. READ_COMMITTED and REPEATABLE_READ are the two isolation modes offered in Red Hat JBoss Data Grid.

  • READ_COMMITTED. This isolation level is applicable to a wide variety of requirements. This is the default value in Remote Client-Server and Library modes.

  • REPEATABLE_READ.

    Important

    The only valid value for locks in Remote Client-Server mode is the default READ_COMMITTED value. The value explicitly specified with the isolation value is ignored.

    If the locking element is not present in the configuration, the default isolation value is READ_COMMITTED.

For isolation mode configuration examples in JBoss Data Grid, see the lock striping configuration samples:

19.2. About READ_COMMITTED

READ_COMMITTED is one of two isolation modes available in Red Hat JBoss Data Grid.

In JBoss Data Grid’s READ_COMMITTED mode, write operations are made to copies of data rather than the data itself. A write operation blocks other data from being written, however writes do not block read operations. As a result, both READ_COMMITTED and REPEATABLE_READ modes permit read operations at any time, regardless of when write operations occur.

In READ_COMMITTED mode multiple reads of the same key within a transaction can return different results due to write operations in different transactions modifying data between reads. This phenomenon is known as non-repeatable reads and is avoided in REPEATABLE_READ mode.

19.3. About REPEATABLE_READ

REPEATABLE_READ is one of two isolation modes available in Red Hat JBoss Data Grid.

Traditionally, REPEATABLE_READ does not allow write operations while read operations are in progress, nor does it allow read operations when write operations occur. This prevents the "non-repeatable read" phenomenon, which occurs when a single transaction has two read operations on the same row but the retrieved values differ (possibly due to a write operation modifying the value between the two read operations).

JBoss Data Grid’s REPEATABLE_READ isolation mode preserves the value of an entry before a modification occurs. As a result, the "non-repeatable read" phenomenon is avoided because a second read operation on the same entry retrieves the preserved value rather than the new modified value. As a result, the two values retrieved by the two read operations in a single transaction will always match, even if a write operation occurs in a different transaction between the two reads.