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.