Transaction isolation

You can use one of three transaction isolation levels to tune the locking semantics that maintain consistency in each cache map: repeatable read, read committed and read uncommitted.

Transaction isolation overview

Transaction isolation defines how the changes that are made by one operation become visible to other concurrent operations.

You can define the following transaction isolation levels to tune the locking semantics that eXtreme Scale uses to maintain consistency in each cache map: repeatable read, read committed and read uncommitted.

You can set the transaction isolation level in one of the following ways:

With the txIsolation attribute in the ObjectGrid descriptor XML file. For more information, see ObjectGrid descriptor XML file.
On the Session interface with the setTransactionIsolation method. The transaction isolation can be changed any time during the life of the session, if a transaction is not currently in progress.
On the IGridTransaction interface with the TransactionIsolationLevel property.

The product enforces the various transaction isolation semantics by adjusting the way in which shared (S) locks are requested and held. Transaction isolation has no effect on maps that are configured to use the optimistic locking or no locking or when upgradeable (U) locks are acquired.

Repeatable read with pessimistic locking

The repeatable read transaction isolation level is the default. This isolation level prevents dirty reads and non-repeatable reads, but does not prevent phantom reads. A dirty read is a read operation that occurs on data that has been modified by a transaction but has not been committed. A non-repeatable read might occur when read locks are not acquired when performing a read operation. A phantom read can occur when two identical read operations are performed, but two different sets of results are returned because an update has occurred on the data between the read operations. In Java applications, phantom reads are possible when you are using queries or indexes because locks are not acquired for ranges of data, only for the cache entries that match the index or query criteria. The product achieves a repeatable read by holding onto any S locks until the transaction that owns the lock completes. Because an X lock is not granted until all S locks are released, all transactions holding the S lock are guaranteed to see the same value when re-read.

Read committed with pessimistic locking

The read committed transaction isolation level can be used with eXtreme Scale, which prevents dirty reads, but does not prevent non-repeatable reads or phantom reads, so eXtreme Scale continues to use S locks to read data from the cache map, but immediately releases the locks.

Read uncommitted with pessimistic locking

The read uncommitted transaction isolation level can be used with eXtreme Scale, which is a level that allows dirty reads, non-repeatable reads and phantom reads.