Chapter 25. Set Up Transactions

25.1. About Transactions

A transaction consists of a collection of interdependent or related operations or tasks. All operations within a single transaction must succeed for the overall success of the transaction. If any operations within a transaction fail, the transaction as a whole fails and rolls back any changes. Transactions are particularly useful when dealing with a series of changes as part of a larger operation.
In Red Hat JBoss Data Grid, transactions are only available in Library mode.

25.1.1. About the Transaction Manager

In Red Hat JBoss Data Grid, the Transaction Manager coordinates transactions across a single or multiple resources. The responsibilities of a Transaction Manager include:
  • initiating and concluding transactions
  • managing information about each transaction
  • coordinating transactions as they operate over multiple resources
  • recovering from a failed transaction by rolling back changes

25.1.2. XA Resources and Synchronizations

XA Resources are fully fledged transaction participants. In the prepare phase (see Section F.7, “About Two Phase Commit (2PC)” for details), the XA Resource returns a vote with either the value OK or ABORT. If the Transaction Manager receives OK votes from all XA Resources, the transaction is committed, otherwise it is rolled back.
Synchronizations are a type of listener that receive notifications about events leading to the transaction life cycle. Synchronizations receive an event before and after the operation completes.
Unless recovery is required, it is not necessary to register as a full XA resource. An advantage of synchronizations is that they allow the Transaction Manager to optimize 2PC (Two Phase Commit) with a 1PC (One Phase Commit) where only one other resource is enlisted with that transaction (last resource commit optimization). This makes registering a synchronization more efficient.
However, if the operation fails in the prepare phase within Red Hat JBoss Data Grid, the transaction is not rolled back and if there are more participants in the transaction, they can ignore this failure and commit. Additionally, errors encountered in the commit phase are not propagated to the application code that commits the transaction.
By default JBoss Data Grid registers to the transaction as a synchronization.

25.1.3. Optimistic and Pessimistic Transactions

Pessimistic transactions acquire the locks when the first write operation on the key executes. After the key is locked, no other transaction can modify the key until this transaction is committed or rolled back. It is up to the application code to acquire the locks in correct order to prevent deadlocks.
With optimistic transactions locks are acquired at transaction prepare time and are held until the transaction commits (or rolls back). Also, Red Hat JBoss Data Grid sorts keys for all entries modified within a transaction automatically, preventing any deadlocks occurring due to the incorrect order of keys being locked. This results in:
  • less messages being sent during the transaction execution
  • locks held for shorter periods
  • improved throughput

Note

Read operations never acquire any locks. Acquiring the lock for a read operation on demand is possible only with pessimistic transactions, using the FORCE_WRITE_LOCK flag with the operation.

25.1.4. Write Skew Checks

A common use case for entries is that they are read and subsequently written in a transaction. However, a third transaction can modify the entry between these two operations. In order to detect such a situation and roll back a transaction Red Hat JBoss Data Grid offers entry versioning and write skew checks. If the modified version is not the same as when it was last read during the transaction, the write skew checks throws an exception and the transaction is rolled back.
Enabling write skew check requires the REPEATABLE_READ isolation level. Also, in clustered mode (distributed or replicated modes), set up entry versioning. For local mode, entry versioning is not required.

Important

With optimistic transactions, write skew checks are required for (atomic) conditional operations.

25.1.5. Transactions Spanning Multiple Cache Instances

Each cache operates as a separate, standalone Java Transaction API (JTA) resource. However, components can be internally shared by Red Hat JBoss Data Grid for optimization, but this sharing does not affect how caches interact with a Java Transaction API (JTA) Manager.