25.3. Transaction Recovery
The Transaction Manager coordinates the recovery process and works with Red Hat JBoss Data Grid to determine which transactions require manual intervention to complete operations. This process is known as transaction recovery.
JBoss Data Grid uses JMX for operations that explicitly force transactions to commit or roll back. These methods receive byte arrays that describe the XID instead of the number associated with the relevant transactions.
The System Administrator can use such JMX operations to facilitate automatic job completion for transactions that require manual intervention. This process uses the Transaction Manager's transaction recovery process and has access to the Transaction Manager's XID objects.
25.3.1. Transaction Recovery Process
The following process outlines the transaction recovery process in Red Hat JBoss Data Grid.
Procedure 25.3. The Transaction Recovery Process
- The Transaction Manager creates a list of transactions that require intervention.
- The system administrator, connected to JBoss Data Grid using JMX, is presented with the list of transactions (including transaction IDs) using email or logs. The status of each transaction is either
COMMITTEDorPREPARED. If some transactions are in bothCOMMITTEDandPREPAREDstates, it indicates that the transaction was committed on some nodes while in the preparation state on others. - The System Administrator visually maps the XID received from the Transaction Manager to a JBoss Data Grid internal ID. This step is necessary because the XID (a byte array) cannot be conveniently passed to the JMX tool and then reassembled by JBoss Data Grid without this mapping.
- The system administrator forces the commit or rollback process for a transaction based on the mapped internal ID.
25.3.2. Transaction Recovery Example
The following example describes how transactions are used in a situation where money is transferred from an account stored in a database to an account stored in Red Hat JBoss Data Grid.
Example 25.2. Money Transfer from an Account Stored in a Database to an Account in JBoss Data Grid
- The
TransactionManager.commit()method is invoked to run the two phase commit protocol between the source (the database) and the destination (JBoss Data Grid) resources. - The
TransactionManagertells the database and JBoss Data Grid to initiate the prepare phase (the first phase of a Two Phase Commit).
During the commit phase, the database applies the changes but JBoss Data Grid fails before receiving the Transaction Manager's commit request. As a result, the system is in an inconsistent state due to an incomplete transaction. Specifically, the amount to be transferred has been subtracted from the database but is not yet visible in JBoss Data Grid because the prepared changes could not be applied.
Transaction recovery is used here to reconcile the inconsistency between the database and JBoss Data Grid entries.
Note
To use JMX to manage transaction recoveries, JMX support must be explicitly enabled.
