MySQL 8.0 Reference Manual Including MySQL NDB Cluster 8.0

13.3.8.3 Restrictions on XA Transactions

XA transaction support is limited to the InnoDB storage engine.

For external XA, a MySQL server acts as a Resource Manager and client programs act as Transaction Managers. For Internal XA, storage engines within a MySQL server act as RMs, and the server itself acts as a TM. Internal XA support is limited by the capabilities of individual storage engines. Internal XA is required for handling XA transactions that involve more than one storage engine. The implementation of internal XA requires that a storage engine support two-phase commit at the table handler level, and currently this is true only for InnoDB.

For XA START, the JOIN and RESUME clauses are recognized but have no effect.

For XA END the SUSPEND [FOR MIGRATE] clause is recognized but has no effect.

The requirement that the bqual part of the xid value be different for each XA transaction within a global transaction is a limitation of the current MySQL XA implementation. It is not part of the XA specification.

An XA transaction is written to the binary log in two parts. When XA PREPARE is issued, the first part of the transaction up to XA PREPARE is written using an initial GTID. A XA_prepare_log_event is used to identify such transactions in the binary log. When XA COMMIT or XA ROLLBACK is issued, a second part of the transaction containing only the XA COMMIT or XA ROLLBACK statement is written using a second GTID. Note that the initial part of the transaction, identified by XA_prepare_log_event, is not necessarily followed by its XA COMMIT or XA ROLLBACK, which can cause interleaved binary logging of any two XA transactions. The two parts of the XA transaction can even appear in different binary log files. This means that an XA transaction in PREPARED state is now persistent until an explicit XA COMMIT or XA ROLLBACK statement is issued, ensuring that XA transactions are compatible with replication.

On a replication slave, immediately after the XA transaction is prepared, it is detached from the slave applier thread, and can be committed or rolled back by any thread on the slave. This means that the same XA transaction can appear in the events_transactions_current table with different states on different threads. The events_transactions_current table displays the current status of the most recent monitored transaction event on the thread, and does not update this status when the thread is idle. So the XA transaction can still be displayed in the PREPARED state for the original applier thread, after it has been processed by another thread. To positively identify XA transactions that are still in the PREPARED state and need to be recovered, use the XA RECOVER statement rather than the Performance Schema transaction tables.

The following restrictions exist for using XA transactions:

Note

Prior to MySQL 5.7.7, XA transactions were not compatible with replication at all. This was because an XA transaction that was in PREPARED state would be rolled back on clean server shutdown or client disconnect. Similarly, an XA transaction that was in PREPARED state would still exist in PREPARED state in case the server was shutdown abnormally and then started again, but the contents of the transaction could not be written to the binary log. In both of these situations the XA transaction could not be replicated correctly.