MySQL 8.0 Release Notes
MySQL 8.0 Source Code Documentation
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 replica, immediately after the XA transaction is prepared,
it is detached from the replication applier thread, and can be
committed or rolled back by any thread on the replica. 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:
Prior to MySQL 8.0.30, XA transactions are not fully resilient to an unexpected halt with respect to the binary log. If there is an unexpected halt while the server is in the middle of executing an
XA PREPARE,XA COMMIT,XA ROLLBACK, orXA COMMIT ... ONE PHASEstatement, the server might not be able to recover to a correct state, leaving the server and the binary log in an inconsistent state. In this situation, the binary log might either contain extra XA transactions that are not applied, or miss XA transactions that are applied. Also, if GTIDs are enabled, after recovery@@GLOBAL.GTID_EXECUTEDmight not correctly describe the transactions that have been applied. Note that if an unexpected halt occurs beforeXA PREPARE, betweenXA PREPAREandXA COMMIT(orXA ROLLBACK), or afterXA COMMIT(orXA ROLLBACK), the server and binary log are correctly recovered and taken to a consistent state.Beginning with MySQL 8.0.30, this is no longer an issue; the server implements
XA PREPAREas a two-phase operation, which maintains the state of the prepare operation between the storage engine and the server, and imposes order of execution between the storage engine and the binary log, so that state is not broadcast before it is consistent and persistent on the server node.You should be aware that, when the same transaction XID is used to execute XA transactions sequentially and a break occurs during the processing of
XA COMMIT ... ONE PHASE, it may no longer be possible to synchronize the state between the binary log and the storage engine. This can occur if the series of events just described takes place after this transaction has been prepared in the storage engine, while theXA COMMITstatement is still executing. This is a known issue.The use of replication filters or binary log filters in combination with XA transactions is not supported. Filtering of tables could cause an XA transaction to be empty on a replica, and empty XA transactions are not supported. Also, with the replica's connection metadata repository and applier metadata repository stored in
InnoDBtables, which became the default in MySQL 8.0, the internal state of the data engine transaction is changed following a filtered XA transaction, and can become inconsistent with the replication transaction context state.The error
ER_XA_REPLICATION_FILTERSis logged whenever an XA transaction is impacted by a replication filter, whether or not the transaction was empty as a result. If the transaction is not empty, the replica is able to continue running, but you should take steps to discontinue the use of replication filters with XA transactions in order to avoid potential issues. If the transaction is empty, the replica stops. In that event, the replica might be in an undetermined state in which the consistency of the replication process might be compromised. In particular, thegtid_executedset on a replica of the replica might be inconsistent with that on the source. To resolve this situation, isolate the source and stop all replication, then check GTID consistency across the replication topology. Undo the XA transaction that generated the error message, then restart replication.XA transactions are considered unsafe for statement-based replication. If two XA transactions committed in parallel on the source are being prepared on the replica in the inverse order, locking dependencies can occur that cannot be safely resolved, and it is possible for replication to fail with deadlock on the replica. This situation can occur for a single-threaded or multithreaded replica. When
binlog_format=STATEMENTis set, a warning is issued for DML statements inside XA transactions. Whenbinlog_format=MIXEDorbinlog_format=ROWis set, DML statements inside XA transactions are logged using row-based replication, and the potential issue is not present.