I.5. Restrictions on XA Transactions
XA transaction support is limited to the InnoDB
storage engine.
The MySQL XA implementation is for “external XA,”
where a MySQL server acts as a Resource Manager and client
programs act as Transaction Managers. “Internal XA”
is not implemented. This would allow individual storage engines
within a MySQL server to act as RMs, and the server itself to act
as a TM. Internal XA is required for handling XA transactions that
involve more than one storage engine. The implementation of
internal XA is incomplete because it 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 not supported.
For XA END, the SUSPEND [FOR
MIGRATE] clause is not supported.
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.
If an XA transaction has reached the PREPARED
state and the MySQL server is killed (for example, with
kill -9 on Unix) or shuts down abnormally, the
transaction can be continued after the server restarts. However,
if the client reconnects and commits the transaction, the
transaction will be absent from the binary log even though it has
been committed. This means the data and the binary log have gone
out of synchrony. An implication is that XA cannot be used safely
together with replication.
It is possible that the server will roll back a pending XA
transaction, even one that has reached the
PREPARED state. This happens if a client
connection terminates and the server continues to run, or if
clients are connected and the server shuts down gracefully. (In
the latter case, the server marks each connection to be
terminated, and then rolls back the PREPARED XA
transaction associated with it.) It should be possible to commit
or roll back a PREPARED XA transaction, but
this cannot be done without changes to the binary logging
mechanism.
