MySQL's XA implementation is broked.

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+# MySQL's Two-Phase Commit Implementation Is Broken
+
+From [the fine
+manual](http://dev.mysql.com/doc/refman/5.5/en/xa-restrictions.html):
+
+> 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**.
+
+(Emphasis mine.)
+
+If you're solving the kinds of problems where two-phase commit and XA
+transaction management look attractive, then you very likely have the kinds of
+uptime requirements that make replication mandatory. "It works, but not with
+replication" is effectively "it doesn't work".
+
+> 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.
+
+XA transaction managers assume that if every resource successfully reaches the
+PREPARED state, then every resource will be able to commit the transaction
+"eventually". Resources that unilaterally roll back PREPARED transactions
+violate this assumption pretty badly.