A question of "MySQL external XA and internal XA"

[Haihao Tang]

Hi,
    I am recently in the study of MySQL XA. In the section
"Restrictions on XA Transactions" of MySQL reference 5.1
(http://dev.mysql.com/doc/refman/5.1/en/xa-restrictions.html), it
wrote:
    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.
    I understand the "external XA" where MySQL server acts as RM, but
the "internal XA" confuse me. How does MySQL server itself act as a TM
and storage engine (It could be only InnoDB, right?) act as RM? How
does MySQL server manage storage engines to accomplish a DTP? Any
examples?
    Thank you!

Best Regards,

                   Robin

                   2010-10-8