[Zope] Transactions and rollback

[Dennis Allison]

I am building a Zope site where a manager enrolls users.  A manager
completes a form that gathers information about the user.  When submitted,
the action routine does two things --

	.. updates a MySQL database with the user information
        .. updates acl_users locally to manage the login

The MySQL is of a vintage that supports transactions so (per the Zope
book) that update will go as a transaction and will either succeed or 
fail.  Likewise, the acl_user update is a transaction and will either 
succeed or fail.

My code needs to keep everything in sync and treat everything as a single
transaction, committing only when both the MySQL and the ZOODB
updates succeed, otherwise backing out.  The problem is figuring out how
to do this.  

Is there a general mechanism?

If there isn't, then another approach is to do the acl_user update first
since it can be undone (assuming I can get a handle on the transaction), 
then the MySQL transaction.  If the latter fails, I can back out of the 
acl_user transaction by undoing it.

I am using Z SQL methods for the MySQL side of things--they provide a
nice wrapper.  A few things are unclear from the docs.   What exception
gets raised by a Z SQL method if it does not commit?

On the acl_users side, the updates are managed through External Methods as
noted in the code.  Currently they just make calls on UserFolder per the
in-line documentation.  This can fail.  For example, what exception is
raised when an attempt is made to add a name already in use.  (Are these
documented somewhere?) I presume that an exception in the External Method
will get exported out to an enclosing <dtml-try> block so it can be
captured and delt with in the web portion of the system.

If I need to back off a committed transaction in the ZOODB, how is that
done programatically.  Presumably I can do the equivalent of the undo 
management screen progrmatically, but to do do, I need the handle of the 
transaction I want/need to undo.  I don't see how to get it.



Thanks for help.