[Zope-DB] Avoid Lost Updates?

Martin Gebert Murphy.Gebert@gmx.de
Wed, 16 Oct 2002 12:57:00 +0200

Hi all!

I didn't found much about this issue, so I'd like to ask this list. 
Please correct me if I'm stating anything wrong...

Zope provides transaction management for RDBMS bound to it's internal 
transactions - begin and end of a TA are marked by Request and Response 
(request for the page until end of uploading it). This surely is a 
convincing and quite secure concept, working for the most practical 
The problem is how to manage longer transactions which spread over two 
or more requests, esp. in combination with locking? Locking is bound to 
commit/rollback cycles, too.

The problem I face is the following:
Consider a person retrieving his personal data from an RDBSM 
(MySQL/InnoDB) via Web (Zope) to change it. After that he presses the 
submit button to write the data back.
If this person takes a while for entering, and some other person (e. g. 
with a manager role) changes his data in the meanwhile and saves 
*before* the other one, this update will be overwritten with the update 
of our first person and is simply lost.
Doing an exclusive lock ("for update") on retrieving isn't of much use, 
for the lock will be set back at the moment the result page was sent to 
the client; the transaction is at it's end there and a new one begins, 
ending with sending the updated data back to the DBMS (or cancelling the 

I know this is an old problem, and due to the statelessness of the HTTP 
protocol. But I've already seen a Zope site which was trying to keep the 
Request up until the submitting of the site, which showed as a 
neverending retrieval of the page (the "Loading..." sign in the browser 
didn't stop until you either hit the "Stop" button, went to another 
page, or pressed "Submit").

My question is:
Is there any way in Zope to extend TAs over a couple Requests, e. g. to 
keep the Request open as described above? Is this a practical solution, 
and what are the problems (there are at least timeout issues)? May there 
be a solution which is based on the DBMS (ignoring commits to keep the 
TA open)??? Or is this one of the long-known-and-so-far-unresolved 
problems of Web programming?

Thank you for reading that far!