[Zope-DB] Re: Trying to catching ZPsycopg exceptions aborts the transaction

Josef Meile jmeile at hotmail.com
Tue Nov 30 17:54:07 EST 2004


> Every time I see FASTER in connection with databases I get sceptical. FASTER 
> in what sense? Execution speed is in my experience not generally an issue 
> (of course, there are lots of cases where it is important), FASTER 
> maintenance, is.
Perhaps you are right. I haven't done a real test, so, I can assure

 > I think you need to rethink your application. CONSTRAINTS is to
 > enforce data integrity and should not be part of an application. There
 > are times for wanting to go outside Zope's transaction machinery but
 > this is easy enough using an external method and well-documented in
 > these archives. But that isn't the issue, I think.
Isn't the same if you do this from a python product, which is what I'm
actually doing?

> This seems unnecessary complex and, dare I say it, redundant and thus likely 
> to cause problems.
Yes, I know it is redundant, but it was a way to find overlapping
ranges. I'm not happy with it either.

> Why will it take longer? How much longer will it take? Does it really matter 
> here? What about if these requests are running concurrently?
> You need to consider your model carefully and check for the existance of 
> others - there must be because this is a standard situation.
> As Dieter suggests explicit implentation of your logic: checking for 
> availability before booking makes more sense. You can then return 
> appropriate information to the user to act upon.
> "RESOURCE XYZ is not available for the requested time.
> It is available from AA:BB hours to CC:DD hours"
> Actually you might want to consider the more general issue and whether a 
> rules engine might not be helpful. After all: how do hotels deal with this 
> problem?
Yes, I'm also wondering how do they deal with the race conditions. You
said that one could return the appropiate information to the user, but
as you know html is static and if the user takes, let's say, 5 seconds 
to do the reservation, the information may be outdated. Anyway, I guess
after all one has to catch the exception some how.

> Good luck!
Thanks ;-)

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