[Zope] Double and Triple Posts

[Chris McDonough]

This is almost certainly due to ZODB conflict errors, which are an
artifact of ZODB using an "optimistic concurrency" strategy for
concurrent updates.  A lot of times  people will come to the list and
say "My application is sending two or three mails when it should only
send one!" and that's a giveaway symptom of the code that sends the mail
experiencing a conflict error and retrying.

Conflict errors are normal to a degree.  If a ZODB conflict error
happens, Zope retries the request up to three times.  The retry is
"invisible"; it happens within Zope itself.  Most Zope apps can deal
with it because they use transactional data storages only, which get
rolled back, and then the request is replayed.  It makes things slower,
but it eventually works.

Conflict errors are common during sessioning usage (because sessions use
ZODB "under the hood" and people who use sessions may not have a lot of
experience writing "conflict-resistant" code), but they can be mitigated
somewhat.

The best way to fix these is to change your code to maybe not do as much
work, to do its work faster, or to always use transactional data
storages instead of nontransactional ones (like syslog or MailHost, or
so forth).  In the case of sessioning, using multiple frames/windows
that access the session at the same time is a common source of conflict
errors too.  Someone "leaning" on their browser's reload button could
also cause conflict errors to occur.  Also, if you store your session
data in a ZEO storage, conflict errors are likely to occur more
frequently due to increased latency when committing transactions.

Unfortunately, there's no magic wand here and reviewing your code for
"conflict-resistance" borders on consulting work rather than simple
maillist help (it's particularly time-consuming and usually requires
actually running the application), so unless you ditch Zope sessions,
implement a more conflict resistant sessioning storage, or hire somebody
who is intimiately familiar with ZODB, you will likely need to learn
what causes these errors and how to mitigate them (or at least work
around them), maybe even by trial and error.

It's worthwhile to know that any access to a Zope session actually has
the potential to *write* to the ZODB (even though it may appear you are
just reading), so heavy session usage is a prime candidate for
generating conflict errors.  However, I've reduced them to an acceptable
level in my own projects, so I know it's possible.

See also "Conflict Errors" within
http://www.plope.com/Books/2_7Edition/Sessions.stx .. this work by
Michael Dunstan might also be interesting to you..
http://www.plope.com/Members/dunny/conflicts/view

HTH,

- C

PS: please don't send HTML email to the list, it makes it hard for
people searching the maillist archives to read threads; thanks!

On Tue, 2005-03-15 at 22:06, Andy Yates wrote:
> I have a form that posts to a python script which can take several
> seconds to run.  Sometimes the script gets called 2 or 3 times in the
> same request.  It is not easy to reproduce but it seems to happen at
> least once every 20 requests.  This is happening on two different
> versions of Zope and in two completely different projects.
> 
>  
> 
> Zope only logs a single request.  However, I know it is getting called
> more than once because the script writes to syslog.  
> 
>  
> 
> I’ve packet sniffed the browser and it is only sending one post.  It
> happens in IE and FireFox but seems to be easier to reproduce in IE. 
> IE will let you click more than once on the submit button.  When I
> click the submit button twice sometimes I get 3 or 4 runs of the
> script.  I would expect to get only two.  It seems to happen much more
> often when I do this.  (FireFox will only let you press the submit
> button once)
> 
>  
> 
> It seems to happen if the script reads and writes values to
> context.REQUEST.SESSION.  When I take this out it seems to stop.  (It
> can be hard to reproduce)
> 
>  
> 
> To reproduce this I created a script that calls an external method to
> syslog a message.  Then it reads and writes to
> context.REQUEST.SESSION.   Then it calls another external method to
> sleep 15 seconds.   Finally it prints a message and returns.
> 
>  
> 
>  
> 
> I see this behavior on Zope-2.7.3 and Zope-2.6.1.  
> 
>  
> 
>  
> 
> context.zsyslog('Snooze test' )
> 
>  
> 
> foo = context.REQUEST.SESSION.get('foo', 1)
> 
> if foo==1 :
> 
>     context.REQUEST.SESSION.set('foo', 0)
> 
>  
> 
> context.REQUEST.SESSION.set('bar',123)
> 
>  
> 
> context.Snooze(15)
> 
>  
> 
> print "I'm awake now!"
> 
> return printed
> 
>  
> 
> Please let me know if this is happening anywhere else.  
> 
>  
> 
> Andy
> 
> 
>  
> 
> 
> 
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