----- Original Message ----- From: "Dennis Allison" <[EMAIL PROTECTED]>
To: "Jonathan" <[EMAIL PROTECTED]>
Cc: <zope@zope.org>
Sent: Tuesday, July 04, 2006 1:27 PM
Subject: Re: [Zope] Trying to trap ConflictError


I have been lurking, following your thread, but not paying full
attention.  Excuse me if thes points have been made before.

There are many long threads on Conflict Errors.  You should read them.
Note that there have been significant changes in management strategy
beginning with Zope 2.9.X.

A "Conflict Error" is not really an error in the usual sense;  it is an
indication of conflicting transactions in the ZODB, that is, transactions
have violated the optimistic locking used by the ZODB.  In general,
Conflict Errors are resolved without problem.  The resolution usually
involves rolling back the older transaction and committing the newer
transaction.  When the actions associated with the rolled back transaction
are not themselves fully transactional (for example, a file write) there
can be unanticipated side effects.

Your conflicts appear to be in Temporary Storage which is used for
Session Variables.  The version that ships with Zope is flexible and
complex and, IMHO, overkill.  Moreover, it has some unexpected properties,
the most significant is that, even though session variables are stored in
RAM, it occasionally uses the ZODB to manage session variable expiration.
You can often decrease the number of session variable induced conflicts by
increasing the lifetime of session variables.

As both you and Andreas have recommended, I have started researching 'ZODB Conflict Resolution' and if I come up with generic/reusable external method application-level solution I will post it.

By way of providing more information:

I am not using sessions. I am using TemporaryStorage to hold small, custom-made, short-lived image objects (the use case requires that each user views a static image with a custom overlay, and that the user accesses this customized image via an html img tag). The custom images are viewed only once, and then can be deleted. Hence the use of TemporaryStorage (to eliminate many disk writes and ZODB bloating). The image objects stored in TemporaryStorage are never overwritten, edited etc (which is causing me some confusion as to why the conflict errors are occurring), they are simply created, stored in the TemporaryStorage folder (actually a BTreeFolder2 folder contained within the TemporaryStorage folder) and then 2 minutes later are deleted. I am guessing that the conflicts are occuring as the TemporaryStorage is trying to delete old images while new image object 'add requests' are piling up, but I have yet to determine the cause of the conflict with 100% certainty (conflicts only occur under heavy loading, and not always in the same place in the load testing).

Thanks again for everyone's help!


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