On Wednesday, November 30, 2011 11:44:20 AM Adam GROSZER wrote: > Hello, > > I thought BTrees have conflict resolution code. > > We have here a weird conflict here, the BTree in question is a > SessionDataContainer. The conflict seems to appear when one transaction > removes and the other adds an entry.
I am working with Adam on this, so here is some more detail. First of all, this clearly does not happen all the time. It seems to happen only infrequently, which I think is in cases where the buckets of the BTree get reorganized due to a deletion sweep. I think the reason why this is a relatively common scenario is: 1. There is a lot of activity on the site at some point. 2. Then there is a prolonged period of inactivity, which causes a bunch of sessions to time out. 3. Then activity is starting again, causing the first transaction to do a large session removal sweep (which causes the BTree restructuring) and the next concurrent transaction to add a new session causing a conflict. At least for sessions, conflict resolution is not that difficult, because we are always dealing with unique keys and keys are never re-assigned to another object. So it effectively becomes a set conflict resolution issue. Also, I think that BTrees might be overkill for this problem, since the amount of active authenticated users on a system is usually small. And even if there is a large amount of users, the key->session mapping will always reside in the ZODB object cache (though there is some overhead to distributing new versions of the mapping due to the relatively high number of writes.) I am tempted to write a conflict resolving dictionary for this, but I would love hear whether someone has done something like this already. Regards, Stephan -- Entrepreneur and Software Geek Google me. "Zope Stephan Richter" _______________________________________________ For more information about ZODB, see http://zodb.org/ ZODB-Dev mailing list - ZODB-Dev@zope.org https://mail.zope.org/mailman/listinfo/zodb-dev