Conflict resolution in ZODB

Question

I do run parallel write requests on my ZODB. I do have multiple BTree instances inside my ZODB. Once the server accesses the same objects inside such a BTree, I get a ConflictError for the IOBucket class. For all my Django bases classes I do have _p_resolveconflict set up, but can't implement it for IOBucket 'cause its a C based class.

I did a deeper analysis, but still don't understand why it complains about the IOBucket class and what it writes into it. Additionally, what would be the right strategy to resolve it?

Thousand thanks for any help!

Answer 1

IOBucket is part of the persistence structure of a BTree; it exists to try and reduce conflict errors, and it does try and resolve conflicts where possible.

That said, conflicts are not always avoidable, and you should restart your transaction. In Zope, for example, the whole request is re-run up to 5 times if a ConflictError is raised. Conflicts are ZODB's way of handling the (hopefully rare) occasion where two different requests tried to change the exact same data structure.

Restarting your transaction means calling transaction.begin() and applying the same changes again. The .begin() will fetch any changes made by the other process and your commit will be based on the fresh data.