How to get rid of ConflictError on ZEO workers?

Question

Looking at my ZEO workers I get to see quite a lot of:

2013-10-18T11:59:54 INFO ZPublisher.Conflict ConflictError at
/VirtualHostBase/http/www.domain.com:80/Plone/VirtualHostRoot/:
database conflict error (oid 0x533cd5, class
persistent.mapping.PersistentMapping) (78 conflicts (0 unresolved)
since startup at Mon Oct 14 04:09:45 2013)

As they are logged as INFO should I assume that is not harmful at all?

And I guess that if there is a conflict is because there are too much writes on the ZODB?

Answer 1

The conflicts are indeed caused because two requests are trying to change a PersistentMapping at the same time. One of these is then forced to retry the commit.

Use these entries to pinpoint bottlenecks in your application; perhaps replace the specific mapping with a BTree.OOBTree which minimizes conflicts by spreading key-value pairs out over separate persistent buckets.

Without traffic data and what that specific PersistentMapping holds or what your application does with it, it is impossible to say if 78 conflicts in 4 days is a lot or a little, and if it is worth your while switching to a different container.

Answer 2

Conflict errors are not -- in themselves -- harmful. The ZEO server will retry several times to resolve the error. But they are a sign of write-contention in the database, and a lot of them will indicate that you have a bottleneck in your current configuration. Your users soon will be complaining of poor performance.

You should probably begin analysis to determine if you've some add-on package that's doing excessive or very inefficient writes to the database. The worst case, for example, would be some code that's trying to write to the database on every page load like a traffic logger. The ZODB is optimized for reading, not writing, and those operations should be redesigned to put their data stores somewhere other than the ZODB.

If it's just content writes that are the problem, look to reduce catalog indexes and metadata. If at all possible, replace old Archetypes-style content with Dexterity content types. Dexterity is far more efficient in content creation.