Zope: Weird "couldn't install" error

Question

I get a weird error in my log-files when I start a Zope instance. The instance is running with a ZEO server and the Zope installation is a virtualenv (in /home/myUser/opt). I get this error with several Products, but Zope is working fine and these products are installed. Here is an example with the product BTreeFolder2:

2014-01-22T12:38:13 ERROR Application Couldn't install BTreeFolder2
Traceback (most recent call last):
  File "/home/myUser/opt/Zope2-2.13.21/local/lib/python2.7/site-packages/Zope2-2.13.21-py2.7.egg/OFS/Application.py", line 693, in install_product
    transaction.commit()
  File "/home/myUser/opt/Zope2-2.13.21/local/lib/python2.7/site-packages/transaction-1.1.1-py2.7.egg/transaction/_manager.py", line 89, in commit
    return self.get().commit()
  File "/home/myUser/opt/Zope2-2.13.21/local/lib/python2.7/site-packages/transaction-1.1.1-py2.7.egg/transaction/_transaction.py", line 329, in commit
    self._commitResources()
  File "/home/myUser/opt/Zope2-2.13.21/local/lib/python2.7/site-packages/transaction-1.1.1-py2.7.egg/transaction/_transaction.py", line 446, in _commitResources
    rm.tpc_vote(self)
  File "/home/myUser/opt/Zope2-2.13.21/local/lib/python2.7/site-packages/ZODB3-3.10.5-py2.7-linux-x86_64.egg/ZODB/Connection.py", line 781, in tpc_vote
    s = vote(transaction)
  File "/home/myUser/opt/Zope2-2.13.21/local/lib/python2.7/site-packages/ZODB3-3.10.5-py2.7-linux-x86_64.egg/ZEO/ClientStorage.py", line 1098, in tpc_vote
    return self._check_serials()
  File "/home/myUser/opt/Zope2-2.13.21/local/lib/python2.7/site-packages/ZODB3-3.10.5-py2.7-linux-x86_64.egg/ZEO/ClientStorage.py", line 929, in _check_serials
    raise s
ConflictError: database conflict error (oid 0x01, class OFS.Application.Application, serial this txn started with 0x03a449da3a7b1e44 2014-01-22 11:38:13.706468, serial currently committed 0x03a449da3af74dee 2014-01-22 11:38:13.820164)

I'd like to fix this, even if it does not affect the functionality of my site, but I don't know where to look. Any suggestions? :)

Or does this just mean that the cached objects have to be renewed with the data of the ZEO server?

Answer 1

When a Zope instances starts up, it registers extensions (Products) in the ZODB. When you run multiple instances sharing a ZEO server, however, and they all start up roughly the same time, you run into conflicts during this stage. The conflicts are essentially harmless, they only occur because another instance did succeed in installing the persistent components.

The solution is to configure instances not to register products, except for one (usually one per machine in a multi-machine cluster); then on restart, only one of the instances does the registration, the rest, running the same software stack, won't have to.

Note that in more recent Zope installations, the persistent parts for Products have been deprecated and mostly disabled; if you don't use Through-The-Web products or ZClasses, you probably don't need to enable this at all.

When you do need it, set the enable-product-installation configuration in zope.conf to on for just one instance, off for the rest. If you use buildout, and the plone.recipe.zope2instance recipe, you can specify this setting in the buildout recipe configuration.