Database and transactions troubleshooting


How to debug and fix ZODB database problems in Plone


This document contains information to fix and debug ZODB databases with Plone.

BLOBs and POSKeyErrors

The Plone CMS from version 4.x onwards stores files and images uploaded to the ZODB as blob. They exist in a var/blobstorage folder structure on the file system, files being named after (opaque) persistent object ids. When using the default backend, the objects themselves, without file payload, are stored in an append-only database file called filestorage and usually the name of this file is Data.fs.

If you copy the Plone site database object data (Data.fs) and forget to copy the blobstorage folder(s), or if data gets out of the sync during the copy, various problems appear on the Plone site:

  • You cannot access a content item for which the a corresponding blob file is missing from the file system;
  • you cannot rebuild the portal_catalog indexes;
  • database packing may fail.

Instead, you'll see something like this - an evil POSKeyError exception (POS referring to Persistent Object Storage):

Traceback (most recent call last):
  File "/fast/xxx/eggs/ZODB3-3.10.3-py2.6-macosx-10.6-i386.egg/ZODB/", line 860, in setstate
  File "/fast/xxx/eggs/ZODB3-3.10.3-py2.6-macosx-10.6-i386.egg/ZODB/", line 922, in _setstate
    obj._p_blob_committed = self._storage.loadBlob(obj._p_oid, serial)
  File "/fast/xxx/eggs/ZODB3-3.10.3-py2.6-macosx-10.6-i386.egg/ZODB/", line 644, in loadBlob
    raise POSKeyError("No blob file", oid, serial)
POSKeyError: 'No blob file'

The proper solution to this problem is to:

  • Re-copy blobstorage folder;
  • restart Plone twice in foreground mode (sometimes a freshly copied blobstorage folder does not get picked up - some kind of timestamp issue?). Restarting ZEO clients once seems to be enough.
  • Copy a Plone site

However you may have failed. You may have damaged or lost your blobstorage forever. To get the Plone site to a working state, all content with bad BLOB data must be deleted (which usually entails losing some site images and uploaded files).

Below is Python code for a BrowserView which you can drop in to your own Plone. It creates an admin view which you can call directly via an URL. This code will walk through all the content on your Plone site and try to delete bad content items with BLOBs missing.

The code handles both Archetypes and Dexterity subsystems' content types.


Fixing Dexterity blobs with this code has never been tested - please feel free to update the code in collective.developermanual on GitHub if you find it not working properly.

The code,


    A Zope command line script to delete content with missing BLOB in Plone, causing
    POSKeyErrors when content is being accessed or during portal_catalog rebuild.

    Tested on Plone 4.1 + Dexterity 1.1.

    Also see:


# Zope imports
from ZODB.POSException import POSKeyError
from zope.component import queryUtility
from Products.CMFCore.interfaces import IPropertiesTool
from Products.CMFCore.interfaces import IFolderish

# Plone imports
from Products.Five import BrowserView
from Products.Archetypes.Field import FileField
from Products.Archetypes.interfaces import IBaseContent
from plone.namedfile.interfaces import INamedFile
from plone.dexterity.content import DexterityContent

def check_at_blobs(context):
    """ Archetypes content checker.

    Return True if purge needed

    if IBaseContent.providedBy(context):

        schema = context.Schema()
        for field in schema.fields():
            id = field.getName()
            if isinstance(field, FileField):
                except POSKeyError:
                    print "Found damaged AT FileField %s on %s" % (id, context.absolute_url())
                    return True

    return False

def check_dexterity_blobs(context):
    """ Check Dexterity content for damaged blob fields


    Return True if purge needed

    # Assume dexterity contennt inherits from Item
    if isinstance(context, DexterityContent):

        # Iterate through all Python object attributes
        # XXX: Might be smarter to use zope.schema introspection here?
        for key, value in context.__dict__.items():
            # Ignore non-contentish attributes to speed up us a bit
            if not key.startswith("_"):
                if INamedFile.providedBy(value):
                    except POSKeyError:
                        print "Found damaged Dexterity %s on %s" % (key, context.absolute_url())
                        return True
    return False

def fix_blobs(context):
    Iterate through the object variables and see if they are blob fields
    and if the field loading fails then poof

    if check_at_blobs(context) or check_dexterity_blobs(context):
        print "Bad blobs found on %s" % context.absolute_url() + " -> deleting"
        parent = context.aq_parent

def recurse(tree):
    """ Walk through all the content on a Plone site """
    for id, child in tree.contentItems():


        if IFolderish.providedBy(child):

class FixBlobs(BrowserView):
    A management view to clean up content with damaged BLOB files

    You can call this view by

    1) Starting Plone in debug mode (console output available)

    2) Visit URL

    def disable_integrity_check(self):
        """  Content HTML may have references to this broken image - we cannot fix that HTML
        but link integrity check will yell if we try to delete the bad image.

        ptool = queryUtility(IPropertiesTool)
        props = getattr(ptool, 'site_properties', None)
        self.old_check = props.getProperty('enable_link_integrity_checks', False)
        props.enable_link_integrity_checks = False

    def enable_integrity_check(self):
        """ """
        ptool = queryUtility(IPropertiesTool)
        props = getattr(ptool, 'site_properties', None)
        props.enable_link_integrity_checks = self.old_check

    def render(self):
        #plone = getMultiAdapter((self.context, self.request), name="plone_portal_state")
        print "Checking blobs"
        portal = self.context
        print "All done"
        return "OK - check console for status messages"

Registering the view in ZCML:


More info


Transactions are usually problematic only when many ZEO front-end clients are used.


When the site gets more load, ConflictErrors start to occur. Zope tries to solve the situation by replaying HTTP requests for ConflictErrors and has a default threshold (3) of how many times the request is replayed.

More info

How to debug which object causes ConflictErrors

ConflictErrors are caused by concurrent transactions trying to write to the same object(s) - usually portal_catalog. They are harmless, but slow down badly coded sites. Plone will retry the HTTP request and transaction three times before giving up.

The OID is visible in the ConflictError traceback.

You can turn OID back to the corresponding Python object, as mentioned by A. Jung:

from ZODB.utils import p64

If every transaction appears as write transaction

If you are not careful, you may accidentally write code which turns all transactions to write transactions. This typically happens when you call some method without realizing that that method eventually modifies a persistent object, causing a database write.


  • Your Undo tab in ZMI will be full of entries, one added per page request.
  • If you run the server in single Zope server mode, it is slow.
  • If you run the server in ZEO mode you get the exceptions like one below. It may happen even with one user. This is because each page load requres more than one HTTP request: HTML load, image load, CSS load and so on. Browser makes many requests per page and those transactions are conflicting, because they are all write transactions.

Traceback example:

    * Module ZPublisher.Publish, line 202, in publish_module_standard
    * Module ZPublisher.Publish, line 170, in publish
    * Module ZPublisher.Publish, line 170, in publish
    * Module ZPublisher.Publish, line 170, in publish
    * Module ZPublisher.Publish, line 157, in publish
    * Module, line 15, in zpublisher_exception_hook_wrapper
    * Module ZPublisher.Publish, line 125, in publish
    * Module Zope2.App.startup, line 238, in commit
    * Module transaction._manager, line 96, in commit
    * Module transaction._transaction, line 395, in commit
    * Module transaction._transaction, line 495, in _commitResources
    * Module ZODB.Connection, line 510, in commit
    * Module ZODB.Connection, line 547, in _commit

ConflictError: database conflict error (oid 0x2b92, class Products.CMFPlone.PropertiesTool.SimpleItemWithProperties)

How to debug it

Zope 2 doesn't have many well-documented ZODB debugging tools. Below is one snippet to examine the contents of the last transactions of an offline Data.fs file. It is an evolved version of this original script.

  • Do something on a badly behaving site.
  • Stop Zope instance.
  • Run the script below ( on the Data.fs file to see what objects have been changed.
  • Guess the badly behaving code from the object class name.

Example how to run the script for the last 30 transaction under a Zope egg environment using the zopepy script:

bin/zopepy -n 30 Data.fs


The following is obsolete with current Zope. FileIterator does not take a pos argument any more.

Code for

# Copyright (c) 2001, 2002 Zope Corporation and Contributors.
# All Rights Reserved.
# This software is subject to the provisions of the Zope Public License,
# Version 2.1 (ZPL).  A copy of the ZPL should accompany this distribution.
"""Tool to dump the last few transactions from a FileStorage."""

from ZODB.fstools import prev_txn
from ZODB.serialize import ObjectReader, get_refs
from persistent.TimeStamp import TimeStamp
from ZODB.FileStorage.FileStorage import FileIterator
import cStringIO, cPickle
import optparse, getopt
import sys

class Nonce(object): pass

class Reader(ObjectReader):

    def __init__(self):
        self.identity = None

    def _get_unpickler(self, pickle):
        file = cStringIO.StringIO(pickle)
        unpickler = cPickle.Unpickler(file)
        unpickler.persistent_load = self._persistent_load

        def find_global(modulename, name):
            self.identity ="%s.%s"%(modulename, name)
            return Nonce

        unpickler.find_global = find_global

        return unpickler

    def getIdentity(self, pickle ):
        self.identity = None
        unpickler = self._get_unpickler( pickle )
        return self.identity

    def getObject(self, pickle):
        unpickler = self._get_unpickler( pickle )
        ob = unpickler.load()
        return ob

def pretty_size( size ):
    if size < 1024:
        return "%sB"%(size)
    kb = size / 1024.0
    if kb < 1024.0:
        return '%0.1fKb'%kb
        mb = kb/1024.0
        return '%0.1fMb'%mb

def run(path, ntxn):
    f = open(path, "rb"), 2)

    th = prev_txn(f)
    for i in range(ntxn):
        th = th.prev_txn()
    reader = Reader()
    iterator = FileIterator(path, pos=th._pos)
    for i in iterator:
        print "Transaction ", TimeStamp(i.tid), i.user, i.description
        object_types = {}
        for o in i:
            ot = reader.getIdentity(
            if ot in object_types:
                size, count = object_types[ot]
                object_types[ot] = (size+len(, count+1)
                object_types[ot] = (len(,1)

            ob = cPickle.loads(

            print "Object data for :" + str(o)

            # Not sure why some objects are stored as tuple (object, ())
            if type(ob) == tuple and len(ob) == 2:
                ob = ob[0]

            if hasattr(ob, "__dict__"):
                for i in ob.__dict__.items():
                    if not callable(i[1]):
                        print i
                print "can't extract:" + str(ob)

            print "-------------------------------------------------------"

        keys = object_types.keys()
        for k in keys:
            # count, class, size (aggregate)
            print " - ", object_types[k][1], k, pretty_size(object_types[k][0])

def main():
    ntxn = 20
    opts, args = getopt.getopt(sys.argv[1:], "n:")
    path, = args
    for k, v in opts:
        if k == '-n':
            ntxn = int(v)
    run(path, ntxn)

if __name__ == "__main__":

zeostorage Client has seen newer transactions than server

If you get:

ClientStorageError: zeostorage Client has seen newer transactions than server!

, you can fix it by removing cache-data.zec from parts/instace/var/.

Updating objects created by older code

In the course of development, classes may be renamed or moved. When an object is read from the ZODB, the class required to unpickle the serialized object is named in the pickle data. If this name cannot be imported, you have a broken object on your hands.

In the Zope event log that will show up as, for example:

2014-06-19 11:04:04 WARNING OFS.Uninstalled Could not import class 'ATSimpleStringCriterion' from module 'Products.ATContentTypes.types.criteria.ATSimpleStringCriterion'

To make the object usable again, the reference needs to be updated to refer to a class that can instantiate this object. One tool that can help you with this is zodbupdate

In this case, the ATSimpleStringCriterion class in question has moved from Products.ATContentTypes.types.criteria.ATSimpleStringCriterion to Products.ATContentTypes.criteria.simplestring.

To make zodbupdate handle this, add a zodbupdate entry point to ATContentTypes. Depending on your configuration, that may look like this:

$ cat .../buildout-cache/eggs/Products.ATContentTypes-2.1.13-py2.7.egg/EGG-INFO/entry_points.txt

Next, define rename_dict in the of the named package, e.g.:


In this case, our ``rename_dict`` will look like this:
rename_dict = {
    'Products.ATContentTypes.types.criteria.ATSimpleStringCriterion ATSimpleStringCriterion':
    'Products.ATContentTypes.criteria.simplestring ATSimpleStringCriterion'}


As always, work on a copy of your data first, before working on the live site.