Plone 4: Local Zeo blobs conflict with Plone instance

As the title suggests, there’s a conflict between a Zeo database instance being run together with a Plone 4/Zope instance, and them trying to share the same blob storage folder.  But, this only happens if you misunderstand or incorrectly set the option of  shared-blob = off, and blob-storage to be the same location as for Zeo in your buildout. If that previous set of (seemingly jumbled) thoughts doesn’t make any sense to you, then move along, nothing to see here.

By default, at the time of writing, blob support within Zeo runs with the bushylayout, and creates this format by default when starting up for the first time. Plone 4, however, when trying to stream blobs from your Zeo database, uses a customised zeocache blob layout. See where I’m going with this? You try and inadvertently use these aspects together and you’ve got a conflict.

If you’re having this problem, then you’ll get weird errors concerning the layout of your blobs. Starting the Zope instance before the Zeo server results in Zope setting the .layout file for the blob storage to zeocache. Once your Zope instance errors out and you realise that your Zeo instance isn’t running, you’ll inevitably try and start Zeo up. When you do, you get an error about the layout type not existing:

Traceback (most recent call last):
  File "/home/david/buildout/eggs/ZODB3-3.9.5-py2.6-linux-i686.egg/ZODB/", line 714, in __init__
    self._blob_init(base_directory, layout)
  File "/home/david/buildout/eggs/ZODB3-3.9.5-py2.6-linux-i686.egg/ZODB/", line 600, in _blob_init
    self.fshelper = FilesystemHelper(blob_dir, layout)
  File "/home/david/buildout/eggs/ZODB3-3.9.5-py2.6-linux-i686.egg/ZODB/", line 339, in __init__
    self.layout = LAYOUTS[layout_name]
KeyError: 'zeocache'

Now, if you try to fix this by dumping your now-existing blob storage folder (a method mentioned to work elsewhere on the web) and starting your Zeo server first, you’ll still get problems. The blob storage gets created correctly with the ‘bushy’ layout by Zeo, but trying to start the Zope instance fails, because Zeo’s created its storage accordingly in the same spot. This is part of the error:

Traceback (most recent call last):
  File "/home/david/buildout/eggs/ZODB3-3.9.5-py2.6-linux-i686.egg/ZODB/", line 362, in create
    (self.layout_name, self.base_dir, layout))
ValueError: Directory layout `zeocache` selected for blob directory /home/david/buildout/var/blobstorage/, but marker found for layout `bushy`

The two ways of solving this are to:

  1. Change the shared-blob option to be on. Both Zeo and Zope will exist in harmony again, both using the bushy layout.
  2. Changing the blob-storage location for either the Zeo server or the Zope instance. This will result in two separate folders and likely duplicate files and HDD wastage, but hey, it’ll work, right? This is probably a useful option for testing.

The zeocache layout is new with Plone 4 (aka, I haven’t seen it before so it probably is), so watch out for this when upgrading.

Stay tuned for more in this series of Plone 4 upgrade notes.

Go Top
comments powered by Disqus