Watch out: Python 2.4 ZEO client and Python 2.6 ZEO Server

// under database fs plone server zeo zope

Today was interesting when an issue with saving content on a Plone 3.3.5 site got reported to me. The system was producing undecipherable error messages about ZEO disconnections and database conflicts whenever someone would try and create some content within Plone (folders, particularly). Errors looked like:

Module ZEO.zrpc.connection, line 796, in wait. DisconnectedError" and "raise ReadConflictError().
ReadConflictError: database read conflict error"

and also:

ERROR ZEO.zrpc (2515) can't decode message: '(K\x11K\x01U\tserialnos(]((U\x08\x00\x00\x00\x00\x00X\x...'

— essentially the issue described here. My ZEO server was running ZODB3 3.9.5 with Python 2.6, and my client running ZODB3 3.8.3 with Python 2.4; almost everything worked.

The long and the short of this issue appears to be, as the ZODB documentation mentions, is that Python 2.4 servers work with 2.5+ clients, but not the other way around. It’s slightly misleading when you read further and see that:

ZODB ZEO clients from ZODB 3.2 on can talk to ZODB 3.9 servers. ZODB
ZEO Clients can talk to ZODB 3.8 and 3.9 ZEO servers.

So, despite this seeming interoperability, dang - I can’t use my pre-existing ZEO server setup (based on the Plone 4 buildout for versions, etc) which is currently in place and running.

Thankfully, building being what it is, the solution for me was to simply re-use that buildout for any Plone instances that are Python 2.4 clients. This was as simple as re-cloning my git repository where I keep the buildouts, firing up a brand new virtualenv for Python 2.4, and moving the data across.

One more thing…

There is one final thing worth mentioning, which I got awfully caught out by. I was finding that my ZEO processes, under this new environment, just would not start - essentially, they would keep respawning again and again until they stopped (triggered by some ‘quick-respawn’ safety mechanism, seemingly). I thought this was some issue concerning the fact I’d had my databases and files loaded against newer ZEO versions, but this actually not. This was a by-product of not having my ZEO authentication database present - because there was no file, presumably the processes kept crashing. Sadly, no error message, so hence this note. Take care with this one.

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