[Zope-dev] [Solved] Cannot load state: None has no attribute 'load'

[Dieter Maurer]

Recently, I reported a strange effect where Zope suddenly
and without apparent reason reports lots of
"Could not load state: None object has no attribute 'load'.

I think, I now understand why this happens and can work around it:

  We use GUF (Generic User Folder) to maintain users in
  an Oracle database. Although Oracle is quite fast
  with the kind of queries involved in authentication
  (with others it is extremely slow), the complete
  authentication process was slow: 0.2 to 0.5 seconds
  I wanted to improve on this and implemented a user cache.

  For simplicity and speed I cached the completely wrapped
  user object.
  Not a good idea, if one thinks carefully about it!

  Apparently, everything worked well, but suddenly, non-deterministically
  after many hours, Zope reports "Could not load..." in the log
  file and raises really erratic exceptions in Web requests....


What have I learned....

  *NEVER* cache persistent objects (or objects referencing a
  persistent one!).

  They contain a hidden reference to the ZODB connection
  that created them (and that is used to load subobjects that
  have not yet been loaded).
  This reference is valid only in the thread (and probably
  also only the request) that made it.

  Invalid uses can show very interesting effects, like
  the above "Could not load...", "TypeError, 'Results' instances
  do not support subscription", "AttributeError, '__len__'", .....


What will I do:

  I will cache the bare non-persistent user object (after I scrapped
  all acquisition wrappers). I will rewrap it into the
  UserFolder context, when the cached object is looked up.


Special thanks to Chris for his help with the original problem report.


Dieter