On Apr 5, 2008, at 6:30 PM, Alan Runyan wrote:
Here is something more ZODB and less Zope related (kinda).

I was talking with Benji a few weeks ago about a problem that should
be easy to debug but was not.  Here is the scenerio:

 - Customer has software on a remote machine.  They are seeing
unnecessary transaction commits. Just like the guy 'Analyzing a ZODB'.

- Customer is completely incapable of doing anything other than putting
 a script on the filesystem.

- Benji and I thought about it and he proposed 'the simplest thing that c/should work, monkey patch persistent.__setstate__' so that I could see
 what objects were being mutated.

That doesn't show you which objects are being mutated. It only shows you which objects are being loaded.

If you want to see what objects are actually being mutated, it would be better, IMO, to set up some sort of event channel on object invalidations.

If you want to see what code is mutating objects, it would be better to set breakpoints in Connection.register.

  - Unfortunately ZODB 3.x does not have a Python fallback of
persistent.Persistent -- its in C. The customer did not have a C compiler
 on their box.

IIRC how I solved it was increase ZEO event log to see the oid's. Then I
walked him through loading the oid up to see what object was being
mutated.  It was more painful than it should have been.

Yup, but automating something like this might have been informative without being painful.

Question:  Is it possible for ZODB 3.9 to have a pure python
implementation of persistent.Persistent?


 Maybe this would be a good
ZODB GSOC project?

The current C-level persistence implementation is rather nasty because of the close coupling with the persistent cache implementation.

My goal is to come up with a new persistence implementation that is much simpler and more flexible with both Python and C implementations. I haven't had time to work on this though. :( If someone wanted to work on this, I'd be happy to go into more detail.


 - increasing zeo server log level and watching oid's being changed
is sort-of the equivalent of turning on RDBMS logging to see SQL
stmt's being executed.  Unfortunately I believe without having a
hook in persistent.Persistent we can never really get that level
of granularity (i.e. __getattribute__ is only accessible in client)
with only ZEO server logs.

Regardless of whether it's implementation is in C or Python, the current implementation is complex and highly coupled. A more flexible system that I envision, would make it easier to experiment with alternate cache implementations and would also make it easier to substitute implementations that did more logging for debugging purposes.

This is a transparent plea for a "new feature". But I believe it would
significantly help people writing ZODB applications.

Maybe people should always have a base class that you override
those methods which delegate to persistent.Persistent.  i.e.
class MyMixin(persistent.Persistent) and mixin MyMixin
instead of mixin persistent.Persistent directly?  Then
you can instrument the MyMixin with the logging?

That would introduce lots of overhead and complexity that I don't think is warranted.


Jim Fulton
Zope Corporation

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