[Zope] Re: Global variables in ExternalMethod modules

[Ben Last (Zope)]

Tres Seaver wrote:
> Persistent objects are held in per-thread caches;  module-level stuff is
> ~ *not* part of that cache.  People sometimes fake out thread-level
> caches by assigning "volatile" attributes to persistent objects
> (attributes whose names start with '_v_');  the downside is that such
> attributes go away whenever the persistent object is "ghostified", which
> may occur at unpredictable times (e.g., at transaction or subtransaction
> boundaries).
Thanks very much for this, Tres; much appreciated.
I've sorted the problem out thus:
Added a module called ThreadShared which provides a thread-local storage 
dict, obtainable by any thread calling ThreadShared.getTLS()
I've then used that to store the per-thread objects I wanted 
(MySQLdb.Connection objects, primarily)
This is working very well now from within ExternalMethods.

> Remind me again what you need a per-thread cache for?  Could you
> scribble on the REQUEST object instead?
As above; initially for MySQLdb.Connection objects (actually, we have a 
class that wraps them to make us more db-independent).  I do also 
scribble on the REQUEST object with a _v_ attribute because that makes 
obtaining the per-thread connection somewhat faster as the request code 
calls the various database methods.  But the connections need to persist 
between requests (to save continual database re-connection), so I needed 
  somewhere thread-local to store them.

We also have a large number of configuration settings that are read at 
startup; I've just added code to cache these with the date and time they 
were read, allowing the cached versions to be used until they expire and 
are re-read.  The database load has been significantly reduced with all 
of this.

Regards
Ben