[ZODB-Dev] Threads and Connections

[Chris McDonough]

FWIW I believe by default at least, open ZODB connections are tied to  
the thread which did the opening (they are returned to a pool when  
closed and reused, possibly in another thread).  And indeed each  
connection does have a cache; caching is one of the primary  
responsibilities of a connection object.

See the documentation in ZODB/interfaces.py for "IConnection".

I *think* what is happening here is that you are committing the  
transaction devoted to the current connection/thread, and trying to  
close a connection that has pending changes from another thread.  So  
when you do transaction.abort(), it's aborting the transaction  
involving the current thread, not the one associated with the other  
connection.

That said, I'm not entirely sure what to tell you to do here; there  
used to be an API named "setLocalTransaction" or somesuch that  
allowed you to control the one-thread-per-connection policy wrt to a  
transaction minimally, IIRC.  This API has disappeared, probably  
replaced with something more flexible, but I'm not sure what that  
is.  I suspect it may have something to do with the  
transaction_manager parameter to DB.open() however.

- C


On Jul 25, 2006, at 11:40 PM, Chris S wrote:

> The problem is the only objects I need to grab are the ones loaded by
> other connections. I use Python's garbage collector to track
> references to instances of classes that have been updated. Aside from
> the strange error below, the live update functions correctly. It's
> almost like the connection is keeping a special cache that's
> conflicting with the new objects.
>
> Chris
>
> On 7/25/06, Mika, David P (GE, Research) <mika at crd.ge.com> wrote:
>> I don't know the answer to your question, but when I have separate  
>> threads,
>> I always have each thread independently control their own connection
>> (open and close).  Perhaps in your situation, you could have the
>> update function open it's own connection, grab the object in question
>> from the ZODB, change it, commit it and close.
>> Dave
>>
>> -----Original Message-----
>> From: zodb-dev-bounces+mika=research.ge.com at zope.org
>> [mailto:zodb-dev-bounces+mika=research.ge.com at zope.org]On Behalf Of
>> Chris S
>> Sent: Friday, July 21, 2006 6:29 PM
>> To: zodb-dev at zope.org
>> Subject: [ZODB-Dev] Threads and Connections
>>
>>
>> Is it possible to close a connection used in one thread from another
>> thread? My program has a locking mechanism, so I can ensure threads
>> don't use their connection while it's closed, but whenever I try to
>> close one thread's connection from another thread, I get errors like:
>>
>>     self.conn.close()
>>   File "C:\Program
>> Files\Python24\lib\site-packages\ZODB\Connection.py", line 252, in
>> close
>>     self.transaction_manager.unregisterSynch(self)
>>   File "C:\Program
>> Files\Python24\Lib\site-packages\transaction\_manager.py", line 169,
>> in unre
>> gisterSynch
>>     ws.remove(synch)
>>   File "C:\Program Files\Python24\Lib\site-packages\ZODB\utils.py",
>> line 236, in remove
>>     del self.data[id(obj)]
>>   File "C:\Program Files\Python24\lib\UserDict.py", line 19, in  
>> __delitem__
>>     def __delitem__(self, key): del self.data[key]
>> KeyError: 26403280
>>
>> The reason why I'm doing this is because I'm trying to update the
>> classes of persistent objects loaded into memory. I'm able to load  
>> the
>> new classes just fine, but when I try to close the connection at
>> program termination, I get the error:
>>
>> transaction.commit()
>> conn.close()
>> ConnectionStateError: Cannot close a connection joined to a  
>> transaction
>>
>> The only way I'm able to avoid this strange error is if I close all
>> connections during the update, but since my update function runs in a
>> different thread, it can't close the connections. Is there a way
>> around this, or am I misunderstanding something?
>>
>> Chris
>> _______________________________________________
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>> http://www.zope.org/Wikis/ZODB/
>>
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