On Feb 11, 2007, at 7:29 PM, Manuel Vazquez Acosta wrote:
I'm writing a small application for which I want to use ZODB's
persistence machinery. I have a couple of question, though.
I have read that each thread should have its own connection to the
DB. However, in my case, each thread should be aware of what is
actually in the DB at all times. So I wonder if I can shared the
connection between those threads as long as I take the means to
protect it (i.e RLock).
I am not certain, actually. The mechanism for doing this is in
there. The interfaces.py file states:
A Connection instance is not thread-safe. It is designed to
support a thread model where each thread has its own transaction.
If an application has more than one thread that uses the
connection or the transaction the connection is registered with,
the application should provide locking.
And it goes on to talk about the sync=False parameter vaguely.
But the story doesnt really end there... there is also the capability
to register a different "transaction manager" at DB open time.. the
default one assumes one transaction per thread.
I'd need to dig through all this code to reunderstand it, but it
My scenario is akin a consumer-producer with shared buffer.
Consumers pull items from the buffer whilst producers put items in
the buffer. The buffer is an OOBTree along with an IOBTree which
gives "serial" numbers to the keys of the OOBTree.
The typical way to do this using one-connection-per-thread and a
threaded txn manager would be to call transaction.commit() when
mutating the shared data structures. To detect changes to the
objects provided by a connection (due to other threads committing
during the connection's transaction), you can connection.sync().
Zope calls transaction.begin() at the beginning of a request and
either transaction.abort() (on failure) or transaction.commit() (on
success) at its termination. It never needs to do an explict
connection.sync() because it never wants to do a "dirty read" (each
request has its own connection and sees its connection data as
canonical for the duration of its existence). Zope handles conflicts
by retrying the request. But Zope has it easy because the web has
natural transaction boundaries that don't always exist in GUI apps or
other apps with a different request/response cycle.
The other question is about compiling ZODB without using the out-of-
the-box distutils installation procedure. I'm also playing with
Zope and Plone, so I have several instances on the same machine. I
think installing with distutils may cause conflicts with the Zope
instances. Am I right? If so, then how should I install ZODB side-
by-side the consumer-producer application?
The easiest thing to do here is to install a separate Python instance
for each project. Alternately if you're on UNIX, you can use
"virtual python" (http://peak.telecommunity.com/dist/virtual-
python.py) which makes a tree of symlinks that acts like a new Python
instance but has its own "site packages" directory in which distutils
stuff is installed.
If you're on Windows, I believe Zope/Plone ship with their own copy
of Python on that platform, so installing to the system python
shouldnt create a conflict.
For more information about ZODB, see the ZODB Wiki:
ZODB-Dev mailing list - ZODB-Dev@zope.org