On Feb 11, 2007, at 7:29 PM, Manuel Vazquez Acosta wrote:

Hi all,

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 looks possible.

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.

- C

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