[orm-devel] Unified Connection Strings

Diedrich Vorberg orm-devel@mailman.tux4web.de
Mon, 2 Dec 2002 10:50:11 +0100

Hi Ross,

>Hmm, what is a ORMDatasourcePool Product? I was just looking at what it
>would take use orm from inside other Zope products, using a Zope SQL DA
>as the datasource. Is that anything like what you're up to, or something
>even more integrated?

This is my solution using a Zope SQL DA (ZPsycopgDA), just 
subclassing the PostgreSQL datasource. 

class datasource(orm.adapters.psql.datasource.datasource):
    We're using Zope's mechanism for database connections here.
    This is a subclass of orm's psycopg adapter.
    def __init__(self, context):
        zope_ds = context.restrictedTraverse("zope_ds")        
        self._conn = zope_ds().db
        self._encoding = "iso-8859-1"
        self._dsn = ""

The problem I can see is, that there is only one connection to the 
database. PostgreSQL will maintain transactions connection-wise. So 
if one of my threads runs a couple of queries on one connection and 
another runs a rollback() on that connection my changes are lost. 
Even an unwanted commit() might lead to trouble.

The Zope Product I'm currently working on is in the CVS contrib 
directory, but be warned it, is rather primitive :-) It provides
(besides the Zope Management facilities) two methods: aquire() and 
release(). There is a minium and maximum number of connections the 
Pool will create to the RDBMS. aquire() will return one to you if 
one is available or open a new one for you. If the maximum number of 
open connections is reached it will make you wait until one is 

This is untested, I'm going to start working with it today.

There is a rather nasty problem with ORM and Zope. ORM requires 
Python 2.2, because of Python's builtin classmethod(). Zope will warn 
about 2.1 being the preferred version to run it with, but to my 
experience will run fine. The problem: starting Zope using Python 2.2
takes forever, which is a pain when developing a product.

My solution (er... workaround) is to have two version of my 
Application, one that runs on 2.2 and one that runs on 2.1 using a 
defunc version of ORM.

You can use 2.2 and Zope just make sure you have some kind of way to 
reload() all your modules without restarting Zope.

           _..._                            Diedrich Vorberg
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