[Zope] Database Connections for ZOPE 1.10.3

[Ross J. Reedstrom]

On Tue, Aug 10, 1999 at 09:07:10AM -0400, Jim Fulton wrote:
> Theodore Patrick wrote:
> > 
> > ZOPE 1.10.3
> > 
> > I have two separate database connections to an oracle database. The
> > connections are in separate folders and are attached to the same ORACLE
> > database.
> > 
> > Are there any hidden do's and don'ts for multiple database connections?
> 
> The transaction machinery in Oracle, or any database that supports
> transactions can lead to a deadlock situation if you have multiple
> connections to the same database.  For example, suppose you have
> 

<snipped Jim's cogent description of DB table lock contention>

I'd just like to point out the 'or any database that supports
transactions' isn't strictly true. The latest version of PostgreSQL (6.5)
implements something called multi-version coherence control, or MVCC. In
the situation Jim described (transaction T1 writing to a table, another
(T2) reading from it) there's no locks used at all: all readers see
the table in it's pre-T1 state, until T1 commits, or rolls back. Two
transactions can even write to the same table, as long as they don't write
the same row. Hmm, this sounds a bit like Zope Versions (er Sessions?),
doesn't it?

One practical benefit from this is live db backups: since the dump runs
in a transaction, it gets a consistent snapshot of the db.

Gee, I better check on how work is going on the ZPyGreSQLDA, hmm? (I've
got some people helping me, btw)

Ross
-- 
Ross J. Reedstrom, Ph.D., <reedstrm@rice.edu> 
NSBRI Research Scientist/Programmer
Computer and Information Technology Institute
Rice University, 6100 S. Main St.,  Houston, TX 77005