There's a somewhat subtle issue using DCOracle2 with DBPool. DCOracle2
defines it's threadsafety as 3 because technically connections can be shared
across threads. However, invoking <connection>.commit() will commit any
pending transactions on the connection. Since DBPool could allow multiple
threads to share the same connection (with threadsafety 3), then one thread
could commit another thread's changes.
For example with 10 pooled connections:
- thread 0 obtains connection 0 from the DBPool and goes to work on a long
series of transactions
- meanwhile ... connections 1-9 are allocated to other threads
- thread x comes along and obtains connection 0 from the DBPool as it is the
next in the rotation
- thread x can do a simple insert and commit() before thread 0 has finished
all it's transactions
- thread 0 excounters an error and tries to rollback, but the rollback only
covers what happened AFTER thread x's commit.
- so the rollback succeeds but the changes are already committed. doh!
I have a homegrown db pool for use with DCOracle2 that uses the Queue
method. A better solution though, might be a subclass of DBPool which allows
for both threadsafe and unthreadsafe retrieval of db connections. That way,
when threads need to modify the db they can truly reserve a connection,
while threads that need to fetch data can obtain a threadsafe connection.
Another solution is to simply have one's application maintain 2 DBPools, one
for querying and one for DML.
Any thoughts on this little snag?
PS - Should this thread be moved into discuss?
[mailto:webware-devel-admin@... Behalf Of Ian
Sent: Tuesday, April 22, 2003 10:16 AM
To: Uwe Schmitt
Cc: Jeff Johnson; Webware devel
Subject: Re: [mailinglist] Re: [Webware-devel] pooling of
On Tue, 2003-04-22 at 08:52, Uwe Schmitt wrote:
> I found DBPool meanwhile, but did not know how to share one instance of
> this class. That is the reason why I asked about creation of servlet
You can just put it in a global variable, like:
import DBPool, MySQLdb
pool = DBPool.DBPool(MySQLdb, 10, user=...)
from SiteDatabase import pool
conn = pool.getConnection()
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