[Twisted-Python] Creating a new selectable (for Pg async interface)
haering_python at gmx.de
Sun Dec 15 18:47:22 EST 2002
* Itamar Shtull-Trauring <twisted at itamarst.org> [2002-12-15 12:30 -0500]:
> Gerhard Häring <haering_python at gmx.de> wrote:
> > [...] I believe my PgAsyncConn would need to be registered with the
> > reactor?! [...]
> If you look at abstract.FileDescriptor.__init__, you'll see it wants
> to get a reactor as an argument, and if not uses the global one.
> In general the code seems ok, except that you really ought to be using
> Deferreds instead of callback/errback pairs.
Tell that to whoever wrote twisted.enterprise.adbapi. That's where I got
the idea for this interface from ;-)
> See http://twistedmatrix.com/documents/howto/defer for details.
> > Btw. I'll try to keep my new baby interface-compatible with
> > twisted.enterprise.adbapi as much as possible.
Even cooler is that this now works basically :-D Even with a simplistic
connection pool built upon.
Perhaps you have a tip on how to improve my connection pool? Here's some
rough code I produced a few minutes ago:
def __init__(self, reactor=None, connstr="", cp_min=3, cp_max=5):
self.connections = 
for i in range(cp_min):
conn = PgAsyncConn(reactor)
# Whoa. Quite likely race condition here :-/
for conn in self.connections:
print "using connection", conn
def query(self, callback, errback, *args, **kwargs):
conn = self.findConn()
assert conn is not None
[snip more code]
Now the "assert conn is not None" isn't very helpful. What is best done
if all connections are busy? I could detect the availability of a free
connection in the doRead method of one of the PgAsyncConn in the
connection pool. But where to go from this knowledge as far as
PgConnPool is concerned? :-)
(Btw. the connection pool in adbapi seems to be a fake one. I don't see
where it'd respect the upper and lower limits for the pool.)
> > Next question: I'd like to try my code out. Is there a good way to
> > test this under Twisted, apart from writing a little server like the
> > Echo example of the docs, and trying to call PgAsyncConn from it?
> Well, look at how we do tests in twisted.test - you can write a unit
> test suite for your code, with the limitation of course that you'd
> need a live postgresql server to test it.
Uh, this seems hard. I believe I'd need to start a reactor in my test
case, as this is a client-side API only and there's no way I'll just
quickly implement PostgreSQL in Python. Any hints on how to do this?
Favourite database: http://www.postgresql.org/
Favourite programming language: http://www.python.org/
Combine the two: http://pypgsql.sf.net/
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