Re: [HACKERS] please? - Mailing list pgsql-hackers

> First, thanks all for the feedback and good luck with the new
> release!
> 
> > 
> > > A possible approach is for your clients to maintain more than one
> > > backend connection, and use one of the backends to do the stuff 
> > > that might block while using another one to do the stuff that won't.
> 
> Yes. Same effect as PQreset() if the code is to be ran only once, but
> a lot better if inside a loop!. 
> 
> > Or you could do it outside of the database using a Unix filesystem lock
> > file.  There are symantics for no-blocking lock stuff in flock():
> > 
> >            #define LOCK_SH 0x01    /* shared file lock */
> >            #define LOCK_EX 0x02    /* exclusive file lock */
> >            #define LOCK_NB 0x04    /* don't block when locking */
> >            #define LOCK_UN 0x08    /* unlock file */
> 
> Exactly what's wanted in this case. The unix flock() locks a file or,
> if already locked, either waits or fails depending on what you
> requested. The lock is released by either an unlock operation or the
> death of the locking process. It would solve my problem, except it 
> requires all clients to share a filesystem. 

Sharing file systems.  Good point.  You could have a table you use to
lock.  Lock the table, view the value, possibly modify, and unlock. 
This does not handle the case where someone died and did not remove
their entry from the lock table.  

> 
> > I don't know of any SQL databases that allow non-blocking lock requests.
> 
> I'm not very familiar with full-scale SQL but seems odd not to have
> such things. I guess from the language point of view there ought to be
> a way to know when an item is unavailable/undefined (it's been locked 
> for writing), if you don't want to wait a long time to get a value.
> 
> Imagine I go to the store at 11am and can't buy soap - because
> the price of soap is unknown because it's a heavy trading day for soap 
> at the ny stock exchange. Even if the shop's definition may not allow for
> soap to be sold before the stock market closes and the final price is
> known, I shouldn't be forced to wait there doing nothing. I can do 
> other shopping around and come back later for my soap!  ;-)

Yes, I can see why having such a facility would be nice.

--  Bruce Momjian                        |  http://www.op.net/~candle maillist@candle.pha.pa.us            |  (610)
853-3000+  If your life is a hard drive,     |  830 Blythe Avenue +  Christ can be your backup.        |  Drexel Hill,
Pennsylvania19026