Re: [PHP] database abstraction layer

On Wed, 2010-02-03 at 00:17 +0100, Rene Veerman wrote:

> and after the sleep(rand(1,3)) it might need a short loop like this;
> $rnd = rand(1,99999); $a=0;
> for ($i=0; $i<$rnd; $i++) { $a++ }
> 
> to further randomize the retry attempt..
> 
> 
> On Wed, Feb 3, 2010 at 12:05 AM, Rene Veerman <[email protected]> wrote:
> 
> > i haven't had the pleasure yet of writing for sites that generate so many
> > hits/sec that
> > they'd update the max value of any table at exactly the same time.
> >
> > i usually ask for the max value about 2 milliseconds before doing the
> > insert.
> > And if the insert fails, i can auto-retry via a wrapper function after
> > sleep(rand(1,3));
> > I dare say i could work this way at facebook <g> (not that i really want
> > to, happy with where i am)
> >
> >
> > On Tue, Feb 2, 2010 at 11:46 PM, Ashley Sheridan <[email protected]
> > > wrote:
> >
> >>  On Tue, 2010-02-02 at 23:19 +0100, Rene Veerman wrote:
> >>
> >> function getMax($table, $field)
> >>
> >>
> >> If I saw this sort of code I'd be appalled! It's possibly the worst way to
> >> get the auto increment value. You won't notice it testing the site out on
> >> your own, but all hell will break loose when you start getting a lot of
> >> hits, and two people cause an auto increment at the same time!
> >>
> >>   Thanks,
> >> Ash
> >> http://www.ashleysheridan.co.uk
> >>
> >>
> >>
> >


The problem is where 2 people choose the same instant to perform an
action on your site that inserts a record into your db. The db engine
inserts them one after the other, and then responds about the max(id) to
your PHP script. Then, you now have 2 people who have the same max(id)
retrieved, but one of the values is wrong.

Thanks,
Ash
http://www.ashleysheridan.co.uk