race conditions and network latency

[Kevin Webb]

Jehniah

Sorry, i should have been more clear about the work being performed.
It was just a test to see if this method works in principle. In
practice the work I'm doing is much more complex than increment - I've
got queues that I'm pushing and poping messages out of, etc.

Kevin

On 3/5/06, Jehiah Czebotar <jehiah at gmail.com> wrote:
> why don't you just do
>
> data = memcached->incr(data_name)
>
> then the server handles the locking for you with it's increment /
> decrement functions, and latency doesn't matter.
>
>
> On 3/5/06, Kevin Webb <kevin at tackledesign.com> wrote:
> > Hi,
> >
> > I've been developing a shared memory architecture with locking using
> > memcached's PHP client. It's working wonderfully on a single machine
> > (with many concurrent accesses) or machines on a local network.
> > However, over the Internet, clients accessing the server with
> > significant latency appear collide.  Are there any known issues with
> > latency and use of the Add call?
> >
> > In pseudo code I'm doing this:
> >
> > lock_id = random()
> > while(!memcached->add(lock_name, lock_id))
> > {
> >    wait()
> > }
> >
> > data = memcached->get(data_name)
> > data++;
> > memcached->set(data_name, data)
> >
> > memcached->delete(lock_name)
> >
> >
> > When things break down it appears that two clients are able to add
> > different "lock_id" values for the same lock_name. It's very hard to
> > track down the concurrency of this given the latency however, based on
> > the way the program runs, I can see values getting skipped as "data"
> > increments. On a local machine, no matter how many clients I run or
> > how fast they increment values don't get skipped.
> >
> > If you have any thoughts or suggestion on why I might be having issues
> > with high latency clients I'd very intersted to know.
> >
> > Thanks for your help!
> > Kevin
> >
>