Re: Can a script run twice?

Robert Cummings wrote:

These are called race conditions for a reason. They are racing against all kinds of things. Network latency, filesystem, processor, etc etc.

One site has take down it's Cisco backbone and replaced it with simple hubs. Problems disappeared overnight! ( That was not just my problems )

Any one of these thigns can slow down one thread just long enough for a newer thread to grab the session data before the older thread saves it. Then the newer thread happily uses the stale session and voila duplicate input. If you absolutely, definitely, for sure want to prevent this... use some kind of locking mechanism. I'd suggest locking the table that manages the form submission unique IDs until you've checked and, if necessary, updated it. There's a reason pages load sequentially in multiple frames when using PHP sessions... it's because PHP uses locking. Are you using PHP native sessions or your own home cooked system? Are sessions even activated? Did the user disabled sessions? Why am I teaching you second year comp sci? :)

    http://en.wikipedia.org/wiki/Race_condition

The double click 'problem' was a sideline to the original problem, which I found while trying to track things. The original problem *IS* that PHP can run two copies of a script in parallel and the second copy does NOT see the 'locking' in place on the first copy. I had always thought that PHP was sequential, but I'm not so sure now. I've put in a trap for the double click ( and yes it *IS* browser dependent ) and that works fine, but we will have to see if it makes any difference to the original 'race' problem :( -- Lester Caine - G8HFL ----------------------------- L.S.Caine Electronic Services - http://home.lsces.co.uk Model Engineers Digital Workshop - http://home.lsces.co.uk/ModelEngineersDigitalWorkshop/ Treasurer - Firebird Foundation Inc. - http://www.firebirdsql.org/index.php