"Rasmus Lerdorf" <rasmus@xxxxxxxxxxx> wrote in message news:42435332.70807@xxxxxxxxxxxxxx
Joshua Beall wrote:
I am doing some work where I want to do locking, and prevent scripts from running in parallel. I see that I could use the semaphore mechanism, but I'd like for my code to be portable, and that extension is not enabled in many places.
Sort of defeats the whole concept of a web server, but to answer just your process id question, use getmypid()
http://php.net/manual/en/function.getmypid.php
It says "Process IDs are not unique"
I really only need it to be unique at any given instant. I can do sha1(microtime().getmypid()) to generate a unique ID. But of course it is only guaranteed to be unique if indeed the process ID is not shared.
pids are not unique over time. They get re-used, but in any instant on a single server, the pid is unique.
The problem I am having is that people are double-submitting certain transactions. My first attempt to prevent this was to store a flag in the session record indicating whether or not certain transactions had been completed, but this turned out to be insufficient at times because users could try and initiate a second transaction before the first transaction had finished (and thus the system had not yet flagged the transaction completed in the session record). They then both completed in parallel, and voila, duplicate transactions again.
But a double-submit is likely to come from separate Apache processes, so I don't see where the pid comes into the picture. If I reload a page and resend the post data, that POST request is going to be processed a second time most likely by a different httpd process. What you need to do is put a unique (you can use the uniqid function) in the actual transaction data and not allow the transaction if that token is already present in your datastore.
-Rasmus
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