[TriLUG] Aligning of the crazy numbers

[Josh Vickery]

Well, if you are reading from /dev/urandom, why not read from a file
with a counter in it instead?  You can then use file locking to make
sure that only one instance at time reads from it, and you can be
guaranteed that the numbers do not overlap.  If that won't work, you
could use file locking on /dev/urandom.  Get an exclusive lock before
you seed the generator, then release it after its been seeded.  This
won't guaranty that you won't get duplicates though, because urandom
could still spit out the same number twice in a row.

The PID bit would help, but wouldn't work if the program uses multiple threads.

Josh

On 8/17/06, Owen Berry <oberry at trilug.org> wrote:
> I have a CGI program that needs to generate a unique identifier each
> time it gets executed. The problem is that it can get executed multiple
> times per second (duh ... CGI), and requirements limit me from having a
> central source from which to generate a unique id. Besides, I have a
> much simpler solution ... well I thought I did. Take the time in seconds
> since the beginning of the epoch, the number of microseconds in the
> current second, and a 3 digit random number, and concatenate them
> together with delimiters. Sounds reasonable, right? Maybe even a little
> excessive with the random number. Well, 3 times in the past month we've
> seen the same id generated by 2 requests running simultaneously!
>
> It's Perl code, but according to the documentation the seconds and
> microseconds are grabbed using the standard gettimeofday system
> function, and the random number generator is seeded by /dev/urandom. So
> they should both work pretty well, and seem to when tested.
>
> The only partial explanation I can think of is that this is a dual CPU
> system and both requests were literally running at the same time, down
> to the microsecond. Anyone know if there is any locking on /dev/urandom
> to prevent 2 processes grabbing the same data at the same time?
>
> Anyway, I have a simple solution ... add the process id to the mix. That
> should be unique amongst concurrently executing processes, right? ;-)
>
> Owen
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