[TriLUG] Aligning of the crazy numbers

[Owen Berry]

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