Re: Sinister variable caching problem with rand()

Michael Ludwig wrote:
> André Warnier schrieb am 15.05.2010 um 22:47:14 (+0200):
> 
>> A tip : to get a really unique identifier, I often use
>> "yyyymmddhhmmssrrrrr", where rrrrr is the rand() result,
>> and the prefix is the date/time. Unless you process more
>> than 99,999 requests per second, it will always be unique.
> 
> I'm probably missing something trivial, but how do you
> enforce uniqueness over rrrrr for 99,999 consecutive calls
> to rand?
> 
> The perldoc doesn't promise any uniqueness, only randomness,
> which isn't uniqueness.
> 
You are right, I should not have said "always".  It is still not 
guaranteed, and does not "enforce" uniqueness.
But yyyymmddhhmmssrrrrr means that you would need a duplicate within the 
same second, which is quite a bit less likely than over a much longer 
period of time.
I have never done the math to calculate exactly how likely it was that I 
would get 2 identical identifiers for any given number of 
requests/second. Nor what adding one rand() digit would do to this 
probability.
Let's say this : I have a dozen websites where the above is being used. 
None of them even approaches 99,999 requests/second, and none of them 
necessarily generates a unique-id at each request.
But the setup is such that, should a duplicate identifier be generated, 
the application stops with an error. I did it so because I was curious, 
and because my sites are not Google.
Over a period of about 5 years, it has never happened.
Probabilities being what they are, it does not mean that it cannot be 
happening at this very second.  But then also, the Sun may have turned 
into a supernova 6 minutes ago.

One alternative is to use some strictly incremental counter, shared 
between multiple processes running on potentially multiple systems or 
CPUs.  This requires a common place to store the counter, which survives 
a system restart, and it requires some lock-read-increment-unlock 
mechanism.  I don't know any really fast and efficient way of doing 
this.  I am interested however if anyone knows one.