Re: Sinister variable caching problem with rand()

Adam Prime wrote:
> Perrin Harkins wrote:
>> On Sat, May 15, 2010 at 4:03 PM, Anthony Esposito
>> <[email protected]> wrote:
>>> In one of my programs I started to receive database errors for not 
>>> having a
>>> unique id. I generate unique ids for each of the mysql lines that I 
>>> add to
>>> the database. I realized that the perl variable $idNum was keeping 
>>> the same
>>> random string for multiple executions.
>>
>> You need to call srand() in s a child init handler:
>> http://marc.info/?l=apache-modperl&m=123904225030744&w=1
>>
>> However, I have to ask, why are you generating id numbers randomly?
>> Why not just let mysql  do it with auto_increment?
>>
>> - Perrin
> 
> I think, in theory you shouldn't have to explicitly srand yourself in 
> your startup.pl if you're using mod_perl 1.29 or greater, and perl 5.8.1 
> or greater.
> 
> see:
> 
> http://marc.info/?l=apache-modperl-dev&m=106606815110220&w=2
> 
Also see : perldoc -f srand
and
http://en.wikipedia.org/wiki/Murphy%27s_law

With the second, I mean that even if the rand() sequence is different in 
different Apache children and cgi-bin's or modules called by these 
children, that still does not mean that the same number could not be 
generated occasionally.  Especially if this over a long period of time, 
and the result is stored in a database.
So I would either use what Perrin mentioned, or combine the result of (a 
shorter) rand() call, with time() for example.

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.  In addition, such an identifier is at 
the same time a time stamp, which is often invaluable in tracking down a 
problem.