[TriLUG] Re: Aligning of the crazy numbers

[Lance A. Brown]

What about Data::UUID?

http://search.cpan.org/~rjbs/Data-UUID-0.14/UUID.pm

This module provides a framework for generating UUIDs (Universally
Unique Identifiers, also known as GUIDs (Globally Unique Identifiers). A
UUID is 128 bits long, and is guaranteed to be different from all other
UUIDs/GUIDs generated until 3400 CE.

UUIDs were originally used in the Network Computing System (NCS) and
later in the Open Software Foundation's (OSF) Distributed Computing
Environment. Currently many different technologies rely on UUIDs to
provide unique identity for various software components. Microsoft
COM/DCOM for instance, uses GUIDs very extensively to uniquely identify
classes, applications and components across network-connected systems.

The algorithm for UUID generation, used by this extension, is described
in the Internet Draft "UUIDs and GUIDs" by Paul J. Leach and Rich Salz
(http://hegel.ittc.ku.edu/topics/internet/internet-drafts/draft-l/draft-leach-uuids-guids-01.txt).
It provides reasonably efficient and reliable framework for generating
UUIDs and supports fairly high allocation rates -- 10 million per second
per machine -- and therefore is suitable for identifying both extremely
short-lived and very persistent objects on a given system as well as
across the network.


Owen Berry 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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