In response to Rob Sargent <robjsargent@gmail.com>:
> >
> > In our case (and I expect it's the case with most people considering UUIDs)
> > we're talking about independent devices that occasionally synchronize
> > data between themselves. These devices need to generate a unique ID
> > of some sort without having to check with every other device. This is
> > one of the problems that UUIDs were intended to fix.
>
> Indeed there is a finite space. A very large, but finite space, just as
> sequence has I suspect. If your software cannot handle a rollback for
> whatever reason, you have much bigger problem on your hand than the the
> remote chance of a collision in uuid generation.
You missed the point.
Despite the fact that the chance of a collision is very, very small, there
is no easy way to fix it if it happens. Zero. It can't be done without
shutting the system down, recalling all the remote devices and manually
reconciling the problem ... which is not an option.
Also, it's hard to DETECT the collision. If a device creates a new record
with a duplicate UUID, how do we tell that apart, during synchronization,
from an update to the record? Now I have your SSN or private medical
data included as part of my record, which violates laws, could lead to
incorrect medical care that could kill someone ... etc.
So, despite the chance being very small, the consequences are HUGE. I
can't just detect it and roll back.
If you have a single DB backend, these problem are easy to solve -- so
easy, in fact, that using UUIDs is overkill.
But when you have mission-critical data that must be correct and available
under every circumstance, you have to consider that UUIDs are not a
guarantee. And when a method that DOES have a guarantee is one of the
options, why would you take ANY risk at all, no matter how small?
(BTW: I hope that the people who think that the risk is acceptable aren't
writing medical software. Even if it only kills one person every 10,000
years because they were given the wrong medicine, that's too often in
my opinion)
--
Bill Moran
http://www.potentialtech.com
http://people.collaborativefusion.com/~wmoran/