[Qt-interest] QUuid with QUuid::Time

[Scott Aron Bloom]

-----Original Message-----
From: qt-interest-bounces at trolltech.com [mailto:qt-interest-bounces at trolltech.com] On Behalf Of Constantin Makshin
Sent: Tuesday, June 15, 2010 8:49 AM
To: Qt Interest
Subject: Re: [Qt-interest] QUuid with QUuid::Time

On Tuesday 15 June 2010 18:58:04 Oliver.Knoll at comit.ch wrote:
> Constantin Makshin wrote on Monday, June 14, 2010 9:50 PM:
> 
> > IMHO, even 7 duplicates per 15 million values (~0.00005% chance of
> > collision) is [more than] acceptable result for most, if not all,
> > applications. :)
> 
> But can we agree upon the following: 
> 
> Given propability p1 of a "UUID collision", as suggested to be "infinitely small" by descriptions such as:
> 
>   "For example, consider the observable universe, which contains about 5×10^22 stars; every star could then have 6.8×10^15 universally unique GUIDs."
> 
> and given probability p2 = ~0.00005% of a real world UUID generator implementation, that there are, well, "light years" (as to stay in the figurative speech of the http://en.wikipedia.org/wiki/Globally_Unique_Identifier article) between p1 and p2?
Of course, 0.00005% is much more than theoretical 1/2^128 (~3e-37%), but still [generally] acceptable.

> > I agree with one of the posters from that discussion:
> > "chances are so low that you really should stress about something
> > else- [...]"
> 
> I also generally agree on that :) It is always a question of "how bad" the impact of a duplicate really is and whether it is worth bothering. But for example if - and I stress the IF here - you'd rely on a UUID for identifying finanical transactions it could turn out disastrous... Or would you trust a bank which would officially state that "our online banking/transactions are (technically) 0.99995% safe"? ;)
I'd *never* trust a bank with 0.99995% "transaction safety". ~1 successful transaction per 100 attempts would quickly turn anyone into a bankrupt. :)

> Cheers, Oliver
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Of course you need to take into account the likelihood of collision WRT any data safety...

If you are only doing 10k transactions/day, and the transaction id safety is 1 in 30 million (not 1 in 100 as you imply), its gonna take 3000 days for your duplicate to hit...  

I would worry at that scale.. Since if Im working on server side large app, hopefully it will be in process for 8 years or so...

But large scale or critical applications always have other issues that come up... 

However, for a smaller client side app, where you are only creating 100-500 unique transactions a day, the 30,000->60,000 days its gonna take to hit the bug I can live with, and focus on other more likely scenarios of failures.

Scott