When are you truly forced to use UUID as part of the design?

Question

I don\'t really see the point of UUID. I know the probability of a collision is effectively nil, but effectively nil is not even close to impossible.

Answer 1

The thing that UUIDs buy you that is very difficult to do otherwise is to get a unique identifier without having to consult or coordinate with a central authority. The general problem of being able to get such a thing without some sort of managed infrastructure is the problem the UUIDs solve.

I've read that according to the birthday paradox the chance of a UUID collision occuring is 50% once 2^64 UUIDs have been generated. Now 2^64 is a pretty big number, but a 50% chance of collision seems far too risky (for example, how many UUIDs need to exist before there's a 5% chance of collision - even that seems like too large of a probability).

The problem with that analysis is twofold:

1. UUIDs are not entirely random - there are major components of the UUID that are time and/or location-based. So to have any real chance at a collision, the colliding UUIDs need tobe generated at the exact same time from different UUID generators. I'd say that while there is a reasonable chance that several UUID's might be generated at the same time, there's enough other gunk (including location info or random bits) to make the likeyhood of a collision between this very small set of UUIDs nearly impossible.

2. strictly speaking, UUIDs only need to be unique among the set of other UUIDs that they might be compared against. If you're generating a UUID to use as a database key, it doesn't matter if somewhere else in an evil alternate universe that the same UUID is being used to identify a COM interface. Just like it'll cause no confusion if there's someone (or something) else named "Michael Burr" on Alpha-Centauri.