Is a GUID unique 100% of the time?

Question

Is a GUID unique 100% of the time?

Will it stay unique over multiple threads?

Answer 1

In a more general sense, this is known as the "birthday problem" or "birthday paradox". Wikipedia has a pretty good overview at: Wikipedia - Birthday Problem

In very rough terms, the square root of the size of the pool is a rough approximation of when you can expect a 50% chance of a duplicate. The article includes a probability table of pool size and various probabilities, including a row for 2^128. So for a 1% probability of collision you would expect to randomly pick 2.6*10^18 128-bit numbers. A 50% chance requires 2.2*10^19 picks, while SQRT(2^128) is 1.8*10^19.

Of course, that is just the ideal case of a truly random process. As others mentioned, a lot is riding on the that random aspect - just how good is the generator and seed? It would be nice if there was some hardware support to assist with this process which would be more bullet-proof except that anything can be spoofed or virtualized. I suspect that might be the reason why MAC addresses/time-stamps are no longer incorporated.