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

Is a GUID unique 100% of the time?

Not guaranteed, since there are several ways of generating one. However, you can try to calculate the chance of creating two GUIDs that are identical and you get the idea: a GUID has 128 bits, hence, there are 2¹²⁸ distinct GUIDs – much more than there are stars in the known universe. Read the wikipedia article for more details.

Answer 2

Eric Lippert has written a very interesting series of articles about GUIDs.

There are on the order 2³⁰ personal computers in the world (and of course lots of hand-held devices or non-PC computing devices that have more or less the same levels of computing power, but lets ignore those). Let's assume that we put all those PCs in the world to the task of generating GUIDs; if each one can generate, say, 2²⁰ GUIDs per second then after only about 2⁷² seconds -- one hundred and fifty trillion years -- you'll have a very high chance of generating a collision with your specific GUID. And the odds of collision get pretty good after only thirty trillion years.

• GUID Guide, part one
• GUID Guide, part two
• GUID Guide, part three

Answer 3

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.

Answer 4

The simple answer is yes.

Raymond Chen wrote a great article on GUIDs and why substrings of GUIDs are not guaranteed unique. The article goes in to some depth as to the way GUIDs are generated and the data they use to ensure uniqueness, which should go to some length in explaining why they are :-)

Answer 5

As a side note, I was playing around with Volume GUIDs in Windows XP. This is a very obscure partition layout with three disks and fourteen volumes.

\\?\Volume{23005604-eb1b-11de-85ba-806d6172696f}\ (F:)
\\?\Volume{23005605-eb1b-11de-85ba-806d6172696f}\ (G:)
\\?\Volume{23005606-eb1b-11de-85ba-806d6172696f}\ (H:)
\\?\Volume{23005607-eb1b-11de-85ba-806d6172696f}\ (J:)
\\?\Volume{23005608-eb1b-11de-85ba-806d6172696f}\ (D:)
\\?\Volume{23005609-eb1b-11de-85ba-806d6172696f}\ (P:)
\\?\Volume{2300560b-eb1b-11de-85ba-806d6172696f}\ (K:)
\\?\Volume{2300560c-eb1b-11de-85ba-806d6172696f}\ (L:)
\\?\Volume{2300560d-eb1b-11de-85ba-806d6172696f}\ (M:)
\\?\Volume{2300560e-eb1b-11de-85ba-806d6172696f}\ (N:)
\\?\Volume{2300560f-eb1b-11de-85ba-806d6172696f}\ (O:)
\\?\Volume{23005610-eb1b-11de-85ba-806d6172696f}\ (E:)
\\?\Volume{23005611-eb1b-11de-85ba-806d6172696f}\ (R:)
                                     | | | | |
                                     | | | | +-- 6f = o
                                     | | | +---- 69 = i
                                     | | +------ 72 = r
                                     | +-------- 61 = a
                                     +---------- 6d = m

It's not that the GUIDs are very similar but the fact that all GUIDs have the string "mario" in them. Is that a coincidence or is there an explanation behind this?

Now, when googling for part 4 in the GUID I found approx 125.000 hits with volume GUIDs.

Conclusion: When it comes to Volume GUIDs they aren't as unique as other GUIDs.

Answer 6

Guids are statistically unique. The odds of two different clients generating the same Guid are infinitesimally small (assuming no bugs in the Guid generating code). You may as well worry about your processor glitching due to a cosmic ray and deciding that 2+2=5 today.

Multiple threads allocating new guids will get unique values, but you should get that the function you are calling is thread safe. Which environment is this in?