Question

I want to generate a uuid which should be used as unique identifier across different systems/databases. I read the examples but i don’t understand how i can be sure that the generated id’s are unique over different systems and databases.

I hope you can help me out here.

Best regards

Answer 1

The idea behind a UUID is — depending on how they are generated — that there are so many values representable with 122-bits* that the chance of accidental collisions — again, depending on how they are generated — is very, very, very, very, very, very, very, very, small.

An excerpt from Wikipedia for the UUID version 4 (Leach-Salz Random):

In other words, only after generating 1 billion UUIDs every second for the next 100 years, the probability of creating just one duplicate would be about 50%. The probability of one duplicate would be about 50% if every person on earth owns 600 million UUIDs.

… however, these probabilities only hold when the UUIDs are generated using sufficient entropy.

Of course, there are other UUID generation schemes and "well-known GUIDs", not all of which may be suitable for "globally-unique" usage. (Additionally, non-specialized UUIDs tend not to work well for primary/clustered keys due to fragmentation on insert: SQL Server has NEWSEQUENTIALID to help with that issue.)

Happy coding.

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*There is a maximum of 128-bits in a UUID, however some UUID versions use some of the bits internally. I do not know what boost uses but I suspect it is also UUIDv4.