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
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.
*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.