The traditional way to handle it is to have a central database the assigns numbers and increments a counter. The problem with is the central database may be inaccessible. Further, the central database becomes a crucial bottleneck.
You can also have an in-ram unique number server with a thread safe access.
I suggest a partially decentralised variant in my Unique Number Server Student Project.
Another way to do it is to use large pseudorandom numbers and count on the improbability of two numbers being picked the same. This technique is fully decentralised approach. No communication between number assigners is needed. This is how java.util. UUID (Universally Unique Identifier) works.
Let’s say you needed to assign ticket numbers. The easiest way is to have a central counter you increment to serve ascending integers. If you want a discourage counterfeiting ticket numbers, other than my making duplicates of actual tickets, you need to make the set of number you use sparce, so if someone randomly picks a number it will be an invalid number. You want a simple mathematical way of knowing if a number is valid.
There are two simple algorithms.
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