alt Hacker NewsThe latest UUID (7?) Uses half random gen, half timestamp. This not only makes it sortable by creation, but would also make a collision like this impossible.
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ffsm8 • yesterday at 5:38 PM
Considering the context I think it's worth pointing out that it's technically not impossible - it's just even less likely.
Everything in crypto is always a probability - never a certainty
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majorchord • today at 12:39 AM
The spec doesn't require the use of actually random numbers though.
It's still possible in most implementations of UUIDv7.
UUIDv7 assigns the first 48 bits for the timestamp in milliseconds. You can generate a lot of UUID's in a millisecond though!
Then you have another 12 bits that you can use as you wish; "rand_a". The spec has a few methods they suggest on how to use these bits including 12 bits of random data, using it for sub-millisecond timestamps, or creating a monotonic counter, but each have their downsides:
- Purely random data means you can still run into collisions and anything within the same millisecond is unordered
- Sub millisecond you can run into collisions; there's nothing stopping you from generating two UUID's with the same 62 bits of rand_b data in the same sub-millisecond timestamp.
- Monotonic counters can overflow before the next tick, then what? Rollover? Once you roll over it's no longer monotonic and you can generate the same random data within the same monotonic cycle. Also; it's only monotonic to the system that's generating the UUID. If you have a distributed system and they each have their own monotonic cycles then you'll be generating UUID's with the same timestamp + monotonic counter, and again, are relying on not generating the same random data.
You can steal some of the 62 bits in rand_b if you want as well; you can use rand_a for sub-millisecond accuracy, and then use a few bits of rand_b for a monotonic counter. There's still a chance of collision here, but it's exceedingly low at the expense of less truly random data at the end.
If you want truly collision free, you'd also need to assign a couple of bits to identify the subsystem generating the UUID so that the monotonic counter is unique to that subsystem. You lose the ordering part of the monotonic counter this way though, but I guess you could argue that in nearly 100% of cases the accuracy of sub-millisecond order in a distributed system is a lie anyways.