Re: UUID v7

On Sat, Nov 9, 2024 at 9:07 AM Sergey Prokhorenko
<sergeyprokhorenko@yahoo.com.au> wrote:
>
> On Saturday 9 November 2024 at 01:00:15 am GMT+3, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> > the microsecond part is working also as a counter in a sense. IT seems fine to me but I'm slightly concerned that there is no guidance of such implementation in RFC 9562.
>
> In fact, there is guidance of similar implementation in RFC 9562:
> https://datatracker.ietf.org/doc/html/rfc9562#name-monotonicity-and-counters
> "Counter Rollover Handling:"
> "Alternatively, implementations MAY increment the timestamp ahead of the actual time and reinitialize the counter."
>

Indeed, thank you.

> But in the near future, this may not be enough for the highest-performance systems.

Yeah, I'm concerned about this. That time might gradually come. That
being said, as long as rand_a part works also as a counter, it's fine.
Also, 12 bits does not differ much as Andrey Borodin mentioned. I
think in the first version it's better to start with a simple
implementation rather than over-engineering it.

Regarding the implementation, the v30 patch uses only microseconds
precision time even on platforms where nanoseconds precision is
available such as Linux. I think it's better to store the value of
(sub-milliseconds * 4096) into 12-bits of rand_a space instead of
directly storing microseconds into 10 bits space. That way, we can use
nanoseconds precision timestamps where available. On some platforms
such as macOS, the sub-milliseconds precision timestamp is restricted
to microseconds, we can consider it as a kind of special case. If
12-bits of rand_a space is not enough to guarantee monotonically in
the future, it is also possible to improve it by putting a (random)
counter into rand_b.

Regards,

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com