Re: UUID v7

On Thu, Oct 31, 2024 at 9:53 PM Andrey M. Borodin <x4mmm(at)yandex-team(dot)ru> wrote:
>
>
>
> > On 1 Nov 2024, at 03:00, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > Therefore, if the
> > system clock moves backward due to NTP, we cannot guarantee
> > monotonicity and sortability. Is that right?
>
> Not exactly. Monotonicity is ensured for a given backend. We make sure that timestamp is advanced at least for ~250ns forward on each UUID generation. 60 bits of time are unique and ascending for a given backend.
>

Thank you for your explanation. I now understand this code guarantees
the monotonicity:

+/* minimum amount of ns that guarantees step of increased_clock_precision */
+#define SUB_MILLISECOND_STEP (1000000/4096 + 1)
+ ns = get_real_time_ns();
+ if (previous_ns + SUB_MILLISECOND_STEP >= ns)
+ ns = previous_ns + SUB_MILLISECOND_STEP;
+ previous_ns = ns;

I think that one of the most important parts in UUIDv7 implementation
is which method (1, 2, or 3 described in RFC 9562) we use to guarantee
the monotonicity. The current patch employs method 3 with the
assumption that 12 bits of sub-millisecond information is available on
most of the systems we support. However, as far as I tested, on MacOS,
values returned by clock_gettime(CLOCK_REALTIME) are only microsecond
precision, meaning that we could waste some randomness. Has this point
been considered?

Regards,

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