Re: Support for N synchronous standby servers - take 2

On 2016/04/05 16:35, Simon Riggs wrote:
> 6. Meaning of k (n1, n2, n3) with N servers
>
> It's clearly documented that this means k replies IN SEQUENCE. I believe
> the typical meaning of would be “any k out of N”, which would be faster
> than what we have, e.g.
> 3 (n1, n2, n3) would release as soon as (n1, n2) or (n2, n3) or (n1, n3)
> acknowledge.
>
> The “any k” option is not currently possible, but could be fairly easily.
> The syntax should also be easily extensible.
>
> I would call what we have now “first” semantics, and we could have both of
> these...
>
> * first k (n1, n2, n3) – does the same as k (n1, n2, n3) does now
> * any k (n1, n2, n3) – would release waiters as soon as we have the
> responses from k out of N standbys. “any k” would be faster, so is
> desirable for performance and resilience
>
>>>> So I am suggesting we put an extra keyword in front of the “k”, to
> explain how the k responses should be gathered as an extension to the the
> syntax. I also think implementing “any k” is actually fairly trivial and
> could be done for 9.6 (rather than just "first k").

+1 for 'first/any k (...)', with possibly only 'first' supported for now,
if the 'any' case is more involved than we would like to spend time on,
given the time considerations. IMHO, the extra keyword adds to clarity of
the syntax.

Thanks,
Amit