Re: Support for N synchronous standby servers - take 2

On 5 April 2016 at 08:58, Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>
wrote:

> >>>> 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.
>

Further thoughts:

I said "any k" was faster, though what I mean is both faster and more
robust. If you have network peaks from any of the k sync standbys then the
user will wait longer. With "any k", if a network peak occurs, then another
standby response will work just as well. So the performance of "any k" will
be both faster, more consistent and less prone to misconfiguration.

I also didn't explain why I think it is easy to implement "any k".

All we need to do is change SyncRepGetOldestSyncRecPtr() so that it returns
the k'th oldest pointer of any named standby. Then use that to wake up user
backends. So the change requires only slightly modified logic in a very
isolated part of the code, almost all of which would be code inserts to
cope with the new option. The syntax and doc changes would take a couple of
hours.

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Simon Riggs http://www.2ndQuadrant.com/
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