Re: Support for N synchronous standby servers - take 2

On Tue, Apr 5, 2016 at 6:09 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> On 5 April 2016 at 08:58, Amit Langote <Langote_Amit_f8@lab.ntt.co.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.

s/oldest/newest ?

> 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.

Yes. Probably we need to use some time to find what algorithm is the best
for searching the k'th newest pointer.

> The syntax and doc changes would take a couple of
> hours.

Yes, the updates of documentation would need more time.