Re: Support for N synchronous standby servers - take 2

On Mon, Feb 1, 2016 at 11:28 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>wrote:

> [first version]
> Add only synchronous_standby_num which specifies the number of standbys
> that the master must wait for before marking sync replication as completed.
> This version supports simple use cases like "I want to have two synchronous
> standbys".
>
> [second version]
> Add synchronous_replication_method: 'prioriry' and 'quorum'. This version
> additionally supports simple quorum commit case like "I want to ensure
> that WAL is replicated synchronously to at least two standbys from five
> ones listed in s_s_names".
>
> Or
>
> Add something like quorum_replication_num and quorum_standby_names, i.e.,
> the master must wait for at least q_r_num standbys from ones listed in
> q_s_names before marking sync replication as completed. Also the master
> must wait for sync replication according to s_s_num and s_s_num.
> That is, this approach separates 'priority' and 'quorum' to each
> parameters.
> This increases the number of GUC parameters, but ISTM less confusing, and
> it supports a bit complicated case like "there is one local standby and
> three
> remote standbys, then I want to ensure that WAL is replicated synchronously
> to the local standby and at least two remote one", e.g.,
>
> s_s_num = 1, s_s_names = 'local'
> q_s_num = 2, q_s_names = 'remote1, remote2, remote3'
>
> [third version]
> Add the hooks for more complicated sync replication cases.
>
> I'm thinking that the realistic target for 9.6 might be the first one.
>

If we want to get something out for this release, clearly yes, and being
able to specify 2 sync targets is already a win when the two sync standbys
are not exactly at the same location. FWIW, I don't doing coding and/or
review work, that's basically my first patch that needs a bit more love and
polishing, *and* test cases but I am used enough to perl and PostgresNode
these days to produce something based on sanity checks of
pg_stat_replication and my other set of patches that have more basic
routines.

Now I would not mind if we actually jump into the 3rd case if we are fine
with doing nothing for this release, but this requires a lot of design and
background work, so that's not plausible for 9.6. Of course if there are
voices against the scenario proposed by Fujii-san others feel free to speak
up.
--
Michael