Message-ID: <ebed4b9043b222d486ea3f0e44e7dcd242a80781.camel@cybertec.at>
Subject: Re: pg_receivewal documentation
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Michael Paquier <michael@paquier.xyz>, Jesper Pedersen
	 <jesper.pedersen@redhat.com>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>, PostgreSQL-development
	 <pgsql-hackers@postgresql.org>
Date: Tue, 16 Jul 2019 18:28:57 +0200
In-Reply-To: <20190716050555.GA1439@paquier.xyz>
References: <20190710142421.GA28387@alvherre.pgsql>
	 <54779de5-f922-c0cf-629d-b582eded7265@redhat.com>
	 <20190716050555.GA1439@paquier.xyz>
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On Tue, 2019-07-16 at 14:05 +0900, Michael Paquier wrote:
> >> How about
> >>          Note that while WAL will be flushed with this setting,
> >>          <application>pg_receivewal</application> never applies it, so
> >>          <xref linkend="guc-synchronous-commit"/> must not be set to
> >>          <literal>remote_apply</literal> if <application>pg_receivewal</application>
> >>          is the only synchronous standby.
> 
> This is not true in all cases as since 9.6 it is possible to specify
> multiple synchronous standbys.  So if for example pg_receivewal and
> another synchronous standby are set in s_s_names and that the number
> of a FIRST (priority-based) or ANY (quorum set) is two, then the same
> issue exists, but this documentation is incorrect.  I think that we
> should have a more extensive wording  here, like "if pg_receivewal is
> part of a quorum-based or priority-based set of synchronous standbys."

I think this would be overly complicated.
The wording above seems to cover the priority-based base sufficiently
in my opinion.
Maybe a second sentence with more detail would be better:

  ... must not be set to <literal>remote_apply</literal> if
  <application>pg_receivewal</application> is the only synchronous standby.
  Similarly, if <application>pg_receivewal</application> is part of
  a quorum-based set of synchronous standbys, it won't count towards
  the quorum if <xref linkend="guc-synchronous-commit"/> is set to
  <literal>remote_apply</literal>.

Yours,
Laurenz Albe