Date: Wed, 17 Jul 2019 10:38:35 +0900
From: Michael Paquier <michael@paquier.xyz>
To: Jesper Pedersen <jesper.pedersen@redhat.com>
Cc: Laurenz Albe <laurenz.albe@cybertec.at>,
	Alvaro Herrera <alvherre@2ndquadrant.com>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: pg_receivewal documentation
Message-ID: <20190717013835.GA2130@paquier.xyz>
References: <20190710142421.GA28387@alvherre.pgsql>
 <54779de5-f922-c0cf-629d-b582eded7265@redhat.com>
 <20190716050555.GA1439@paquier.xyz>
 <ebed4b9043b222d486ea3f0e44e7dcd242a80781.camel@cybertec.at>
 <65c3ecdf-8995-34cb-83c4-50068877e632@redhat.com>
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On Tue, Jul 16, 2019 at 01:03:12PM -0400, Jesper Pedersen wrote:
> Here is the patch for that.

+       <para>
+        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. 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>.
+       </para>

I think we should really document the caveat with priority-based sets
of standbys as much as quorum-based sets.  For example if a user sets
synchronous_commit = remote_apply in postgresql.conf, and then sets
s_s_names to '2(pg_receivewal, my_connected_standby)' to get a
priority-based set, then you have the same problem, and pg_receivewal
is not the only synchronous standby in this configuration.  The patch
does not cover that case properly.
--
Michael

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