Re: pg_receivewal documentation

[Jesper Pedersen]

Hi,

On 7/16/19 12:28 PM, Laurenz Albe wrote:

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>.

Here is the patch for that.

Best regards,
 Jesper



>From 4fb28d6fe08ddea5a9740c9a81e8e00b94283d78 Mon Sep 17 00:00:00 2001
From: jesperpedersen <jesper.peder...@redhat.com>
Date: Tue, 9 Jul 2019 13:14:25 -0400
Subject: [PATCH] Highlight that pg_receivewal doesn't apply WAL, and as such
 synchronous-commit needs to be remote_write or lower.
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Authors: Álvaro Herrera, Laurenz Albe and Jesper Pedersen
Review-by: Álvaro Herrera, Laurenz Albe and Jesper Pedersen
---
 doc/src/sgml/ref/pg_receivewal.sgml | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/doc/src/sgml/ref/pg_receivewal.sgml b/doc/src/sgml/ref/pg_receivewal.sgml
index 0506120c00..03b20ecd38 100644
--- a/doc/src/sgml/ref/pg_receivewal.sgml
+++ b/doc/src/sgml/ref/pg_receivewal.sgml
@@ -207,6 +207,18 @@ PostgreSQL documentation
         server as a synchronous standby, to ensure that timely feedback is
         sent to the server.
        </para>
+
+       <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>
       </listitem>
      </varlistentry>
 
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2.21.0