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Doc: Clarify interactions of pg_receivewal with remote_apply
Using pg_receivewal with synchronous_commit = remote_apply set in the backend is incompatible if pg_receivewal is a synchronous standby as it never applies WAL, so document this problem and solutions to it. Backpatch to 9.6, where remote_apply has been added. Author: Robert Haas, Jesper Pedersen Reviewed-by: Laurenz Albe, Álvaro Herrera, Michael Paquier Discussion: https://postgr.es/m/1427a2d3-1e51-9335-1931-4f8853d90d5e@redhat.com Backpatch-through: 9.6
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@ -52,7 +52,17 @@ PostgreSQL documentation
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Unlike the WAL receiver of a PostgreSQL standby server, <application>pg_receivewal</application>
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Unlike the WAL receiver of a PostgreSQL standby server, <application>pg_receivewal</application>
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by default flushes WAL data only when a WAL file is closed.
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by default flushes WAL data only when a WAL file is closed.
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The option <option>--synchronous</option> must be specified to flush WAL data
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The option <option>--synchronous</option> must be specified to flush WAL data
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in real time.
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in real time. Since <application>pg_receivewal</application> does not
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apply WAL, you should not allow it to become a synchronous standby when
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<xref linkend="guc-synchronous-commit"/> equals
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<literal>remote_apply</literal>. If it does, it will appear to be a
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standby that never catches up, and will cause transaction commits to
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block. To avoid this, you should either configure an appropriate value
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for <xref linkend="guc-synchronous-standby-names"/>, or specify
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<varname>application_name</varname> for
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<application>pg_receivewal</application> that does not match it, or
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change the value of <varname>synchronous_commit</varname> to
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something other than <literal>remote_apply</literal>.
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</para>
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</para>
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<para>
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<para>
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