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Commita790ed9

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Improve documentation and comment for quorum-based sync replication.
Author: Masahiko Sawada, heavily modified by meDiscussion:http://postgr.es/m/CAHGQGwEKOw=SmPLxJzkBsH6wwDBgOnVz46QjHbtsiZ-d-2RGUg@mail.gmail.com
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‎doc/src/sgml/high-availability.sgml‎

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@@ -1084,8 +1084,8 @@ primary_slot_name = 'node_a_slot'
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In the case that <varname>synchronous_commit</> is set to
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<literal>remote_apply</>, the standby sends reply messages when the commit
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record is replayed, making the transaction visible.
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If the standby is chosen as a synchronous standby,from a priority
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listof <varname>synchronous_standby_names</> on the primary, the reply
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If the standby is chosen as a synchronous standby,according to the setting
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of <varname>synchronous_standby_names</> on the primary, the reply
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messages from that standby will be considered along with those from other
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synchronous standbys to decide when to release transactions waiting for
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confirmation that the commit record has been received. These parameters
@@ -1246,9 +1246,20 @@ synchronous_standby_names = 'FIRST 2 (s1, s2, s3)'
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The best solution for high availability is to ensure you keep as many
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synchronous standbys as requested. This can be achieved by naming multiple
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potential synchronous standbys using <varname>synchronous_standby_names</>.
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The standbys whose names appear earlier in the list will be used as
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synchronous standbys. Standbys listed after these will take over
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the role of synchronous standby if one of current ones should fail.
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</para>
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<para>
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In a priority-based synchronous replication, the standbys whose names
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appear earlier in the list will be used as synchronous standbys.
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Standbys listed after these will take over the role of synchronous standby
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if one of current ones should fail.
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</para>
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<para>
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In a quorum-based synchronous replication, all the standbys appearing
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in the list will be used as candidates for synchronous standbys.
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Even if one of them should fail, the other standbys will keep performing
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the role of candidates of synchronous standby.
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</para>
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<para>

‎src/backend/replication/syncrep.c‎

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@@ -53,6 +53,10 @@
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* in the list. All the standbys appearing in the list are considered as
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* candidates for quorum synchronous standbys.
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*
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* If neither FIRST nor ANY is specified, FIRST is used as the method.
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* This is for backward compatibility with 9.6 or before where only a
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* priority-based sync replication was supported.
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*
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* Before the standbys chosen from synchronous_standby_names can
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* become the synchronous standbys they must have caught up with
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* the primary; that may take some time. Once caught up,
@@ -629,6 +633,7 @@ SyncRepGetNthLatestSyncRecPtr(XLogRecPtr *writePtr, XLogRecPtr *flushPtr,
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i++;
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}
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/* Sort each array in descending order */
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qsort(write_array,len,sizeof(XLogRecPtr),cmp_lsn);
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qsort(flush_array,len,sizeof(XLogRecPtr),cmp_lsn);
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qsort(apply_array,len,sizeof(XLogRecPtr),cmp_lsn);

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