Support quorum-based synchronous replication.

This feature is also known as "quorum commit" especially in discussion
on pgsql-hackers.

This commit adds the following new syntaxes into synchronous_standby_names
GUC. By using FIRST and ANY keywords, users can specify the method to
choose synchronous standbys from the listed servers.

  FIRST num_sync (standby_name [, ...])
  ANY num_sync (standby_name [, ...])

The keyword FIRST specifies a priority-based synchronous replication
which was available also in 9.6 or before. This method makes transaction
commits wait until their WAL records are replicated to num_sync
synchronous standbys chosen based on their priorities.

The keyword ANY specifies a quorum-based synchronous replication
and makes transaction commits wait until their WAL records are
replicated to *at least* num_sync listed standbys. In this method,
the values of sync_state.pg_stat_replication for the listed standbys
are reported as "quorum". The priority is still assigned to each standby,
but not used in this method.

The existing syntaxes having neither FIRST nor ANY keyword are still
supported. They are the same as new syntax with FIRST keyword, i.e.,
a priorirty-based synchronous replication.

Author: Masahiko Sawada
Reviewed-By: Michael Paquier, Amit Kapila and me
Discussion: <CAD21AoAACi9NeC_ecm+Vahm+MMA6nYh=Kqs3KB3np+MBOS_gZg@mail.gmail.com>

Many thanks to the various individuals who were involved in
discussing and developing this feature.

1 files changed, 32 insertions, 1 deletions

diff --git a/src/test/recovery/t/007_sync_rep.pl b/src/test/recovery/t/007_sync_rep.pl
index 0c872263eab..e11b4289d52 100644
--- a/src/test/recovery/t/007_sync_rep.pl
+++ b/src/test/recovery/t/007_sync_rep.pl
@@ -3,7 +3,7 @@ use strict;
 use warnings;
 use PostgresNode;
 use TestLib;
-use Test::More tests => 8;
+use Test::More tests => 11;
 
 # Query checking sync_priority and sync_state of each standby
 my $check_sql =
@@ -172,3 +172,34 @@ test_sync_state(
 standby2|1|sync
 standby4|1|potential),
 	'potential standby found earlier in array is promoted to sync');
+
+# Check that standby1 and standby2 are chosen as sync standbys
+# based on their priorities.
+test_sync_state(
+$node_master, qq(standby1|1|sync
+standby2|2|sync
+standby4|0|async),
+'priority-based sync replication specified by FIRST keyword',
+'FIRST 2(standby1, standby2)');
+
+# Check that all the listed standbys are considered as candidates
+# for sync standbys in a quorum-based sync replication.
+test_sync_state(
+$node_master, qq(standby1|1|quorum
+standby2|2|quorum
+standby4|0|async),
+'2 quorum and 1 async',
+'ANY 2(standby1, standby2)');
+
+# Start Standby3 which will be considered in 'quorum' state.
+$node_standby_3->start;
+
+# Check that the setting of 'ANY 2(*)' chooses all standbys as
+# candidates for quorum sync standbys.
+test_sync_state(
+$node_master, qq(standby1|1|quorum
+standby2|1|quorum
+standby3|1|quorum
+standby4|1|quorum),
+'all standbys are considered as candidates for quorum sync standbys',
+'ANY 2(*)');