Allow the logical_replication_mode to be used on the subscriber.

Extend the existing developer option 'logical_replication_mode' to help
test the parallel apply of large transactions on the subscriber.

When set to 'buffered', the leader sends changes to parallel apply workers
via a shared memory queue. When set to 'immediate', the leader serializes
all changes to files and notifies the parallel apply workers to read and
apply them at the end of the transaction.

This helps in adding tests to cover the serialization code path in
parallel streaming mode.

Author: Hou Zhijie
Reviewed-by: Peter Smith, Kuroda Hayato, Sawada Masahiko, Amit Kapila
Discussion: https://postgr.es/m/CAA4eK1+wyN6zpaHUkCLorEWNx75MG0xhMwcFhvjqm2KURZEAGw@mail.gmail.com

Author: Amit Kapila

doc/src/sgml/config.sgml

@@ -11701,22 +11701,35 @@ LOG:  CleanUpLock: deleting: lock(0xb7acd844) id(24688,24696,0,0,0,1)
       </term>
       <listitem>
        <para>
-        Allows streaming or serializing changes immediately in logical decoding.
-        The allowed values of <varname>logical_replication_mode</varname> are
-        <literal>buffered</literal> and <literal>immediate</literal>. When set
-        to <literal>immediate</literal>, stream each change if
+        The allowed values are <literal>buffered</literal> and
+        <literal>immediate</literal>. The default is <literal>buffered</literal>.
+        This parameter is intended to be used to test logical decoding and
+        replication of large transactions.  The effect of
+        <varname>logical_replication_mode</varname> is different for the
+        publisher and subscriber:
+       </para>
+
+       <para>
+        On the publisher side, <varname>logical_replication_mode</varname>
+        allows streaming or serializing changes immediately in logical decoding.
+        When set to <literal>immediate</literal>, stream each change if the
         <literal>streaming</literal> option (see optional parameters set by
         <link linkend="sql-createsubscription"><command>CREATE SUBSCRIPTION</command></link>)
         is enabled, otherwise, serialize each change.  When set to
-        <literal>buffered</literal>, which is the default, decoding will stream
-        or serialize changes when <varname>logical_decoding_work_mem</varname>
-        is reached.
+        <literal>buffered</literal>, the decoding will stream or serialize
+        changes when <varname>logical_decoding_work_mem</varname> is reached.
        </para>
+
        <para>
-        This parameter is intended to be used to test logical decoding and
-        replication of large transactions for which otherwise we need to
-        generate the changes till <varname>logical_decoding_work_mem</varname>
-        is reached.
+        On the subscriber side, if the <literal>streaming</literal> option is set to
+        <literal>parallel</literal>, <varname>logical_replication_mode</varname>
+        can be used to direct the leader apply worker to send changes to the
+        shared memory queue or to serialize all changes to the file.  When set to
+        <literal>buffered</literal>, the leader sends changes to parallel apply
+        workers via a shared memory queue.  When set to
+        <literal>immediate</literal>, the leader serializes all changes to files
+        and notifies the parallel apply workers to read and apply them at the
+        end of the transaction.
        </para>
       </listitem>
      </varlistentry>

src/backend/replication/logical/applyparallelworker.c

@@ -1149,6 +1149,13 @@ pa_send_data(ParallelApplyWorkerInfo *winfo, Size nbytes, const void *data)
 	Assert(!IsTransactionState());
 	Assert(!winfo->serialize_changes);
 
+	/*
+	 * We don't try to send data to parallel worker for 'immediate' mode. This
+	 * is primarily used for testing purposes.
+	 */
+	if (unlikely(logical_replication_mode == LOGICAL_REP_MODE_IMMEDIATE))
+		return false;
+
 /*
  * This timeout is a bit arbitrary but testing revealed that it is sufficient
  * to send the message unless the parallel apply worker is waiting on some
@@ -1187,12 +1194,7 @@ pa_send_data(ParallelApplyWorkerInfo *winfo, Size nbytes, const void *data)
 			startTime = GetCurrentTimestamp();
 		else if (TimestampDifferenceExceeds(startTime, GetCurrentTimestamp(),
 											SHM_SEND_TIMEOUT_MS))
-		{
-			ereport(LOG,
-					(errmsg("logical replication apply worker will serialize the remaining changes of remote transaction %u to a file",
-							winfo->shared->xid)));
 			return false;
-		}
 	}
 }
 
@@ -1206,6 +1208,10 @@ void
 pa_switch_to_partial_serialize(ParallelApplyWorkerInfo *winfo,
 							   bool stream_locked)
 {
+	ereport(LOG,
+			(errmsg("logical replication apply worker will serialize the remaining changes of remote transaction %u to a file",
+					winfo->shared->xid)));
+
 	/*
 	 * The parallel apply worker could be stuck for some reason (say waiting
 	 * on some lock by other backend), so stop trying to send data directly to

src/backend/utils/misc/guc_tables.c

@@ -4920,8 +4920,10 @@ struct config_enum ConfigureNamesEnum[] =
 
 	{
 		{"logical_replication_mode", PGC_USERSET, DEVELOPER_OPTIONS,
-			gettext_noop("Controls when to replicate each change."),
-			gettext_noop("On the publisher, it allows streaming or serializing each change in logical decoding."),
+			gettext_noop("Controls when to replicate or apply each change."),
+			gettext_noop("On the publisher, it allows streaming or serializing each change in logical decoding. "
+						 "On the subscriber, it allows serialization of all changes to files and notifies the "
+						 "parallel apply workers to read and apply them at the end of the transaction."),
 			GUC_NOT_IN_SAMPLE
 		},
 		&logical_replication_mode,

src/test/subscription/t/015_stream.pl

@@ -312,6 +312,34 @@ sub test_streaming
   $node_subscriber->safe_psql('postgres', "SELECT count(*) FROM test_tab_2");
 is($result, qq(10000), 'data replicated to subscriber after dropping index');
 
+# Test serializing changes to files and notify the parallel apply worker to
+# apply them at the end of the transaction.
+$node_subscriber->append_conf('postgresql.conf',
+	'logical_replication_mode = immediate');
+# Reset the log_min_messages to default.
+$node_subscriber->append_conf('postgresql.conf', "log_min_messages = warning");
+$node_subscriber->reload;
+
+# Run a query to make sure that the reload has taken effect.
+$node_subscriber->safe_psql('postgres', q{SELECT 1});
+
+$offset = -s $node_subscriber->logfile;
+
+$node_publisher->safe_psql('postgres',
+	"INSERT INTO test_tab_2 SELECT i FROM generate_series(1, 5000) s(i)");
+
+# Ensure that the changes are serialized.
+$node_subscriber->wait_for_log(
+	qr/LOG: ( [A-Z0-9]+:)? logical replication apply worker will serialize the remaining changes of remote transaction \d+ to a file/,
+	$offset);
+
+$node_publisher->wait_for_catchup($appname);
+
+# Check that transaction is committed on subscriber
+$result =
+  $node_subscriber->safe_psql('postgres', "SELECT count(*) FROM test_tab_2");
+is($result, qq(15000), 'parallel apply worker replayed all changes from file');
+
 $node_subscriber->stop;
 $node_publisher->stop;
 

src/test/subscription/t/018_stream_subxact_abort.pl

@@ -143,15 +143,17 @@ sub test_streaming
 	"CREATE TABLE test_tab (a int primary key, b varchar)");
 $node_publisher->safe_psql('postgres',
 	"INSERT INTO test_tab VALUES (1, 'foo'), (2, 'bar')");
+$node_publisher->safe_psql('postgres', "CREATE TABLE test_tab_2 (a int)");
 
 # Setup structure on subscriber
 $node_subscriber->safe_psql('postgres',
 	"CREATE TABLE test_tab (a int primary key, b text, c INT, d INT, e INT)");
+$node_subscriber->safe_psql('postgres', "CREATE TABLE test_tab_2 (a int)");
 
 # Setup logical replication
 my $publisher_connstr = $node_publisher->connstr . ' dbname=postgres';
 $node_publisher->safe_psql('postgres',
-	"CREATE PUBLICATION tap_pub FOR TABLE test_tab");
+	"CREATE PUBLICATION tap_pub FOR TABLE test_tab, test_tab_2");
 
 my $appname = 'tap_sub';
 
@@ -198,6 +200,63 @@ sub test_streaming
 
 test_streaming($node_publisher, $node_subscriber, $appname, 1);
 
+# Test serializing changes to files and notify the parallel apply worker to
+# apply them at the end of the transaction.
+$node_subscriber->append_conf('postgresql.conf',
+	'logical_replication_mode = immediate');
+# Reset the log_min_messages to default.
+$node_subscriber->append_conf('postgresql.conf', "log_min_messages = warning");
+$node_subscriber->reload;
+
+# Run a query to make sure that the reload has taken effect.
+$node_subscriber->safe_psql('postgres', q{SELECT 1});
+
+my $offset = -s $node_subscriber->logfile;
+
+$node_publisher->safe_psql(
+	'postgres', q{
+	BEGIN;
+	INSERT INTO test_tab_2 values(1);
+	ROLLBACK;
+	});
+
+# Ensure that the changes are serialized.
+$node_subscriber->wait_for_log(
+	qr/LOG: ( [A-Z0-9]+:)? logical replication apply worker will serialize the remaining changes of remote transaction \d+ to a file/,
+	$offset);
+
+$node_publisher->wait_for_catchup($appname);
+
+# Check that transaction is aborted on subscriber
+$result =
+  $node_subscriber->safe_psql('postgres', "SELECT count(*) FROM test_tab_2");
+is($result, qq(0), 'check rollback was reflected on subscriber');
+
+# Serialize the ABORT sub-transaction.
+$offset = -s $node_subscriber->logfile;
+
+$node_publisher->safe_psql(
+	'postgres', q{
+	BEGIN;
+	INSERT INTO test_tab_2 values(1);
+	SAVEPOINT sp;
+	INSERT INTO test_tab_2 values(1);
+	ROLLBACK TO sp;
+	COMMIT;
+	});
+
+# Ensure that the changes are serialized.
+$node_subscriber->wait_for_log(
+	qr/LOG: ( [A-Z0-9]+:)? logical replication apply worker will serialize the remaining changes of remote transaction \d+ to a file/,
+	$offset);
+
+$node_publisher->wait_for_catchup($appname);
+
+# Check that only sub-transaction is aborted on subscriber.
+$result =
+  $node_subscriber->safe_psql('postgres', "SELECT count(*) FROM test_tab_2");
+is($result, qq(1), 'check rollback to savepoint was reflected on subscriber');
+
 $node_subscriber->stop;
 $node_publisher->stop;
 

src/test/subscription/t/023_twophase_stream.pl

@@ -319,16 +319,18 @@ sub test_streaming
 	"CREATE TABLE test_tab (a int primary key, b varchar)");
 $node_publisher->safe_psql('postgres',
 	"INSERT INTO test_tab VALUES (1, 'foo'), (2, 'bar')");
+$node_publisher->safe_psql('postgres', "CREATE TABLE test_tab_2 (a int)");
 
 # Setup structure on subscriber (columns a and b are compatible with same table name on publisher)
 $node_subscriber->safe_psql('postgres',
 	"CREATE TABLE test_tab (a int primary key, b text, c timestamptz DEFAULT now(), d bigint DEFAULT 999)"
 );
+$node_subscriber->safe_psql('postgres', "CREATE TABLE test_tab_2 (a int)");
 
 # Setup logical replication (streaming = on)
 my $publisher_connstr = $node_publisher->connstr . ' dbname=postgres';
 $node_publisher->safe_psql('postgres',
-	"CREATE PUBLICATION tap_pub FOR TABLE test_tab");
+	"CREATE PUBLICATION tap_pub FOR TABLE test_tab, test_tab_2");
 
 my $appname = 'tap_sub';
 
@@ -384,6 +386,48 @@ sub test_streaming
 
 test_streaming($node_publisher, $node_subscriber, $appname, 1);
 
+# Test serializing changes to files and notify the parallel apply worker to
+# apply them at the end of the transaction.
+$node_subscriber->append_conf('postgresql.conf',
+	'logical_replication_mode = immediate');
+# Reset the log_min_messages to default.
+$node_subscriber->append_conf('postgresql.conf', "log_min_messages = warning");
+$node_subscriber->reload;
+
+# Run a query to make sure that the reload has taken effect.
+$node_subscriber->safe_psql('postgres', q{SELECT 1});
+
+my $offset = -s $node_subscriber->logfile;
+
+$node_publisher->safe_psql(
+	'postgres', q{
+	BEGIN;
+	INSERT INTO test_tab_2 values(1);
+	PREPARE TRANSACTION 'xact';
+	});
+
+# Ensure that the changes are serialized.
+$node_subscriber->wait_for_log(
+	qr/LOG: ( [A-Z0-9]+:)? logical replication apply worker will serialize the remaining changes of remote transaction \d+ to a file/,
+	$offset);
+
+$node_publisher->wait_for_catchup($appname);
+
+# Check that transaction is in prepared state on subscriber
+$result = $node_subscriber->safe_psql('postgres',
+	"SELECT count(*) FROM pg_prepared_xacts;");
+is($result, qq(1), 'transaction is prepared on subscriber');
+
+# Check that 2PC gets committed on subscriber
+$node_publisher->safe_psql('postgres', "COMMIT PREPARED 'xact';");
+
+$node_publisher->wait_for_catchup($appname);
+
+# Check that transaction is committed on subscriber
+$result =
+  $node_subscriber->safe_psql('postgres', "SELECT count(*) FROM test_tab_2");
+is($result, qq(1), 'transaction is committed on subscriber');
+
 ###############################
 # check all the cleanup
 ###############################