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Commitbf279dd

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author
Amit Kapila
committed
Introduce a new GUC 'standby_slot_names'.
This patch provides a way to ensure that physical standbys that arepotential failover candidates have received and flushed changes beforethe primary server making them visible to subscribers. Doing so guaranteesthat the promoted standby server is not lagging behind the subscriberswhen a failover is necessary.The logical walsender now guarantees that all local changes are sent andflushed to the standby servers corresponding to the replication slotsspecified in 'standby_slot_names' before sending those changes to thesubscriber.Additionally, the SQL functions pg_logical_slot_get_changes,pg_logical_slot_peek_changes and pg_replication_slot_advance are modifiedto ensure that they process changes for failover slots only after physicalslots specified in 'standby_slot_names' have confirmed WAL receipt for those.Author: Hou Zhijie and Shveta MalikReviewed-by: Masahiko Sawada, Peter Smith, Bertrand Drouvot, Ajin Cherian, Nisha Moond, Amit KapilaDiscussion:https://postgr.es/m/514f6f2f-6833-4539-39f1-96cd1e011f23@enterprisedb.com
1 parent453c468 commitbf279dd

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18 files changed

+918
-27
lines changed

18 files changed

+918
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lines changed

‎doc/src/sgml/config.sgml

Lines changed: 39 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -4559,6 +4559,45 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"' # Windows
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</listitem>
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</varlistentry>
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<varlistentry id="guc-standby-slot-names" xreflabel="standby_slot_names">
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<term><varname>standby_slot_names</varname> (<type>string</type>)
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<indexterm>
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<primary><varname>standby_slot_names</varname> configuration parameter</primary>
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</indexterm>
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</term>
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<listitem>
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<para>
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A comma-separated list of streaming replication standby server slot names
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that logical WAL sender processes will wait for. Logical WAL sender processes
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will send decoded changes to plugins only after the specified replication
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slots confirm receiving WAL. This guarantees that logical replication
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failover slots do not consume changes until those changes are received
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and flushed to corresponding physical standbys. If a
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logical replication connection is meant to switch to a physical standby
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after the standby is promoted, the physical replication slot for the
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standby should be listed here. Note that logical replication will not
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proceed if the slots specified in the
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<varname>standby_slot_names</varname> do not exist or are invalidated.
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Additionally, the replication management functions
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<link linkend="pg-replication-slot-advance">
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<function>pg_replication_slot_advance</function></link>,
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<link linkend="pg-logical-slot-get-changes">
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<function>pg_logical_slot_get_changes</function></link>, and
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<link linkend="pg-logical-slot-peek-changes">
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<function>pg_logical_slot_peek_changes</function></link>,
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when used with logical failover slots, will block until all
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physical slots specified in <varname>standby_slot_names</varname> have
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confirmed WAL receipt.
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</para>
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<para>
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The standbys corresponding to the physical replication slots in
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<varname>standby_slot_names</varname> must configure
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<literal>sync_replication_slots = true</literal> so they can receive
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logical failover slot changes from the primary.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect2>
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‎doc/src/sgml/func.sgml

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@@ -28150,7 +28150,7 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
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</row>
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<row>
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<entry role="func_table_entry"><para role="func_signature">
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<entryid="pg-logical-slot-get-changes"role="func_table_entry"><para role="func_signature">
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<indexterm>
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<primary>pg_logical_slot_get_changes</primary>
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</indexterm>
@@ -28173,11 +28173,15 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
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the specified value. Note, however, that the actual number of
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rows returned may be larger, since this limit is only checked after
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adding the rows produced when decoding each new transaction commit.
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If the specified slot is a logical failover slot then the function will
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not return until all physical slots specified in
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<link linkend="guc-standby-slot-names"><varname>standby_slot_names</varname></link>
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have confirmed WAL receipt.
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</para></entry>
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</row>
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<row>
28180-
<entry role="func_table_entry"><para role="func_signature">
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<entryid="pg-logical-slot-peek-changes"role="func_table_entry"><para role="func_signature">
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<indexterm>
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<primary>pg_logical_slot_peek_changes</primary>
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</indexterm>
@@ -28232,7 +28236,7 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
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</row>
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<row>
28235-
<entry role="func_table_entry"><para role="func_signature">
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<entryid="pg-replication-slot-advance"role="func_table_entry"><para role="func_signature">
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<indexterm>
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<primary>pg_replication_slot_advance</primary>
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</indexterm>
@@ -28248,7 +28252,11 @@ postgres=# SELECT '0/0'::pg_lsn + pd.segment_number * ps.setting::int + :offset
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the name of the slot and the actual position that it was advanced to.
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The updated slot position information is written out at the next
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checkpoint if any advancing is done. So in the event of a crash, the
28251-
slot may return to an earlier position.
28255+
slot may return to an earlier position. If the specified slot is a
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logical failover slot then the function will not return until all
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physical slots specified in
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<link linkend="guc-standby-slot-names"><varname>standby_slot_names</varname></link>
28259+
have confirmed WAL receipt.
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</para></entry>
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</row>
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‎doc/src/sgml/logicaldecoding.sgml

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Original file line numberDiff line numberDiff line change
@@ -384,6 +384,18 @@ postgres=# select * from pg_logical_slot_get_changes('regression_slot', NULL, NU
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must be enabled on the standby. It is also necessary to specify a valid
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<literal>dbname</literal> in the
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<link linkend="guc-primary-conninfo"><varname>primary_conninfo</varname></link>.
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It's highly recommended that the said physical replication slot is named in
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<link linkend="guc-standby-slot-names"><varname>standby_slot_names</varname></link>
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list on the primary, to prevent the subscriber from consuming changes
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faster than the hot standby. Even when correctly configured, some latency
391+
is expected when sending changes to logical subscribers due to the waiting
392+
on slots named in
393+
<link linkend="guc-standby-slot-names"><varname>standby_slot_names</varname></link>.
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When <varname>standby_slot_names</varname> is utilized, the
395+
primary server will not completely shut down until the corresponding
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standbys, associated with the physical replication slots specified
397+
in <varname>standby_slot_names</varname>, have confirmed
398+
receiving the WAL up to the latest flushed position on the primary server.
387399
</para>
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<para>

‎src/backend/replication/logical/logicalfuncs.c

Lines changed: 12 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -105,6 +105,7 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
105105
MemoryContextper_query_ctx;
106106
MemoryContextoldcontext;
107107
XLogRecPtrend_of_wal;
108+
XLogRecPtrwait_for_wal_lsn;
108109
LogicalDecodingContext*ctx;
109110
ResourceOwnerold_resowner=CurrentResourceOwner;
110111
ArrayType*arr;
@@ -224,6 +225,17 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
224225
NameStr(MyReplicationSlot->data.plugin),
225226
format_procedure(fcinfo->flinfo->fn_oid))));
226227

228+
/*
229+
* Wait for specified streaming replication standby servers (if any)
230+
* to confirm receipt of WAL up to wait_for_wal_lsn.
231+
*/
232+
if (XLogRecPtrIsInvalid(upto_lsn))
233+
wait_for_wal_lsn=end_of_wal;
234+
else
235+
wait_for_wal_lsn=Min(upto_lsn,end_of_wal);
236+
237+
WaitForStandbyConfirmation(wait_for_wal_lsn);
238+
227239
ctx->output_writer_private=p;
228240

229241
/*

‎src/backend/replication/logical/slotsync.c

Lines changed: 11 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -488,6 +488,10 @@ synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid)
488488
latestFlushPtr=GetStandbyFlushRecPtr(NULL);
489489
if (remote_slot->confirmed_lsn>latestFlushPtr)
490490
{
491+
/*
492+
* Can get here only if GUC 'standby_slot_names' on the primary server
493+
* was not configured correctly.
494+
*/
491495
ereport(AmLogicalSlotSyncWorkerProcess() ?LOG :ERROR,
492496
errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
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errmsg("skipping slot synchronization as the received slot sync"
@@ -857,6 +861,13 @@ validate_remote_info(WalReceiverConn *wrconn)
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remote_in_recovery=DatumGetBool(slot_getattr(tupslot,1,&isnull));
858862
Assert(!isnull);
859863

864+
/*
865+
* Slot sync is currently not supported on a cascading standby. This is
866+
* because if we allow it, the primary server needs to wait for all the
867+
* cascading standbys, otherwise, logical subscribers can still be ahead
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* of one of the cascading standbys which we plan to promote. Thus, to
869+
* avoid this additional complexity, we restrict it for the time being.
870+
*/
860871
if (remote_in_recovery)
861872
ereport(ERROR,
862873
errcode(ERRCODE_FEATURE_NOT_SUPPORTED),

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