[CF 5479] v12 - Enable logical decoding when wal_level = 'replica' without a server restart

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Patch(es): https://www.postgresql.org/message-id/CAD21AoAz1RkCfs-VD6Sm9bCFKiDC=9O-KAtcjxXeL76O3z8PaQ@mail.gmail.com
Author(s): Masahiko Sawada

Author: Commitfest Bot

doc/src/sgml/config.sgml

@@ -3034,6 +3034,17 @@ include_dir 'conf.d'
         many <command>UPDATE</command> and <command>DELETE</command> statements are
         executed.
        </para>
+       <para>
+        It is important to note that when <varname>wal_level</varname> is set to
+        <literal>replica</literal> the effective WAL level can automatically change
+        based on the presence of <link linkend="logicaldecoding-replication-slots">
+        logical replication slots</link>. The system automatically increases the
+        effective WAL level to <literal>logical</literal> when creating the first
+        logical replication slot, and decreases it back to <literal>replica</literal>
+        when dropping the last logical replication slot. The current effective WAL
+        level can be monitored through <xref linkend="guc-effective-wal-level"/>
+        parameter.
+       </para>
        <para>
         In releases prior to 9.6, this parameter also allowed the
         values <literal>archive</literal> and <literal>hot_standby</literal>.
@@ -11747,6 +11758,38 @@ dynamic_library_path = '/usr/local/lib/postgresql:$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-effective-wal-level" xreflabel="effective_wal_level">
+      <term><varname>effective_wal_level</varname> (<type>enum</type>)
+      <indexterm>
+       <primary><varname>effective_wal_level</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the actual WAL logging level currently in effect in the
+        system. This parameter shares the same set of values as
+        <xref linkend="guc-wal-level"/>, but reflects the operational WAL
+        level rather than the configured setting. For descriptions of
+        possible values, refer to the <varname>wal_level</varname>
+        parameter documentation.
+       </para>
+       <para>
+        The effective WAL level can differ from the configured
+        <varname>wal_level</varname> in certain situations. For example,
+        when <varname>wal_level</varname> is set to <literal>replica</literal>
+        and the system has one or more logical replication slots,
+        <varname>effective_wal_level</varname> will show <literal>logical</literal>
+        to indicate that the system is maintaining WAL records at
+        <literal>logical</literal> level equivalent.
+       </para>
+       <para>
+        On standby servers, <varname>effective_wal_level</varname> matches
+        the value of <varname>effective_wal_level</varname> from the most
+        upstream server in the replication chain.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-huge-pages-status" xreflabel="huge_pages_status">
       <term><varname>huge_pages_status</varname> (<type>enum</type>)
       <indexterm>

doc/src/sgml/logical-replication.sgml

@@ -2375,7 +2375,7 @@ CONTEXT:  processing remote data for replication origin "pg_16395" during "INSER
 
    <para>
     <link linkend="guc-wal-level"><varname>wal_level</varname></link> must be
-    set to <literal>logical</literal>.
+    set to <literal>replica</literal> or <literal>logical</literal>.
    </para>
 
    <para>
@@ -2496,7 +2496,7 @@ CONTEXT:  processing remote data for replication origin "pg_16395" during "INSER
      <para>
       The new cluster must have
       <link linkend="guc-wal-level"><varname>wal_level</varname></link> as
-      <literal>logical</literal>.
+      <literal>replica</literal> or <literal>logical</literal>.
      </para>
     </listitem>
     <listitem>

doc/src/sgml/logicaldecoding.sgml

@@ -47,7 +47,7 @@
 
    <para>
     Before you can use logical decoding, you must set
-    <xref linkend="guc-wal-level"/> to <literal>logical</literal> and
+    <xref linkend="guc-wal-level"/> to <literal>replica</literal> or higher and
     <xref linkend="guc-max-replication-slots"/> to at least 1.  Then, you
     should connect to the target database (in the example
     below, <literal>postgres</literal>) as a superuser.
@@ -257,6 +257,32 @@ postgres=# select * from pg_logical_slot_get_changes('regression_slot', NULL, NU
      log</link>, which describe changes on a storage level, into an
      application-specific form such as a stream of tuples or SQL statements.
     </para>
+
+    <para>
+     Logical decoding becomes available in two conditions:
+    </para>
+    <itemizedlist>
+     <listitem>
+      <para>
+       When <xref linkend="guc-wal-level"/> is set to <literal>logical</literal>.
+      </para>
+     </listitem>
+     <listitem>
+      <para>
+       When <xref linkend="guc-wal-level"/> is set to <literal>replica</literal>
+       and at least one logical replication slot exists on the system.
+      </para>
+     </listitem>
+    </itemizedlist>
+    <para>
+     When <varname>wal_level</varname> is set to <literal>replica</literal>,
+     logical decoding is automatically activated upon creation of the first
+     logical replication slot. This activation process involves several steps
+     and requires waiting for any concurrent transactions to finish, ensuring
+     system-wide consistency. Conversely, when the last logical replication slot
+     is dropped from a system with <varname>wal_level</varname> set to
+     <literal>replica</literal>, logical decoding is automatically disabled.
+    </para>
    </sect2>
 
    <sect2 id="logicaldecoding-replication-slots">
@@ -328,8 +354,7 @@ postgres=# select * from pg_logical_slot_get_changes('regression_slot', NULL, NU
      that could be needed by the logical decoding on the standby (as it does
      not know about the <literal>catalog_xmin</literal> on the standby).
      Existing logical slots on standby also get invalidated if
-     <varname>wal_level</varname> on the primary is reduced to less than
-     <literal>logical</literal>.
+     logical decoding becomes disabled on the primary.
      This is done as soon as the standby detects such a change in the WAL stream.
      It means that, for walsenders that are lagging (if any), some WAL records up
      to the <varname>wal_level</varname> parameter change on the primary won't be

doc/src/sgml/ref/pg_createsubscriber.sgml

@@ -379,12 +379,12 @@ PostgreSQL documentation
    <para>
     The source server must accept connections from the target server.  The
     source server must not be in recovery. The source server must have <xref
-    linkend="guc-wal-level"/> as <literal>logical</literal>.  The source server
-    must have <xref linkend="guc-max-replication-slots"/> configured to a value
-    greater than or equal to the number of specified databases plus existing
-    replication slots.  The source server must have <xref
-    linkend="guc-max-wal-senders"/> configured to a value greater than or equal
-    to the number of specified databases and existing WAL sender processes.
+    linkend="guc-wal-level"/> as <literal>replica</literal> or <literal>logical</literal>.
+    The source server must have <xref linkend="guc-max-replication-slots"/>
+    configured to a value greater than or equal to the number of specified
+    databases plus existing replication slots.  The source server must have
+    <xref linkend="guc-max-wal-senders"/> configured to a value greater than or
+    equal to the number of specified databases and existing WAL sender processes.
    </para>
   </refsect2>
 

doc/src/sgml/system-views.sgml

@@ -2996,8 +2996,9 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
         <listitem>
          <para>
           <literal>wal_level_insufficient</literal> means that the
-          primary doesn't have a <xref linkend="guc-wal-level"/> sufficient to
-          perform logical decoding.  It is set only for logical slots.
+          logical decoding is disabled on primary due to insufficient
+          <xref linkend="guc-wal-level"/> or no logical slots.  It is set only
+          for logical slots.
          </para>
         </listitem>
         <listitem>

src/backend/access/heap/heapam.c

@@ -8884,8 +8884,8 @@ log_heap_update(Relation reln, Buffer oldbuf,
 	 *
 	 * Skip this if we're taking a full-page image of the new page, as we
 	 * don't include the new tuple in the WAL record in that case.  Also
-	 * disable if wal_level='logical', as logical decoding needs to be able to
-	 * read the new tuple in whole from the WAL record alone.
+	 * disable if logical decoding is enabled, as logical decoding needs to be
+	 * able to read the new tuple in whole from the WAL record alone.
 	 */
 	if (oldbuf == newbuf && !need_tuple_data &&
 		!XLogCheckBufferNeedsBackup(newbuf))
@@ -9057,8 +9057,8 @@ log_heap_update(Relation reln, Buffer oldbuf,
 /*
  * Perform XLogInsert of an XLOG_HEAP2_NEW_CID record
  *
- * This is only used in wal_level >= WAL_LEVEL_LOGICAL, and only for catalog
- * tuples.
+ * This is only used when XLogLogicalInfoActive() is true, and only for
+ * catalog tuples.
  */
 static XLogRecPtr
 log_heap_new_cid(Relation relation, HeapTuple tup)

src/backend/access/rmgrdesc/xlogdesc.c

@@ -66,7 +66,7 @@ xlog_desc(StringInfo buf, XLogReaderState *record)
 		CheckPoint *checkpoint = (CheckPoint *) rec;
 
 		appendStringInfo(buf, "redo %X/%08X; "
-						 "tli %u; prev tli %u; fpw %s; wal_level %s; xid %u:%u; oid %u; multi %u; offset %u; "
+						 "tli %u; prev tli %u; fpw %s; wal_level %s; logical decoding %s; xid %u:%u; oid %u; multi %u; offset %u; "
 						 "oldest xid %u in DB %u; oldest multi %u in DB %u; "
 						 "oldest/newest commit timestamp xid: %u/%u; "
 						 "oldest running xid %u; %s",
@@ -75,6 +75,7 @@ xlog_desc(StringInfo buf, XLogReaderState *record)
 						 checkpoint->PrevTimeLineID,
 						 checkpoint->fullPageWrites ? "true" : "false",
 						 get_wal_level_string(checkpoint->wal_level),
+						 checkpoint->logicalDecodingEnabled ? "true" : "false",
 						 EpochFromFullTransactionId(checkpoint->nextXid),
 						 XidFromFullTransactionId(checkpoint->nextXid),
 						 checkpoint->nextOid,
@@ -167,6 +168,13 @@ xlog_desc(StringInfo buf, XLogReaderState *record)
 		memcpy(&wal_level, rec, sizeof(int));
 		appendStringInfo(buf, "wal_level %s", get_wal_level_string(wal_level));
 	}
+	else if (info == XLOG_LOGICAL_DECODING_STATUS_CHANGE)
+	{
+		bool		enabled;
+
+		memcpy(&enabled, rec, sizeof(bool));
+		appendStringInfoString(buf, enabled ? "true" : "false");
+	}
 }
 
 const char *
@@ -218,6 +226,9 @@ xlog_identify(uint8 info)
 		case XLOG_CHECKPOINT_REDO:
 			id = "CHECKPOINT_REDO";
 			break;
+		case XLOG_LOGICAL_DECODING_STATUS_CHANGE:
+			id = "LOGICAL_DECODING_STATUS_CHANGE";
+			break;
 	}
 
 	return id;

src/backend/access/transam/xact.c

@@ -551,9 +551,9 @@ MarkCurrentTransactionIdLoggedIfAny(void)
  * operation in a subtransaction.  We require that for logical decoding, see
  * LogicalDecodingProcessRecord.
  *
- * This returns true if wal_level >= logical and we are inside a valid
- * subtransaction, for which the assignment was not yet written to any WAL
- * record.
+ * This returns true if XLogLogicalInfoActive() is true and we are inside
+ * a valid subtransaction, for which the assignment was not yet written to
+ * any WAL record.
  */
 bool
 IsSubxactTopXidLogPending(void)
@@ -562,7 +562,7 @@ IsSubxactTopXidLogPending(void)
 	if (CurrentTransactionState->topXidLogged)
 		return false;
 
-	/* wal_level has to be logical */
+	/* effective WAL level has to be logical */
 	if (!XLogLogicalInfoActive())
 		return false;
 
@@ -681,14 +681,14 @@ AssignTransactionId(TransactionState s)
 	}
 
 	/*
-	 * When wal_level=logical, guarantee that a subtransaction's xid can only
-	 * be seen in the WAL stream if its toplevel xid has been logged before.
-	 * If necessary we log an xact_assignment record with fewer than
-	 * PGPROC_MAX_CACHED_SUBXIDS. Note that it is fine if didLogXid isn't set
-	 * for a transaction even though it appears in a WAL record, we just might
-	 * superfluously log something. That can happen when an xid is included
-	 * somewhere inside a wal record, but not in XLogRecord->xl_xid, like in
-	 * xl_standby_locks.
+	 * When XLogLogicalInfoActive() is true, guarantee that a subtransaction's
+	 * xid can only be seen in the WAL stream if its toplevel xid has been
+	 * logged before. If necessary we log an xact_assignment record with fewer
+	 * than PGPROC_MAX_CACHED_SUBXIDS. Note that it is fine if didLogXid isn't
+	 * set for a transaction even though it appears in a WAL record, we just
+	 * might superfluously log something. That can happen when an xid is
+	 * included somewhere inside a wal record, but not in XLogRecord->xl_xid,
+	 * like in xl_standby_locks.
 	 */
 	if (isSubXact && XLogLogicalInfoActive() &&
 		!TopTransactionStateData.didLogXid)