Add GUC option to control maximum active replication origins.

This commit introduces a new GUC option max_active_replication_origins
to control the maximum number of active replication
origins. Previously, this was controlled by
'max_replication_slots'. Having a separate GUC option provides better
flexibility for setting up subscribers, as they may not require
replication slots (for cascading replication) but always require
replication origins.

Author: Euler Taveira <euler@eulerto.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: vignesh C <vignesh21@gmail.com>
Discussion: https://postgr.es/m/b81db436-8262-4575-b7c4-bc0c1551000b@app.fastmail.com

doc/src/sgml/config.sgml

@@ -4476,13 +4476,6 @@ restore_command = 'copy "C:\\server\\archivedir\\%f" "%p"'  # Windows
          to <literal>replica</literal> or higher to allow replication slots to
          be used.
         </para>
-
-        <para>
-         Note that this parameter also applies on the subscriber side, but with
-         a different meaning. See <xref linkend="guc-max-replication-slots-subscriber"/>
-         in <xref linkend="runtime-config-replication-subscriber"/> for more
-         details.
-        </para>
        </listitem>
       </varlistentry>
 
@@ -5228,10 +5221,10 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
 
      <variablelist>
 
-     <varlistentry id="guc-max-replication-slots-subscriber" xreflabel="max_replication_slots">
-      <term><varname>max_replication_slots</varname> (<type>integer</type>)
+     <varlistentry id="guc-max-active-replication-origins" xreflabel="max_active_replication_origins">
+      <term><varname>max_active_replication_origins</varname> (<type>integer</type>)
        <indexterm>
-        <primary><varname>max_replication_slots</varname> configuration parameter</primary>
+        <primary><varname>max_active_replication_origins</varname> configuration parameter</primary>
         <secondary>in a subscriber</secondary>
        </indexterm>
       </term>
@@ -5243,18 +5236,13 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
         be created on the server. Setting it to a lower value than the current
         number of tracked replication origins (reflected in
         <link linkend="view-pg-replication-origin-status">pg_replication_origin_status</link>)
-        will prevent the server from starting.
-        <literal>max_replication_slots</literal> must be set to at least the
+        will prevent the server from starting. It defaults to 10. This parameter
+        can only be set at server start.
+
+        <literal>max_active_replication_origins</literal> must be set to at least the
         number of subscriptions that will be added to the subscriber, plus some
         reserve for table synchronization.
        </para>
-
-       <para>
-        Note that this parameter also applies on a sending server, but with
-        a different meaning. See <xref linkend="guc-max-replication-slots"/>
-        in <xref linkend="runtime-config-replication-sender"/> for more
-        details.
-       </para>
       </listitem>
      </varlistentry>
 

doc/src/sgml/logical-replication.sgml

@@ -2370,10 +2370,8 @@ CONTEXT:  processing remote data for replication origin "pg_16395" during "INSER
   <title>Configuration Settings</title>
 
   <para>
-   Logical replication requires several configuration options to be set. Most
-   options are relevant only on one side of the replication. However,
-   <varname>max_replication_slots</varname> is used on both the publisher and
-   the subscriber, but it has a different meaning for each.
+   Logical replication requires several configuration options to be set. These
+   options are relevant only on one side of the replication.
   </para>
 
   <sect2 id="logical-replication-config-publisher">
@@ -2413,7 +2411,7 @@ CONTEXT:  processing remote data for replication origin "pg_16395" during "INSER
    <title>Subscribers</title>
 
    <para>
-    <link linkend="guc-max-replication-slots-subscriber"><varname>max_replication_slots</varname></link>
+    <link linkend="guc-max-active-replication-origins"><varname>max_active_replication_origins</varname></link>
     must be set to at least the number of subscriptions that will be added to
     the subscriber, plus some reserve for table synchronization.
    </para>
@@ -2580,7 +2578,7 @@ CONTEXT:  processing remote data for replication origin "pg_16395" during "INSER
     <listitem>
      <para>
       The new cluster must have
-      <link linkend="guc-max-replication-slots"><varname>max_replication_slots</varname></link>
+      <link linkend="guc-max-active-replication-origins"><varname>max_active_replication_origins</varname></link>
       configured to a value greater than or equal to the number of
       subscriptions present in the old cluster.
      </para>

doc/src/sgml/ref/pg_createsubscriber.sgml

@@ -334,7 +334,7 @@ PostgreSQL documentation
 
    <para>
     The target server must be used as a physical standby.  The target server
-    must have <xref linkend="guc-max-replication-slots"/> and <xref
+    must have <xref linkend="guc-max-active-replication-origins"/> and <xref
     linkend="guc-max-logical-replication-workers"/> configured to a value
     greater than or equal to the number of specified databases.  The target
     server must have <xref linkend="guc-max-worker-processes"/> configured to a