Adding upstream version 15.4.upstream/15.4upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+<!-- doc/src/sgml/replication-origins.sgml -->
+<chapter id="replication-origins">
+ <title>Replication Progress Tracking</title>
+
+ <indexterm zone="replication-origins">
+  <primary>Replication Progress Tracking</primary>
+ </indexterm>
+ <indexterm zone="replication-origins">
+  <primary>Replication Origins</primary>
+ </indexterm>
+
+ <para>
+  Replication origins are intended to make it easier to implement
+  logical replication solutions on top
+  of <link linkend="logicaldecoding">logical decoding</link>.
+  They provide a solution to two common problems:
+  <itemizedlist>
+   <listitem>
+    <para>How to safely keep track of replication progress</para>
+   </listitem>
+   <listitem>
+    <para>How to change replication behavior based on the
+     origin of a row; for example, to prevent loops in bi-directional
+     replication setups</para>
+   </listitem>
+  </itemizedlist>
+ </para>
+
+ <para>
+  Replication origins have just two properties, a name and an ID. The name,
+  which is what should be used to refer to the origin across systems, is
+  free-form <type>text</type>. It should be used in a way that makes conflicts
+  between replication origins created by different replication solutions
+  unlikely; e.g., by prefixing the replication solution's name to it.
+  The ID is used only to avoid having to store the long version
+  in situations where space efficiency is important. It should never be shared
+  across systems.
+ </para>
+
+ <para>
+  Replication origins can be created using the function
+  <link linkend="pg-replication-origin-create"><function>pg_replication_origin_create()</function></link>;
+  dropped using
+  <link linkend="pg-replication-origin-drop"><function>pg_replication_origin_drop()</function></link>;
+  and seen in the
+  <link linkend="catalog-pg-replication-origin"><structname>pg_replication_origin</structname></link>
+  system catalog.
+ </para>
+
+ <para>
+  One nontrivial part of building a replication solution is to keep track of
+  replay progress in a safe manner. When the applying process, or the whole
+  cluster, dies, it needs to be possible to find out up to where data has
+  successfully been replicated. Naive solutions to this, such as updating a
+  row in a table for every replayed transaction, have problems like run-time
+  overhead and database bloat.
+ </para>
+
+ <para>
+  Using the replication origin infrastructure a session can be
+  marked as replaying from a remote node (using the
+  <link linkend="pg-replication-origin-session-setup"><function>pg_replication_origin_session_setup()</function></link>
+  function). Additionally the <acronym>LSN</acronym> and commit
+  time stamp of every source transaction can be configured on a per
+  transaction basis using
+  <link linkend="pg-replication-origin-xact-setup"><function>pg_replication_origin_xact_setup()</function></link>.
+  If that's done replication progress will persist in a crash safe
+  manner. Replay progress for all replication origins can be seen in the
+  <link linkend="view-pg-replication-origin-status">
+   <structname>pg_replication_origin_status</structname>
+  </link> view. An individual origin's progress, e.g., when resuming
+  replication, can be acquired using
+  <link linkend="pg-replication-origin-progress"><function>pg_replication_origin_progress()</function></link>
+  for any origin or
+  <link linkend="pg-replication-origin-session-progress"><function>pg_replication_origin_session_progress()</function></link>
+  for the origin configured in the current session.
+ </para>
+
+ <para>
+  In replication topologies more complex than replication from exactly one
+  system to one other system, another problem can be that it is hard to avoid
+  replicating replayed rows again. That can lead both to cycles in the
+  replication and inefficiencies. Replication origins provide an optional
+  mechanism to recognize and prevent that. When configured using the functions
+  referenced in the previous paragraph, every change and transaction passed to
+  output plugin callbacks (see <xref linkend="logicaldecoding-output-plugin"/>)
+  generated by the session is tagged with the replication origin of the
+  generating session.  This allows treating them differently in the output
+  plugin, e.g., ignoring all but locally-originating rows.  Additionally
+  the <link linkend="logicaldecoding-output-plugin-filter-origin">
+  <function>filter_by_origin_cb</function></link> callback can be used
+  to filter the logical decoding change stream based on the
+  source. While less flexible, filtering via that callback is
+  considerably more efficient than doing it in the output plugin.
+ </para>
+</chapter>