doc/src/sgml/replication-origins.sgml

<!-- 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 OID. 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 OID 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>