<!-- doc/src/sgml/logical-replication.sgml -->

<chapter id="logical-replication">
 <title>Logical Replication</title>

 <para>
  Logical replication is a method of replicating data objects and their
  changes, based upon their replication identity (usually a primary key).  We
  use the term logical in contrast to physical replication, which uses exact
  block addresses and byte-by-byte replication.  PostgreSQL supports both
  mechanisms concurrently, see <xref linkend="high-availability"/>.  Logical
  replication allows fine-grained control over both data replication and
  security.
 </para>

 <para>
  Logical replication uses a <firstterm>publish</firstterm>
  and <firstterm>subscribe</firstterm> model with one or
  more <firstterm>subscribers</firstterm> subscribing to one or more
  <firstterm>publications</firstterm> on a <firstterm>publisher</firstterm>
  node.  Subscribers pull data from the publications they subscribe to and may
  subsequently re-publish data to allow cascading replication or more complex
  configurations.
 </para>

 <para>
  When logical replication of a table typically starts, PostgreSQL takes
  a snapshot of the table's data on the publisher database and copies it
  to the subscriber.  Once complete, changes on the publisher since the
  initial copy are sent continually to the subscriber.  The subscriber
  applies the data in the same
  order as the publisher so that transactional consistency is guaranteed for
  publications within a single subscription.  This method of data replication
  is sometimes referred to as transactional replication.
 </para>

 <para>
  The typical use-cases for logical replication are:

  <itemizedlist>
   <listitem>
    <para>
     Sending incremental changes in a single database or a subset of a
     database to subscribers as they occur.
    </para>
   </listitem>

   <listitem>
    <para>
     Firing triggers for individual changes as they arrive on the
     subscriber.
    </para>
   </listitem>

   <listitem>
    <para>
     Consolidating multiple databases into a single one (for example for
     analytical purposes).
    </para>
   </listitem>

   <listitem>
    <para>
     Replicating between different major versions of PostgreSQL.
    </para>
   </listitem>

   <listitem>
    <para>
     Replicating between PostgreSQL instances on different platforms (for
     example Linux to Windows)
    </para>
   </listitem>

   <listitem>
    <para>
     Giving access to replicated data to different groups of users.
    </para>
   </listitem>

   <listitem>
    <para>
     Sharing a subset of the database between multiple databases.
    </para>
   </listitem>
  </itemizedlist>
 </para>

 <para>
  The subscriber database behaves in the same way as any other PostgreSQL
  instance and can be used as a publisher for other databases by defining its
  own publications.  When the subscriber is treated as read-only by
  application, there will be no conflicts from a single subscription.  On the
  other hand, if there are other writes done either by an application or by other
  subscribers to the same set of tables, conflicts can arise.
 </para>

 <sect1 id="logical-replication-publication">
  <title>Publication</title>

  <para>
   A <firstterm>publication</firstterm> can be defined on any physical
   replication primary.  The node where a publication is defined is referred to
   as <firstterm>publisher</firstterm>.  A publication is a set of changes
   generated from a table or a group of tables, and might also be described as
   a change set or replication set.  Each publication exists in only one database.
  </para>

  <para>
   Publications are different from schemas and do not affect how the table is
   accessed.  Each table can be added to multiple publications if needed.
   Publications may currently only contain tables and all tables in schema.
   Objects must be added explicitly, except when a publication is created for
   <literal>ALL TABLES</literal>.
  </para>

  <para>