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