replication slots.
   </para>
  </sect1>

  <sect1 id="logicaldecoding-output-plugin">
   <title>Logical Decoding Output Plugins</title>
   <para>
    An example output plugin can be found in the
    <link linkend="test-decoding">
     <filename>contrib/test_decoding</filename>
    </link>
    subdirectory of the PostgreSQL source tree.
   </para>
   <sect2 id="logicaldecoding-output-init">
    <title>Initialization Function</title>
    <indexterm zone="logicaldecoding-output-init">
     <primary>_PG_output_plugin_init</primary>
    </indexterm>
    <para>
     An output plugin is loaded by dynamically loading a shared library with
     the output plugin's name as the library base name. The normal library
     search path is used to locate the library. To provide the required output
     plugin callbacks and to indicate that the library is actually an output
     plugin it needs to provide a function named
     <function>_PG_output_plugin_init</function>. This function is passed a
     struct that needs to be filled with the callback function pointers for
     individual actions.
<programlisting>
typedef struct OutputPluginCallbacks
{
    LogicalDecodeStartupCB startup_cb;
    LogicalDecodeBeginCB begin_cb;
    LogicalDecodeChangeCB change_cb;
    LogicalDecodeTruncateCB truncate_cb;
    LogicalDecodeCommitCB commit_cb;
    LogicalDecodeMessageCB message_cb;
    LogicalDecodeFilterByOriginCB filter_by_origin_cb;
    LogicalDecodeShutdownCB shutdown_cb;
    LogicalDecodeFilterPrepareCB filter_prepare_cb;
    LogicalDecodeBeginPrepareCB begin_prepare_cb;
    LogicalDecodePrepareCB prepare_cb;
    LogicalDecodeCommitPreparedCB commit_prepared_cb;
    LogicalDecodeRollbackPreparedCB rollback_prepared_cb;
    LogicalDecodeStreamStartCB stream_start_cb;
    LogicalDecodeStreamStopCB stream_stop_cb;
    LogicalDecodeStreamAbortCB stream_abort_cb;
    LogicalDecodeStreamPrepareCB stream_prepare_cb;
    LogicalDecodeStreamCommitCB stream_commit_cb;
    LogicalDecodeStreamChangeCB stream_change_cb;
    LogicalDecodeStreamMessageCB stream_message_cb;
    LogicalDecodeStreamTruncateCB stream_truncate_cb;
} OutputPluginCallbacks;

typedef void (*LogicalOutputPluginInit) (struct OutputPluginCallbacks *cb);
</programlisting>
     The <function>begin_cb</function>, <function>change_cb</function>
     and <function>commit_cb</function> callbacks are required,
     while <function>startup_cb</function>, <function>truncate_cb</function>,
     <function>message_cb</function>, <function>filter_by_origin_cb</function>,
     and <function>shutdown_cb</function> are optional.
     If <function>truncate_cb</function> is not set but a
     <command>TRUNCATE</command> is to be decoded, the action will be ignored.
    </para>

    <para>
     An output plugin may also define functions to support streaming of large,
     in-progress transactions. The <function>stream_start_cb</function>,
     <function>stream_stop_cb</function>, <function>stream_abort_cb</function>,
     <function>stream_commit_cb</function>, and <function>stream_change_cb</function>
     are required, while <function>stream_message_cb</function> and
     <function>stream_truncate_cb</function> are optional. The
     <function>stream_prepare_cb</function> is also required if the output
     plugin also support two-phase commits.
    </para>

    <para>
    An output plugin may also define functions to support two-phase commits,
    which allows actions to be decoded on the <command>PREPARE TRANSACTION</command>.
    The <function>begin_prepare_cb</function>, <function>prepare_cb</function>,
    <function>commit_prepared_cb</function> and <function>rollback_prepared_cb</function>
    callbacks are required, while <function>filter_prepare_cb</function> is optional.
    The <function>stream_prepare_cb</function> is also required if the output plugin
    also supports the streaming of large in-progress transactions.
    </para>
   </sect2>

   <sect2 id="logicaldecoding-capabilities">
    <title>Capabilities</title>