ReorderBufferChange *change);
</programlisting>
     </para>
    </sect3>

    <sect3 id="logicaldecoding-output-plugin-stream-message">
     <title>Stream Message Callback</title>
     <para>
      The optional <function>stream_message_cb</function> callback is called when
      sending a generic message in a block of streamed changes (demarcated by
      <function>stream_start_cb</function> and <function>stream_stop_cb</function> calls).
      The message contents for transactional messages are not displayed as the transaction
      can abort at a later point in time and we don't decode changes for aborted
      transactions.
<programlisting>
typedef void (*LogicalDecodeStreamMessageCB) (struct LogicalDecodingContext *ctx,
                                              ReorderBufferTXN *txn,
                                              XLogRecPtr message_lsn,
                                              bool transactional,
                                              const char *prefix,
                                              Size message_size,
                                              const char *message);
</programlisting>
     </para>
    </sect3>

    <sect3 id="logicaldecoding-output-plugin-stream-truncate">
     <title>Stream Truncate Callback</title>
     <para>
      The optional <function>stream_truncate_cb</function> callback is called
      for a <command>TRUNCATE</command> command in a block of streamed changes
      (demarcated by <function>stream_start_cb</function> and
      <function>stream_stop_cb</function> calls).
<programlisting>
typedef void (*LogicalDecodeStreamTruncateCB) (struct LogicalDecodingContext *ctx,
                                               ReorderBufferTXN *txn,
                                               int nrelations,
                                               Relation relations[],
                                               ReorderBufferChange *change);
</programlisting>
      The parameters are analogous to the <function>stream_change_cb</function>
      callback.  However, because <command>TRUNCATE</command> actions on
      tables connected by foreign keys need to be executed together, this
      callback receives an array of relations instead of just a single one.
      See the description of the <xref linkend="sql-truncate"/> statement for
      details.
     </para>
    </sect3>

   </sect2>

   <sect2 id="logicaldecoding-output-plugin-output">
    <title>Functions for Producing Output</title>

    <para>
     To actually produce output, output plugins can write data to
     the <literal>StringInfo</literal> output buffer
     in <literal>ctx-&gt;out</literal> when inside
     the <function>begin_cb</function>, <function>commit_cb</function>,
     or <function>change_cb</function> callbacks. Before writing to the output
     buffer, <function>OutputPluginPrepareWrite(ctx, last_write)</function> has
     to be called, and after finishing writing to the
     buffer, <function>OutputPluginWrite(ctx, last_write)</function> has to be
     called to perform the write. The <parameter>last_write</parameter>
     indicates whether a particular write was the callback's last write.
    </para>

    <para>
     The following example shows how to output data to the consumer of an
     output plugin:
<programlisting>
OutputPluginPrepareWrite(ctx, true);
appendStringInfo(ctx->out, "BEGIN %u", txn->xid);
OutputPluginWrite(ctx, true);
</programlisting>
    </para>
   </sect2>
  </sect1>

  <sect1 id="logicaldecoding-writer">
   <title>Logical Decoding Output Writers</title>

   <para>
    It is possible to add more output methods for logical decoding.
    For details, see
    <filename>src/backend/replication/logical/logicalfuncs.c</filename>.
    Essentially, three functions need to be provided: one to read WAL, one to
    prepare writing output, and one to write the output
    (see <xref linkend="logicaldecoding-output-plugin-output"/>).
   </para>
  </sect1>

  <sect1 id="logicaldecoding-synchronous">