Baoding-FenShuaJiang
/
Green_GoodERP18_FSJ_Standard


			
				
					
						
						
							
							<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>48.6. Logical Decoding Output Plugins</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="logicaldecoding-catalogs.html" title="48.5. System Catalogs Related to Logical Decoding" /><link rel="next" href="logicaldecoding-writer.html" title="48.7. Logical Decoding Output Writers" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">48.6. Logical Decoding Output Plugins</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="logicaldecoding-catalogs.html" title="48.5. System Catalogs Related to Logical Decoding">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="logicaldecoding.html" title="Chapter 48. Logical Decoding">Up</a></td><th width="60%" align="center">Chapter 48. Logical Decoding</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="logicaldecoding-writer.html" title="48.7. Logical Decoding Output Writers">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="LOGICALDECODING-OUTPUT-PLUGIN"><div class="titlepage"><div><div><h2 class="title" style="clear: both">48.6. Logical Decoding Output Plugins</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="logicaldecoding-output-plugin.html#LOGICALDECODING-OUTPUT-INIT">48.6.1. Initialization Function</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-output-plugin.html#LOGICALDECODING-CAPABILITIES">48.6.2. Capabilities</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-output-plugin.html#LOGICALDECODING-OUTPUT-MODE">48.6.3. Output Modes</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-output-plugin.html#LOGICALDECODING-OUTPUT-PLUGIN-CALLBACKS">48.6.4. Output Plugin Callbacks</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-output-plugin.html#LOGICALDECODING-OUTPUT-PLUGIN-OUTPUT">48.6.5. Functions for Producing Output</a></span></dt></dl></div><p>
    An example output plugin can be found in the
    <a class="link" href="test-decoding.html" title="F.40. test_decoding">
     <code class="filename">contrib/test_decoding</code>
    </a>
    subdirectory of the PostgreSQL source tree.
   </p><div class="sect2" id="LOGICALDECODING-OUTPUT-INIT"><div class="titlepage"><div><div><h3 class="title">48.6.1. Initialization Function</h3></div></div></div><a id="id-1.8.14.12.3.2" class="indexterm"></a><p>
     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
     <code class="function">_PG_output_plugin_init</code>. This function is passed a
     struct that needs to be filled with the callback function pointers for
     individual actions.
</p><pre class="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;
} OutputPluginCallbacks;

typedef void (*LogicalOutputPluginInit) (struct OutputPluginCallbacks *cb);
</pre><p>
     The <code class="function">begin_cb</code>, <code class="function">change_cb</code>
     and <code class="function">commit_cb</code> callbacks are required,
     while <code class="function">startup_cb</code>,
     <code class="function">filter_by_origin_cb</code>, <code class="function">truncate_cb</code>,
     and <code class="function">shutdown_cb</code> are optional.
     If <code class="function">truncate_cb</code> is not set but a
     <code class="command">TRUNCATE</code> is to be decoded, the action will be ignored.
    </p></div><div class="sect2" id="LOGICALDECODING-CAPABILITIES"><div class="titlepage"><div><div><h3 class="title">48.6.2. Capabilities</h3></div></div></div><p>
     To decode, format and output changes, output plugins can use most of the
     backend's normal infrastructure, including calling output functions. Read
     only access to relations is permitted as long as only relations are
     accessed that either have been created by <code class="command">initdb</code> in
     the <code class="literal">pg_catalog</code> schema, or have been marked as user
     provided catalog tables using
</p><pre class="programlisting">
ALTER TABLE user_catalog_table SET (user_catalog_table = true);
CREATE TABLE another_catalog_table(data text) WITH (user_catalog_table = true);
</pre><p>
     Any actions leading to transaction ID assignment are prohibited. That, among others,
     includes writing to tables, performing DDL changes, and
     calling <code class="literal">txid_current()</code>.
    </p></div><div class="sect2" id="LOGICALDECODING-OUTPUT-MODE"><div class="titlepage"><div><div><h3 class="title">48.6.3. Output Modes</h3></div></div></div><p>
     Output plugin callbacks can pass data to the consumer in nearly arbitrary
     formats. For some use cases, like viewing the changes via SQL, returning
     data in a data type that can contain arbitrary data (e.g., <code class="type">bytea</code>) is
     cumbersome. If the output plugin only outputs textual data in the
     server's encoding, it can declare that by
     setting <code class="literal">OutputPluginOptions.output_type</code>
     to <code class="literal">OUTPUT_PLUGIN_TEXTUAL_OUTPUT</code> instead
     of <code class="literal">OUTPUT_PLUGIN_BINARY_OUTPUT</code> in
     the <a class="link" href="logicaldecoding-output-plugin.html#LOGICALDECODING-OUTPUT-PLUGIN-STARTUP" title="48.6.4.1. Startup Callback">startup
     callback</a>. In that case, all the data has to be in the server's encoding
     so that a <code class="type">text</code> datum can contain it. This is checked in assertion-enabled
     builds.
    </p></div><div class="sect2" id="LOGICALDECODING-OUTPUT-PLUGIN-CALLBACKS"><div class="titlepage"><div><div><h3 class="title">48.6.4. Output Plugin Callbacks</h3></div></div></div><p>
     An output plugin gets notified about changes that are happening via
     various callbacks it needs to provide.
    </p><p>
     Concurrent transactions are decoded in commit order, and only changes
     belonging to a specific transaction are decoded between
     the <code class="literal">begin</code> and <code class="literal">commit</code>
     callbacks. Transactions that were rolled back explicitly or implicitly
     never get
     decoded. Successful savepoints are
     folded into the transaction containing them in the order they were
     executed within that transaction.
    </p><div class="note"><h3 class="title">Note</h3><p>
      Only transactions that have already safely been flushed to disk will be
      decoded. That can lead to a <code class="command">COMMIT</code> not immediately being decoded in a
      directly following <code class="literal">pg_logical_slot_get_changes()</code>
      when <code class="varname">synchronous_commit</code> is set
      to <code class="literal">off</code>.
     </p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-STARTUP"><div class="titlepage"><div><div><h4 class="title">48.6.4.1. Startup Callback</h4></div></div></div><p>
      The optional <code class="function">startup_cb</code> callback is called whenever
      a replication slot is created or asked to stream changes, independent
      of the number of changes that are ready to be put out.
</p><pre class="programlisting">
typedef void (*LogicalDecodeStartupCB) (struct LogicalDecodingContext *ctx,
                                        OutputPluginOptions *options,
                                        bool is_init);
</pre><p>
      The <code class="literal">is_init</code> parameter will be true when the
      replication slot is being created and false
      otherwise. <em class="parameter"><code>options</code></em> points to a struct of options
      that output plugins can set:
</p><pre class="programlisting">
typedef struct OutputPluginOptions
{
    OutputPluginOutputType output_type;
    bool        receive_rewrites;
} OutputPluginOptions;
</pre><p>
      <code class="literal">output_type</code> has to either be set to
      <code class="literal">OUTPUT_PLUGIN_TEXTUAL_OUTPUT</code>
      or <code class="literal">OUTPUT_PLUGIN_BINARY_OUTPUT</code>. See also
      <a class="xref" href="logicaldecoding-output-plugin.html#LOGICALDECODING-OUTPUT-MODE" title="48.6.3. Output Modes">Section 48.6.3</a>.
      If <code class="literal">receive_rewrites</code> is true, the output plugin will
      also be called for changes made by heap rewrites during certain DDL
      operations.  These are of interest to plugins that handle DDL
      replication, but they require special handling.
     </p><p>
      The startup callback should validate the options present in
      <code class="literal">ctx-&gt;output_plugin_options</code>. If the output plugin
      needs to have a state, it can
      use <code class="literal">ctx-&gt;output_plugin_private</code> to store it.
     </p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-SHUTDOWN"><div class="titlepage"><div><div><h4 class="title">48.6.4.2. Shutdown Callback</h4></div></div></div><p>
      The optional <code class="function">shutdown_cb</code> callback is called
      whenever a formerly active replication slot is not used anymore and can
      be used to deallocate resources private to the output plugin. The slot
      isn't necessarily being dropped, streaming is just being stopped.
</p><pre class="programlisting">
typedef void (*LogicalDecodeShutdownCB) (struct LogicalDecodingContext *ctx);
</pre><p>
     </p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-BEGIN"><div class="titlepage"><div><div><h4 class="title">48.6.4.3. Transaction Begin Callback</h4></div></div></div><p>
      The required <code class="function">begin_cb</code> callback is called whenever a
      start of a committed transaction has been decoded. Aborted transactions
      and their contents never get decoded.
</p><pre class="programlisting">
typedef void (*LogicalDecodeBeginCB) (struct LogicalDecodingContext *ctx,
                                      ReorderBufferTXN *txn);
</pre><p>
      The <em class="parameter"><code>txn</code></em> parameter contains meta information about
      the transaction, like the time stamp at which it has been committed and
      its XID.
     </p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-COMMIT"><div class="titlepage"><div><div><h4 class="title">48.6.4.4. Transaction End Callback</h4></div></div></div><p>
      The required <code class="function">commit_cb</code> callback is called whenever
      a transaction commit has been
      decoded. The <code class="function">change_cb</code> callbacks for all modified
      rows will have been called before this, if there have been any modified
      rows.
</p><pre class="programlisting">
typedef void (*LogicalDecodeCommitCB) (struct LogicalDecodingContext *ctx,
                                       ReorderBufferTXN *txn,
                                       XLogRecPtr commit_lsn);
</pre><p>
     </p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-CHANGE"><div class="titlepage"><div><div><h4 class="title">48.6.4.5. Change Callback</h4></div></div></div><p>
      The required <code class="function">change_cb</code> callback is called for every
      individual row modification inside a transaction, may it be
      an <code class="command">INSERT</code>, <code class="command">UPDATE</code>,
      or <code class="command">DELETE</code>. Even if the original command modified
      several rows at once the callback will be called individually for each
      row.
</p><pre class="programlisting">
typedef void (*LogicalDecodeChangeCB) (struct LogicalDecodingContext *ctx,
                                       ReorderBufferTXN *txn,
                                       Relation relation,
                                       ReorderBufferChange *change);
</pre><p>
      The <em class="parameter"><code>ctx</code></em> and <em class="parameter"><code>txn</code></em> parameters
      have the same contents as for the <code class="function">begin_cb</code>
      and <code class="function">commit_cb</code> callbacks, but additionally the
      relation descriptor <em class="parameter"><code>relation</code></em> points to the
      relation the row belongs to and a struct
      <em class="parameter"><code>change</code></em> describing the row modification are passed
      in.
     </p><div class="note"><h3 class="title">Note</h3><p>
       Only changes in user defined tables that are not unlogged
       (see <a class="xref" href="sql-createtable.html#SQL-CREATETABLE-UNLOGGED"><code class="literal">UNLOGGED</code></a>) and not temporary
       (see <a class="xref" href="sql-createtable.html#SQL-CREATETABLE-TEMPORARY"><code class="literal">TEMPORARY</code> or <code class="literal">TEMP</code></a>) can be extracted using
       logical decoding.
      </p></div></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-TRUNCATE"><div class="titlepage"><div><div><h4 class="title">48.6.4.6. Truncate Callback</h4></div></div></div><p>
      The <code class="function">truncate_cb</code> callback is called for a
      <code class="command">TRUNCATE</code> command.
</p><pre class="programlisting">
typedef void (*LogicalDecodeTruncateCB) (struct LogicalDecodingContext *ctx,
                                         ReorderBufferTXN *txn,
                                         int nrelations,
                                         Relation relations[],
                                         ReorderBufferChange *change);
</pre><p>
      The parameters are analogous to the <code class="function">change_cb</code>
      callback.  However, because <code class="command">TRUNCATE</code> 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 <a class="xref" href="sql-truncate.html" title="TRUNCATE"><span class="refentrytitle">TRUNCATE</span></a> statement for
      details.
     </p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-FILTER-ORIGIN"><div class="titlepage"><div><div><h4 class="title">48.6.4.7. Origin Filter Callback</h4></div></div></div><p>
       The optional <code class="function">filter_by_origin_cb</code> callback
       is called to determine whether data that has been replayed
       from <em class="parameter"><code>origin_id</code></em> is of interest to the
       output plugin.
</p><pre class="programlisting">
typedef bool (*LogicalDecodeFilterByOriginCB) (struct LogicalDecodingContext *ctx,
                                               RepOriginId origin_id);
</pre><p>
      The <em class="parameter"><code>ctx</code></em> parameter has the same contents
      as for the other callbacks. No information but the origin is
      available. To signal that changes originating on the passed in
      node are irrelevant, return true, causing them to be filtered
      away; false otherwise. The other callbacks will not be called
      for transactions and changes that have been filtered away.
     </p><p>
       This is useful when implementing cascading or multidirectional
       replication solutions. Filtering by the origin allows to
       prevent replicating the same changes back and forth in such
       setups.  While transactions and changes also carry information
       about the origin, filtering via this callback is noticeably
       more efficient.
     </p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-MESSAGE"><div class="titlepage"><div><div><h4 class="title">48.6.4.8. Generic Message Callback</h4></div></div></div><p>
      The optional <code class="function">message_cb</code> callback is called whenever
      a logical decoding message has been decoded.
</p><pre class="programlisting">
typedef void (*LogicalDecodeMessageCB) (struct LogicalDecodingContext *ctx,
                                        ReorderBufferTXN *txn,
                                        XLogRecPtr message_lsn,
                                        bool transactional,
                                        const char *prefix,
                                        Size message_size,
                                        const char *message);
</pre><p>
      The <em class="parameter"><code>txn</code></em> parameter contains meta information about
      the transaction, like the time stamp at which it has been committed and
      its XID. Note however that it can be NULL when the message is
      non-transactional and the XID was not assigned yet in the transaction
      which logged the message. The <em class="parameter"><code>lsn</code></em> has WAL
      location of the message. The <em class="parameter"><code>transactional</code></em> says
      if the message was sent as transactional or not.
      The <em class="parameter"><code>prefix</code></em> is arbitrary null-terminated prefix
      which can be used for identifying interesting messages for the current
      plugin. And finally the <em class="parameter"><code>message</code></em> parameter holds
      the actual message of <em class="parameter"><code>message_size</code></em> size.
     </p><p>
      Extra care should be taken to ensure that the prefix the output plugin
      considers interesting is unique. Using name of the extension or the
      output plugin itself is often a good choice.
     </p></div></div><div class="sect2" id="LOGICALDECODING-OUTPUT-PLUGIN-OUTPUT"><div class="titlepage"><div><div><h3 class="title">48.6.5. Functions for Producing Output</h3></div></div></div><p>
     To actually produce output, output plugins can write data to
     the <code class="literal">StringInfo</code> output buffer
     in <code class="literal">ctx-&gt;out</code> when inside
     the <code class="function">begin_cb</code>, <code class="function">commit_cb</code>,
     or <code class="function">change_cb</code> callbacks. Before writing to the output
     buffer, <code class="function">OutputPluginPrepareWrite(ctx, last_write)</code> has
     to be called, and after finishing writing to the
     buffer, <code class="function">OutputPluginWrite(ctx, last_write)</code> has to be
     called to perform the write. The <em class="parameter"><code>last_write</code></em>
     indicates whether a particular write was the callback's last write.
    </p><p>
     The following example shows how to output data to the consumer of an
     output plugin:
</p><pre class="programlisting">
OutputPluginPrepareWrite(ctx, true);
appendStringInfo(ctx-&gt;out, "BEGIN %u", txn-&gt;xid);
OutputPluginWrite(ctx, true);
</pre><p>
    </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="logicaldecoding-catalogs.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="logicaldecoding.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="logicaldecoding-writer.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">48.5. System Catalogs Related to Logical Decoding </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 48.7. Logical Decoding Output Writers</td></tr></table></div></body></html>