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<!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>49.6. 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 Vsnapshot" /><link rel="prev" href="logicaldecoding-catalogs.html" title="49.5. System Catalogs Related to Logical Decoding" /><link rel="next" href="logicaldecoding-writer.html" title="49.7. 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">49.6. Logical Decoding Output Plugins</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="logicaldecoding-catalogs.html" title="49.5. System Catalogs Related to Logical Decoding">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="logicaldecoding.html" title="Chapter 49. Logical Decoding">Up</a></td><th width="60%" align="center">Chapter 49. Logical Decoding</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="logicaldecoding-writer.html" title="49.7. 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">49.6. 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">49.6.1. Initialization Function</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-output-plugin.html#LOGICALDECODING-CAPABILITIES">49.6.2. Capabilities</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-output-plugin.html#LOGICALDECODING-OUTPUT-MODE">49.6.3. Output Modes</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-output-plugin.html#LOGICALDECODING-OUTPUT-PLUGIN-CALLBACKS">49.6.4. Output Plugin Callbacks</a></span></dt><dt><span class="sect2"><a href="logicaldecoding-output-plugin.html#LOGICALDECODING-OUTPUT-PLUGIN-OUTPUT">49.6.5. 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">49.6.1. 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">49.6.2. 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">49.6.3. 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="49.6.4.1. 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">49.6.4. 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">49.6.4.1. 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="49.6.3. Output Modes">Section 49.6.3</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->output_plugin_options</code>. If the output plugin
needs to have a state, it can
use <code class="literal">ctx->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">49.6.4.2. 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">49.6.4.3. 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">49.6.4.4. 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">49.6.4.5. 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">49.6.4.6. 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">49.6.4.7. 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">49.6.4.8. 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">49.6.5. 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->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->out, "BEGIN %u", txn->xid);
OutputPluginWrite(ctx, true);
</pre><p>
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