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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 id="docContent" class="container-fluid col-10"><div 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 16.3 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 /></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 <a href="#LOGICALDECODING-OUTPUT-PLUGIN" class="id_link">#</a></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.45. test_decoding — SQL-based test/example module for WAL logical 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 <a href="#LOGICALDECODING-OUTPUT-INIT" class="id_link">#</a></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;
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);
</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">truncate_cb</code>,
<code class="function">message_cb</code>, <code class="function">filter_by_origin_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><p>
An output plugin may also define functions to support streaming of large,
in-progress transactions. The <code class="function">stream_start_cb</code>,
<code class="function">stream_stop_cb</code>, <code class="function">stream_abort_cb</code>,
<code class="function">stream_commit_cb</code>, and <code class="function">stream_change_cb</code>
are required, while <code class="function">stream_message_cb</code> and
<code class="function">stream_truncate_cb</code> are optional. The
<code class="function">stream_prepare_cb</code> is also required if the output
plugin also support two-phase commits.
</p><p>
An output plugin may also define functions to support two-phase commits,
which allows actions to be decoded on the <code class="command">PREPARE TRANSACTION</code>.
The <code class="function">begin_prepare_cb</code>, <code class="function">prepare_cb</code>,
<code class="function">commit_prepared_cb</code> and <code class="function">rollback_prepared_cb</code>
callbacks are required, while <code class="function">filter_prepare_cb</code> is optional.
The <code class="function">stream_prepare_cb</code> is also required if the output plugin
also supports the streaming of large in-progress transactions.
</p></div><div class="sect2" id="LOGICALDECODING-CAPABILITIES"><div class="titlepage"><div><div><h3 class="title">49.6.2. Capabilities <a href="#LOGICALDECODING-CAPABILITIES" class="id_link">#</a></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>
Note that access to user catalog tables or regular system catalog tables
in the output plugins has to be done via the <code class="literal">systable_*</code>
scan APIs only. Access via the <code class="literal">heap_*</code> scan APIs will
error out. Additionally, any actions leading to transaction ID assignment
are prohibited. That, among others, includes writing to tables, performing
DDL changes, and calling <code class="literal">pg_current_xact_id()</code>.
</p></div><div class="sect2" id="LOGICALDECODING-OUTPUT-MODE"><div class="titlepage"><div><div><h3 class="title">49.6.3. Output Modes <a href="#LOGICALDECODING-OUTPUT-MODE" class="id_link">#</a></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 <a href="#LOGICALDECODING-OUTPUT-PLUGIN-CALLBACKS" class="id_link">#</a></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. A transaction that is prepared for
a two-phase commit using <code class="command">PREPARE TRANSACTION</code> will
also be decoded if the output plugin callbacks needed for decoding
them are provided. It is possible that the current prepared transaction
which is being decoded is aborted concurrently via a
<code class="command">ROLLBACK PREPARED</code> command. In that case, the logical
decoding of this transaction will be aborted too. All the changes of such
a transaction are skipped once the abort is detected and the
<code class="function">prepare_cb</code> callback is invoked. Thus even in case of
a concurrent abort, enough information is provided to the output plugin
for it to properly deal with <code class="command">ROLLBACK PREPARED</code> once
that is decoded.
</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 <a href="#LOGICALDECODING-OUTPUT-PLUGIN-STARTUP" class="id_link">#</a></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 <a href="#LOGICALDECODING-OUTPUT-PLUGIN-SHUTDOWN" class="id_link">#</a></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 <a href="#LOGICALDECODING-OUTPUT-PLUGIN-BEGIN" class="id_link">#</a></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 <a href="#LOGICALDECODING-OUTPUT-PLUGIN-COMMIT" class="id_link">#</a></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 <a href="#LOGICALDECODING-OUTPUT-PLUGIN-CHANGE" class="id_link">#</a></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. The <code class="function">change_cb</code> callback may access system or
user catalog tables to aid in the process of outputting the row
modification details. In case of decoding a prepared (but yet
uncommitted) transaction or decoding of an uncommitted transaction, this
change callback might also error out due to simultaneous rollback of
this very same transaction. In that case, the logical decoding of this
aborted transaction is stopped gracefully.
</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 <a href="#LOGICALDECODING-OUTPUT-PLUGIN-TRUNCATE" class="id_link">#</a></h4></div></div></div><p>
The optional <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 <a href="#LOGICALDECODING-OUTPUT-PLUGIN-FILTER-ORIGIN" class="id_link">#</a></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 <a href="#LOGICALDECODING-OUTPUT-PLUGIN-MESSAGE" class="id_link">#</a></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. Similar to the change
callback, in case of decoding a prepared (but yet uncommitted)
transaction or decoding of an uncommitted transaction, this message
callback might also error out due to simultaneous rollback of
this very same transaction. In that case, the logical decoding of this
aborted transaction is stopped gracefully.
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 class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-FILTER-PREPARE"><div class="titlepage"><div><div><h4 class="title">49.6.4.9. Prepare Filter Callback <a href="#LOGICALDECODING-OUTPUT-PLUGIN-FILTER-PREPARE" class="id_link">#</a></h4></div></div></div><p>
The optional <code class="function">filter_prepare_cb</code> callback
is called to determine whether data that is part of the current
two-phase commit transaction should be considered for decoding
at this prepare stage or later as a regular one-phase transaction at
<code class="command">COMMIT PREPARED</code> time. To signal that
decoding should be skipped, return <code class="literal">true</code>;
<code class="literal">false</code> otherwise. When the callback is not
defined, <code class="literal">false</code> is assumed (i.e. no filtering, all
transactions using two-phase commit are decoded in two phases as well).
</p><pre class="programlisting">
typedef bool (*LogicalDecodeFilterPrepareCB) (struct LogicalDecodingContext *ctx,
TransactionId xid,
const char *gid);
</pre><p>
The <em class="parameter"><code>ctx</code></em> parameter has the same contents as for
the other callbacks. The parameters <em class="parameter"><code>xid</code></em>
and <em class="parameter"><code>gid</code></em> provide two different ways to identify
the transaction. The later <code class="command">COMMIT PREPARED</code> or
<code class="command">ROLLBACK PREPARED</code> carries both identifiers,
providing an output plugin the choice of what to use.
</p><p>
The callback may be invoked multiple times per transaction to decode
and must provide the same static answer for a given pair of
<em class="parameter"><code>xid</code></em> and <em class="parameter"><code>gid</code></em> every time
it is called.
</p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-BEGIN-PREPARE"><div class="titlepage"><div><div><h4 class="title">49.6.4.10. Transaction Begin Prepare Callback <a href="#LOGICALDECODING-OUTPUT-PLUGIN-BEGIN-PREPARE" class="id_link">#</a></h4></div></div></div><p>
The required <code class="function">begin_prepare_cb</code> callback is called
whenever the start of a prepared transaction has been decoded. The
<em class="parameter"><code>gid</code></em> field, which is part of the
<em class="parameter"><code>txn</code></em> parameter, can be used in this callback to
check if the plugin has already received this <code class="command">PREPARE</code>
in which case it can either error out or skip the remaining changes of
the transaction.
</p><pre class="programlisting">
typedef void (*LogicalDecodeBeginPrepareCB) (struct LogicalDecodingContext *ctx,
ReorderBufferTXN *txn);
</pre><p>
</p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-PREPARE"><div class="titlepage"><div><div><h4 class="title">49.6.4.11. Transaction Prepare Callback <a href="#LOGICALDECODING-OUTPUT-PLUGIN-PREPARE" class="id_link">#</a></h4></div></div></div><p>
The required <code class="function">prepare_cb</code> callback is called whenever
a transaction which is prepared for two-phase commit has been
decoded. The <code class="function">change_cb</code> callback for all modified
rows will have been called before this, if there have been any modified
rows. The <em class="parameter"><code>gid</code></em> field, which is part of the
<em class="parameter"><code>txn</code></em> parameter, can be used in this callback.
</p><pre class="programlisting">
typedef void (*LogicalDecodePrepareCB) (struct LogicalDecodingContext *ctx,
ReorderBufferTXN *txn,
XLogRecPtr prepare_lsn);
</pre><p>
</p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-COMMIT-PREPARED"><div class="titlepage"><div><div><h4 class="title">49.6.4.12. Transaction Commit Prepared Callback <a href="#LOGICALDECODING-OUTPUT-PLUGIN-COMMIT-PREPARED" class="id_link">#</a></h4></div></div></div><p>
The required <code class="function">commit_prepared_cb</code> callback is called
whenever a transaction <code class="command">COMMIT PREPARED</code> has been decoded.
The <em class="parameter"><code>gid</code></em> field, which is part of the
<em class="parameter"><code>txn</code></em> parameter, can be used in this callback.
</p><pre class="programlisting">
typedef void (*LogicalDecodeCommitPreparedCB) (struct LogicalDecodingContext *ctx,
ReorderBufferTXN *txn,
XLogRecPtr commit_lsn);
</pre><p>
</p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-ROLLBACK-PREPARED"><div class="titlepage"><div><div><h4 class="title">49.6.4.13. Transaction Rollback Prepared Callback <a href="#LOGICALDECODING-OUTPUT-PLUGIN-ROLLBACK-PREPARED" class="id_link">#</a></h4></div></div></div><p>
The required <code class="function">rollback_prepared_cb</code> callback is called
whenever a transaction <code class="command">ROLLBACK PREPARED</code> has been
decoded. The <em class="parameter"><code>gid</code></em> field, which is part of the
<em class="parameter"><code>txn</code></em> parameter, can be used in this callback. The
parameters <em class="parameter"><code>prepare_end_lsn</code></em> and
<em class="parameter"><code>prepare_time</code></em> can be used to check if the plugin
has received this <code class="command">PREPARE TRANSACTION</code> in which case
it can apply the rollback, otherwise, it can skip the rollback operation. The
<em class="parameter"><code>gid</code></em> alone is not sufficient because the downstream
node can have a prepared transaction with same identifier.
</p><pre class="programlisting">
typedef void (*LogicalDecodeRollbackPreparedCB) (struct LogicalDecodingContext *ctx,
ReorderBufferTXN *txn,
XLogRecPtr prepare_end_lsn,
TimestampTz prepare_time);
</pre><p>
</p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-STREAM-START"><div class="titlepage"><div><div><h4 class="title">49.6.4.14. Stream Start Callback <a href="#LOGICALDECODING-OUTPUT-PLUGIN-STREAM-START" class="id_link">#</a></h4></div></div></div><p>
The required <code class="function">stream_start_cb</code> callback is called when
opening a block of streamed changes from an in-progress transaction.
</p><pre class="programlisting">
typedef void (*LogicalDecodeStreamStartCB) (struct LogicalDecodingContext *ctx,
ReorderBufferTXN *txn);
</pre><p>
</p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-STREAM-STOP"><div class="titlepage"><div><div><h4 class="title">49.6.4.15. Stream Stop Callback <a href="#LOGICALDECODING-OUTPUT-PLUGIN-STREAM-STOP" class="id_link">#</a></h4></div></div></div><p>
The required <code class="function">stream_stop_cb</code> callback is called when
closing a block of streamed changes from an in-progress transaction.
</p><pre class="programlisting">
typedef void (*LogicalDecodeStreamStopCB) (struct LogicalDecodingContext *ctx,
ReorderBufferTXN *txn);
</pre><p>
</p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-STREAM-ABORT"><div class="titlepage"><div><div><h4 class="title">49.6.4.16. Stream Abort Callback <a href="#LOGICALDECODING-OUTPUT-PLUGIN-STREAM-ABORT" class="id_link">#</a></h4></div></div></div><p>
The required <code class="function">stream_abort_cb</code> callback is called to
abort a previously streamed transaction.
</p><pre class="programlisting">
typedef void (*LogicalDecodeStreamAbortCB) (struct LogicalDecodingContext *ctx,
ReorderBufferTXN *txn,
XLogRecPtr abort_lsn);
</pre><p>
</p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-STREAM-PREPARE"><div class="titlepage"><div><div><h4 class="title">49.6.4.17. Stream Prepare Callback <a href="#LOGICALDECODING-OUTPUT-PLUGIN-STREAM-PREPARE" class="id_link">#</a></h4></div></div></div><p>
The <code class="function">stream_prepare_cb</code> callback is called to prepare
a previously streamed transaction as part of a two-phase commit. This
callback is required when the output plugin supports both the streaming
of large in-progress transactions and two-phase commits.
</p><pre class="programlisting">
typedef void (*LogicalDecodeStreamPrepareCB) (struct LogicalDecodingContext *ctx,
ReorderBufferTXN *txn,
XLogRecPtr prepare_lsn);
</pre><p>
</p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-STREAM-COMMIT"><div class="titlepage"><div><div><h4 class="title">49.6.4.18. Stream Commit Callback <a href="#LOGICALDECODING-OUTPUT-PLUGIN-STREAM-COMMIT" class="id_link">#</a></h4></div></div></div><p>
The required <code class="function">stream_commit_cb</code> callback is called to
commit a previously streamed transaction.
</p><pre class="programlisting">
typedef void (*LogicalDecodeStreamCommitCB) (struct LogicalDecodingContext *ctx,
ReorderBufferTXN *txn,
XLogRecPtr commit_lsn);
</pre><p>
</p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-STREAM-CHANGE"><div class="titlepage"><div><div><h4 class="title">49.6.4.19. Stream Change Callback <a href="#LOGICALDECODING-OUTPUT-PLUGIN-STREAM-CHANGE" class="id_link">#</a></h4></div></div></div><p>
The required <code class="function">stream_change_cb</code> callback is called
when sending a change in a block of streamed changes (demarcated by
<code class="function">stream_start_cb</code> and <code class="function">stream_stop_cb</code> calls).
The actual changes are not displayed as the transaction can abort at a later
point in time and we don't decode changes for aborted transactions.
</p><pre class="programlisting">
typedef void (*LogicalDecodeStreamChangeCB) (struct LogicalDecodingContext *ctx,
ReorderBufferTXN *txn,
Relation relation,
ReorderBufferChange *change);
</pre><p>
</p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-STREAM-MESSAGE"><div class="titlepage"><div><div><h4 class="title">49.6.4.20. Stream Message Callback <a href="#LOGICALDECODING-OUTPUT-PLUGIN-STREAM-MESSAGE" class="id_link">#</a></h4></div></div></div><p>
The optional <code class="function">stream_message_cb</code> callback is called when
sending a generic message in a block of streamed changes (demarcated by
<code class="function">stream_start_cb</code> and <code class="function">stream_stop_cb</code> 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.
</p><pre class="programlisting">
typedef void (*LogicalDecodeStreamMessageCB) (struct LogicalDecodingContext *ctx,
ReorderBufferTXN *txn,
XLogRecPtr message_lsn,
bool transactional,
const char *prefix,
Size message_size,
const char *message);
</pre><p>
</p></div><div class="sect3" id="LOGICALDECODING-OUTPUT-PLUGIN-STREAM-TRUNCATE"><div class="titlepage"><div><div><h4 class="title">49.6.4.21. Stream Truncate Callback <a href="#LOGICALDECODING-OUTPUT-PLUGIN-STREAM-TRUNCATE" class="id_link">#</a></h4></div></div></div><p>
The optional <code class="function">stream_truncate_cb</code> callback is called
for a <code class="command">TRUNCATE</code> command in a block of streamed changes
(demarcated by <code class="function">stream_start_cb</code> and
<code class="function">stream_stop_cb</code> calls).
</p><pre class="programlisting">
typedef void (*LogicalDecodeStreamTruncateCB) (struct LogicalDecodingContext *ctx,
ReorderBufferTXN *txn,
int nrelations,
Relation relations[],
ReorderBufferChange *change);
</pre><p>
The parameters are analogous to the <code class="function">stream_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><div class="sect2" id="LOGICALDECODING-OUTPUT-PLUGIN-OUTPUT"><div class="titlepage"><div><div><h3 class="title">49.6.5. Functions for Producing Output <a href="#LOGICALDECODING-OUTPUT-PLUGIN-OUTPUT" class="id_link">#</a></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>
</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" title="49.5. System Catalogs Related to Logical Decoding">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="logicaldecoding.html" title="Chapter 49. Logical Decoding">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="logicaldecoding-writer.html" title="49.7. Logical Decoding Output Writers">Next</a></td></tr><tr><td width="40%" align="left" valign="top">49.5. System Catalogs Related to Logical Decoding </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 49.7. Logical Decoding Output Writers</td></tr></table></div></body></html>