path: root/doc/src/sgml/logicaldecoding.sgml

<!-- doc/src/sgml/logicaldecoding.sgml -->
 <chapter id="logicaldecoding">
  <title>Logical Decoding</title>
  <indexterm zone="logicaldecoding">
   <primary>Logical Decoding</primary>
  </indexterm>
  <para>
   PostgreSQL provides infrastructure to stream the modifications performed
   via SQL to external consumers.  This functionality can be used for a
   variety of purposes, including replication solutions and auditing.
  </para>

  <para>
   Changes are sent out in streams identified by logical replication slots.
  </para>

  <para>
   The format in which those changes are streamed is determined by the output
   plugin used.  An example plugin is provided in the PostgreSQL distribution.
   Additional plugins can be
   written to extend the choice of available formats without modifying any
   core code.
   Every output plugin has access to each individual new row produced
   by <command>INSERT</command> and the new row version created
   by <command>UPDATE</command>.  Availability of old row versions for
   <command>UPDATE</command> and <command>DELETE</command> depends on
   the configured replica identity (see <xref linkend="sql-createtable-replica-identity"/>).
  </para>

  <para>
   Changes can be consumed either using the streaming replication protocol
   (see <xref linkend="protocol-replication"/> and
   <xref linkend="logicaldecoding-walsender"/>), or by calling functions
   via SQL (see <xref linkend="logicaldecoding-sql"/>). It is also possible
   to write additional methods of consuming the output of a replication slot
   without modifying core code
   (see <xref linkend="logicaldecoding-writer"/>).
  </para>

  <sect1 id="logicaldecoding-example">
   <title>Logical Decoding Examples</title>

   <para>
    The following example demonstrates controlling logical decoding using the
    SQL interface.
   </para>

   <para>
    Before you can use logical decoding, you must set
    <xref linkend="guc-wal-level"/> to <literal>logical</literal> and
    <xref linkend="guc-max-replication-slots"/> to at least 1.  Then, you
    should connect to the target database (in the example
    below, <literal>postgres</literal>) as a superuser.
   </para>

<programlisting>
postgres=# -- Create a slot named 'regression_slot' using the output plugin 'test_decoding'
postgres=# SELECT * FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');
    slot_name    |    lsn
-----------------+-----------
 regression_slot | 0/16B1970
(1 row)

postgres=# SELECT slot_name, plugin, slot_type, database, active, restart_lsn, confirmed_flush_lsn FROM pg_replication_slots;
    slot_name    |    plugin     | slot_type | database | active | restart_lsn | confirmed_flush_lsn
-----------------+---------------+-----------+----------+--------+-------------+-----------------
 regression_slot | test_decoding | logical   | postgres | f      | 0/16A4408   | 0/16A4440
(1 row)

postgres=# -- There are no changes to see yet
postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
 lsn | xid | data 
-----+-----+------
(0 rows)

postgres=# CREATE TABLE data(id serial primary key, data text);
CREATE TABLE

postgres=# -- DDL isn't replicated, so all you'll see is the transaction
postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
    lsn    |  xid  |     data     
-----------+-------+--------------
 0/BA2DA58 | 10297 | BEGIN 10297
 0/BA5A5A0 | 10297 | COMMIT 10297
(2 rows)

postgres=# -- Once changes are read, they're consumed and not emitted
postgres=# -- in a subsequent call:
postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
 lsn | xid | data 
-----+-----+------
(0 rows)

postgres=# BEGIN;
postgres=*# INSERT INTO data(data) VALUES('1');
postgres=*# INSERT INTO data(data) VALUES('2');
postgres=*# COMMIT;

postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
    lsn    |  xid  |                          data                           
-----------+-------+---------------------------------------------------------
 0/BA5A688 | 10298 | BEGIN 10298
 0/BA5A6F0 | 10298 | table public.data: INSERT: id[integer]:1 data[text]:'1'
 0/BA5A7F8 | 10298 | table public.data: INSERT: id[integer]:2 data[text]:'2'
 0/BA5A8A8 | 10298 | COMMIT 10298
(4 rows)

postgres=# INSERT INTO data(data) VALUES('3');

postgres=# -- You can also peek ahead in the change stream without consuming changes
postgres=# SELECT * FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL);
    lsn    |  xid  |                          data                           
-----------+-------+---------------------------------------------------------
 0/BA5A8E0 | 10299 | BEGIN 10299
 0/BA5A8E0 | 10299 | table public.data: INSERT: id[integer]:3 data[text]:'3'
 0/BA5A990 | 10299 | COMMIT 10299
(3 rows)

postgres=# -- The next call to pg_logical_slot_peek_changes() returns the same changes again
postgres=# SELECT * FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL);
    lsn    |  xid  |                          data                           
-----------+-------+---------------------------------------------------------
 0/BA5A8E0 | 10299 | BEGIN 10299
 0/BA5A8E0 | 10299 | table public.data: INSERT: id[integer]:3 data[text]:'3'
 0/BA5A990 | 10299 | COMMIT 10299
(3 rows)

postgres=# -- options can be passed to output plugin, to influence the formatting
postgres=# SELECT * FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL, 'include-timestamp', 'on');
    lsn    |  xid  |                          data                           
-----------+-------+---------------------------------------------------------
 0/BA5A8E0 | 10299 | BEGIN 10299
 0/BA5A8E0 | 10299 | table public.data: INSERT: id[integer]:3 data[text]:'3'
 0/BA5A990 | 10299 | COMMIT 10299 (at 2017-05-10 12:07:21.272494-04)
(3 rows)

postgres=# -- Remember to destroy a slot you no longer need to stop it consuming
postgres=# -- server resources:
postgres=# SELECT pg_drop_replication_slot('regression_slot');
 pg_drop_replication_slot
-----------------------

(1 row)
</programlisting>

   <para>
    The following example shows how logical decoding is controlled over the
    streaming replication protocol, using the
    program <xref linkend="app-pgrecvlogical"/> included in the PostgreSQL
    distribution.  This requires that client authentication is set up to allow
    replication connections
    (see <xref linkend="streaming-replication-authentication"/>) and
    that <varname>max_wal_senders</varname> is set sufficiently high to allow
    an additional connection.
   </para>
<programlisting>
$ pg_recvlogical -d postgres --slot=test --create-slot
$ pg_recvlogical -d postgres --slot=test --start -f -
<keycombo action="simul"><keycap>Control</keycap><keycap>Z</keycap></keycombo>
$ psql -d postgres -c "INSERT INTO data(data) VALUES('4');"
$ fg
BEGIN 693
table public.data: INSERT: id[integer]:4 data[text]:'4'
COMMIT 693
<keycombo action="simul"><keycap>Control</keycap><keycap>C</keycap></keycombo>
$ pg_recvlogical -d postgres --slot=test --drop-slot
</programlisting>
  </sect1>

  <sect1 id="logicaldecoding-explanation">
   <title>Logical Decoding Concepts</title>
   <sect2>
    <title>Logical Decoding</title>

    <indexterm>
     <primary>Logical Decoding</primary>
    </indexterm>

    <para>
     Logical decoding is the process of extracting all persistent changes
     to a database's tables into a coherent, easy to understand format which
     can be interpreted without detailed knowledge of the database's internal
     state.
    </para>

    <para>
     In <productname>PostgreSQL</productname>, logical decoding is implemented
     by decoding the contents of the <link linkend="wal">write-ahead
     log</link>, which describe changes on a storage level, into an
     application-specific form such as a stream of tuples or SQL statements.
    </para>
   </sect2>

   <sect2 id="logicaldecoding-replication-slots">
    <title>Replication Slots</title>

    <indexterm>
     <primary>replication slot</primary>
     <secondary>logical replication</secondary>
    </indexterm>

    <para>
     In the context of logical replication, a slot represents a stream of
     changes that can be replayed to a client in the order they were made on
     the origin server. Each slot streams a sequence of changes from a single
     database.
    </para>

    <note>
     <para><productname>PostgreSQL</productname> also has streaming replication slots
     (see <xref linkend="streaming-replication"/>), but they are used somewhat
     differently there.
     </para>
    </note>

    <para>
     A replication slot has an identifier that is unique across all databases
     in a <productname>PostgreSQL</productname> cluster. Slots persist
     independently of the connection using them and are crash-safe.
    </para>

    <para>
     A logical slot will emit each change just once in normal operation.
     The current position of each slot is persisted only at checkpoint, so in
     the case of a crash the slot may return to an earlier LSN, which will
     then cause recent changes to be sent again when the server restarts.
     Logical decoding clients are responsible for avoiding ill effects from
     handling the same message more than once.  Clients may wish to record
     the last LSN they saw when decoding and skip over any repeated data or
     (when using the replication protocol) request that decoding start from
     that LSN rather than letting the server determine the start point.
     The Replication Progress Tracking feature is designed for this purpose,
     refer to <link linkend="replication-origins">replication origins</link>.
    </para>

    <para>
     Multiple independent slots may exist for a single database. Each slot has
     its own state, allowing different consumers to receive changes from
     different points in the database change stream. For most applications, a
     separate slot will be required for each consumer.
    </para>

    <para>
     A logical replication slot knows nothing about the state of the
     receiver(s).  It's even possible to have multiple different receivers using
     the same slot at different times; they'll just get the changes following
     on from when the last receiver stopped consuming them. Only one receiver
     may consume changes from a slot at any given time.
    </para>

    <caution>
     <para>
      Replication slots persist across crashes and know nothing about the state
      of their consumer(s). They will prevent removal of required resources
      even when there is no connection using them. This consumes storage
      because neither required WAL nor required rows from the system catalogs
      can be removed by <command>VACUUM</command> as long as they are required by a replication
      slot.  In extreme cases this could cause the database to shut down to prevent
      transaction ID wraparound (see <xref linkend="vacuum-for-wraparound"/>).
      So if a slot is no longer required it should be dropped.
     </para>
    </caution>
   </sect2>

   <sect2>
    <title>Output Plugins</title>
    <para>
     Output plugins transform the data from the write-ahead log's internal
     representation into the format the consumer of a replication slot desires.
    </para>
   </sect2>

   <sect2>
    <title>Exported Snapshots</title>
    <para>
     When a new replication slot is created using the streaming replication
     interface (see <xref linkend="protocol-replication-create-slot"/>), a
     snapshot is exported
     (see <xref linkend="functions-snapshot-synchronization"/>), which will show
     exactly the state of the database after which all changes will be
     included in the change stream. This can be used to create a new replica by
     using <link linkend="sql-set-transaction"><literal>SET TRANSACTION
     SNAPSHOT</literal></link> to read the state of the database at the moment
     the slot was created. This transaction can then be used to dump the
     database's state at that point in time, which afterwards can be updated
     using the slot's contents without losing any changes.
    </para>
    <para>
     Creation of a snapshot is not always possible.  In particular, it will
     fail when connected to a hot standby.  Applications that do not require
     snapshot export may suppress it with the <literal>NOEXPORT_SNAPSHOT</literal>
     option.
    </para>
   </sect2>
  </sect1>

  <sect1 id="logicaldecoding-walsender">
   <title>Streaming Replication Protocol Interface</title>

   <para>
    The commands
    <itemizedlist>
     <listitem>
      <para><literal>CREATE_REPLICATION_SLOT <replaceable>slot_name</replaceable> LOGICAL <replaceable>output_plugin</replaceable></literal></para>
     </listitem>

     <listitem>
      <para><literal>DROP_REPLICATION_SLOT <replaceable>slot_name</replaceable></literal> <optional> <literal>WAIT</literal> </optional></para>
     </listitem>

     <listitem>
      <para><literal>START_REPLICATION SLOT <replaceable>slot_name</replaceable> LOGICAL ...</literal></para>
     </listitem>
    </itemizedlist>
    are used to create, drop, and stream changes from a replication
    slot, respectively. These commands are only available over a replication
    connection; they cannot be used via SQL.
    See <xref linkend="protocol-replication"/> for details on these commands.
   </para>

   <para>
    The command <xref linkend="app-pgrecvlogical"/> can be used to control
    logical decoding over a streaming replication connection.  (It uses
    these commands internally.)
   </para>
  </sect1>

  <sect1 id="logicaldecoding-sql">
   <title>Logical Decoding <acronym>SQL</acronym> Interface</title>

   <para>
     See <xref linkend="functions-replication"/> for detailed documentation on
     the SQL-level API for interacting with logical decoding.
   </para>

   <para>
    Synchronous replication (see <xref linkend="synchronous-replication"/>) is
    only supported on replication slots used over the streaming replication interface. The
    function interface and additional, non-core interfaces do not support
    synchronous replication.
   </para>
  </sect1>

  <sect1 id="logicaldecoding-catalogs">
   <title>System Catalogs Related to Logical Decoding</title>

   <para>
    The <link linkend="view-pg-replication-slots"><structname>pg_replication_slots</structname></link>
    view and the
    <link linkend="monitoring-pg-stat-replication-view">
    <structname>pg_stat_replication</structname></link>
    view provide information about the current state of replication slots and
    streaming replication connections respectively. These views apply to both physical and
    logical replication.
   </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;
} 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>filter_by_origin_cb</function>, <function>truncate_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>
   </sect2>

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

    <para>
     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 <command>initdb</command> in
     the <literal>pg_catalog</literal> schema, or have been marked as user
     provided catalog tables using
<programlisting>
ALTER TABLE user_catalog_table SET (user_catalog_table = true);
CREATE TABLE another_catalog_table(data text) WITH (user_catalog_table = true);
</programlisting>
     Any actions leading to transaction ID assignment are prohibited. That, among others,
     includes writing to tables, performing DDL changes, and
     calling <literal>pg_current_xact_id()</literal>.
    </para>
   </sect2>

   <sect2 id="logicaldecoding-output-mode">
    <title>Output Modes</title>

    <para>
     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., <type>bytea</type>) is
     cumbersome. If the output plugin only outputs textual data in the
     server's encoding, it can declare that by
     setting <literal>OutputPluginOptions.output_type</literal>
     to <literal>OUTPUT_PLUGIN_TEXTUAL_OUTPUT</literal> instead
     of <literal>OUTPUT_PLUGIN_BINARY_OUTPUT</literal> in
     the <link linkend="logicaldecoding-output-plugin-startup">startup
     callback</link>. In that case, all the data has to be in the server's encoding
     so that a <type>text</type> datum can contain it. This is checked in assertion-enabled
     builds.
    </para>
   </sect2>

   <sect2 id="logicaldecoding-output-plugin-callbacks">
    <title>Output Plugin Callbacks</title>

    <para>
     An output plugin gets notified about changes that are happening via
     various callbacks it needs to provide.
    </para>

    <para>
     Concurrent transactions are decoded in commit order, and only changes
     belonging to a specific transaction are decoded between
     the <literal>begin</literal> and <literal>commit</literal>
     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.
    </para>

    <note>
     <para>
      Only transactions that have already safely been flushed to disk will be
      decoded. That can lead to a <command>COMMIT</command> not immediately being decoded in a
      directly following <literal>pg_logical_slot_get_changes()</literal>
      when <varname>synchronous_commit</varname> is set
      to <literal>off</literal>.
     </para>
    </note>

    <sect3 id="logicaldecoding-output-plugin-startup">
     <title>Startup Callback</title>
     <para>
      The optional <function>startup_cb</function> 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.
<programlisting>
typedef void (*LogicalDecodeStartupCB) (struct LogicalDecodingContext *ctx,
                                        OutputPluginOptions *options,
                                        bool is_init);
</programlisting>
      The <literal>is_init</literal> parameter will be true when the
      replication slot is being created and false
      otherwise. <parameter>options</parameter> points to a struct of options
      that output plugins can set:
<programlisting>
typedef struct OutputPluginOptions
{
    OutputPluginOutputType output_type;
    bool        receive_rewrites;
} OutputPluginOptions;
</programlisting>
      <literal>output_type</literal> has to either be set to
      <literal>OUTPUT_PLUGIN_TEXTUAL_OUTPUT</literal>
      or <literal>OUTPUT_PLUGIN_BINARY_OUTPUT</literal>. See also
      <xref linkend="logicaldecoding-output-mode"/>.
      If <literal>receive_rewrites</literal> 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.
     </para>

     <para>
      The startup callback should validate the options present in
      <literal>ctx-&gt;output_plugin_options</literal>. If the output plugin
      needs to have a state, it can
      use <literal>ctx-&gt;output_plugin_private</literal> to store it.
     </para>
    </sect3>

    <sect3 id="logicaldecoding-output-plugin-shutdown">
     <title>Shutdown Callback</title>

     <para>
      The optional <function>shutdown_cb</function> 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.
<programlisting>
typedef void (*LogicalDecodeShutdownCB) (struct LogicalDecodingContext *ctx);
</programlisting>
     </para>
    </sect3>

    <sect3 id="logicaldecoding-output-plugin-begin">
     <title>Transaction Begin Callback</title>

     <para>
      The required <function>begin_cb</function> callback is called whenever a
      start of a committed transaction has been decoded. Aborted transactions
      and their contents never get decoded.
<programlisting>
typedef void (*LogicalDecodeBeginCB) (struct LogicalDecodingContext *ctx,
                                      ReorderBufferTXN *txn);
</programlisting>
      The <parameter>txn</parameter> parameter contains meta information about
      the transaction, like the time stamp at which it has been committed and
      its XID.
     </para>
    </sect3>

    <sect3 id="logicaldecoding-output-plugin-commit">
     <title>Transaction End Callback</title>

     <para>
      The required <function>commit_cb</function> callback is called whenever
      a transaction commit has been
      decoded. The <function>change_cb</function> callbacks for all modified
      rows will have been called before this, if there have been any modified
      rows.
<programlisting>
typedef void (*LogicalDecodeCommitCB) (struct LogicalDecodingContext *ctx,
                                       ReorderBufferTXN *txn,
                                       XLogRecPtr commit_lsn);
</programlisting>
     </para>
    </sect3>

    <sect3 id="logicaldecoding-output-plugin-change">
     <title>Change Callback</title>

     <para>
      The required <function>change_cb</function> callback is called for every
      individual row modification inside a transaction, may it be
      an <command>INSERT</command>, <command>UPDATE</command>,
      or <command>DELETE</command>. Even if the original command modified
      several rows at once the callback will be called individually for each
      row.
<programlisting>
typedef void (*LogicalDecodeChangeCB) (struct LogicalDecodingContext *ctx,
                                       ReorderBufferTXN *txn,
                                       Relation relation,
                                       ReorderBufferChange *change);
</programlisting>
      The <parameter>ctx</parameter> and <parameter>txn</parameter> parameters
      have the same contents as for the <function>begin_cb</function>
      and <function>commit_cb</function> callbacks, but additionally the
      relation descriptor <parameter>relation</parameter> points to the
      relation the row belongs to and a struct
      <parameter>change</parameter> describing the row modification are passed
      in.
     </para>

     <note>
      <para>
       Only changes in user defined tables that are not unlogged
       (see <xref linkend="sql-createtable-unlogged"/>) and not temporary
       (see <xref linkend="sql-createtable-temporary"/>) can be extracted using
       logical decoding.
      </para>
     </note>
    </sect3>

    <sect3 id="logicaldecoding-output-plugin-truncate">
     <title>Truncate Callback</title>

     <para>
      The <function>truncate_cb</function> callback is called for a
      <command>TRUNCATE</command> command.
<programlisting>
typedef void (*LogicalDecodeTruncateCB) (struct LogicalDecodingContext *ctx,
                                         ReorderBufferTXN *txn,
                                         int nrelations,
                                         Relation relations[],
                                         ReorderBufferChange *change);
</programlisting>
      The parameters are analogous to the <function>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>

     <sect3 id="logicaldecoding-output-plugin-filter-origin">
     <title>Origin Filter Callback</title>

     <para>
       The optional <function>filter_by_origin_cb</function> callback
       is called to determine whether data that has been replayed
       from <parameter>origin_id</parameter> is of interest to the
       output plugin.
<programlisting>
typedef bool (*LogicalDecodeFilterByOriginCB) (struct LogicalDecodingContext *ctx,
                                               RepOriginId origin_id);
</programlisting>
      The <parameter>ctx</parameter> 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.
     </para>
     <para>
       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.
     </para>
     </sect3>

    <sect3 id="logicaldecoding-output-plugin-message">
     <title>Generic Message Callback</title>

     <para>
      The optional <function>message_cb</function> callback is called whenever
      a logical decoding message has been decoded.
<programlisting>
typedef void (*LogicalDecodeMessageCB) (struct LogicalDecodingContext *ctx,
                                        ReorderBufferTXN *txn,
                                        XLogRecPtr message_lsn,
                                        bool transactional,
                                        const char *prefix,
                                        Size message_size,
                                        const char *message);
</programlisting>
      The <parameter>txn</parameter> 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 <parameter>lsn</parameter> has WAL
      location of the message. The <parameter>transactional</parameter> says
      if the message was sent as transactional or not.
      The <parameter>prefix</parameter> is arbitrary null-terminated prefix
      which can be used for identifying interesting messages for the current
      plugin. And finally the <parameter>message</parameter> parameter holds
      the actual message of <parameter>message_size</parameter> size.
     </para>
     <para>
      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.
     </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">
   <title>Synchronous Replication Support for Logical Decoding</title>
   <sect2>
    <title>Overview</title>

    <para>
     Logical decoding can be used to build
     <link linkend="synchronous-replication">synchronous
     replication</link> solutions with the same user interface as synchronous
     replication for <link linkend="streaming-replication">streaming
     replication</link>.  To do this, the streaming replication interface
     (see <xref linkend="logicaldecoding-walsender"/>) must be used to stream out
     data. Clients have to send <literal>Standby status update (F)</literal>
     (see <xref linkend="protocol-replication"/>) messages, just like streaming
     replication clients do.
    </para>

    <note>
     <para>
      A synchronous replica receiving changes via logical decoding will work in
      the scope of a single database. Since, in contrast to
      that, <parameter>synchronous_standby_names</parameter> currently is
      server wide, this means this technique will not work properly if more
      than one database is actively used.
     </para>
    </note>
   </sect2>

   <sect2 id="logicaldecoding-synchronous-caveats">
    <title>Caveats</title>

    <para>
     In synchronous replication setup, a deadlock can happen, if the transaction
     has locked [user] catalog tables exclusively. See
     <xref linkend="logicaldecoding-capabilities"/> for information on user
     catalog tables. This is because logical decoding of transactions can lock
     catalog tables to access them. To avoid this users must refrain from taking
     an exclusive lock on [user] catalog tables. This can happen in the following
     ways:

     <itemizedlist>
      <listitem>
       <para>
        Issuing an explicit <command>LOCK</command> on <structname>pg_class</structname>
        in a transaction.
       </para>
      </listitem>

      <listitem>
       <para>
        Perform <command>CLUSTER</command> on <structname>pg_class</structname> in a
        transaction.
       </para>
      </listitem>

      <listitem>
       <para>
        Executing <command>TRUNCATE</command> on [user] catalog table in a
        transaction.
       </para>
      </listitem>
     </itemizedlist>

     Note that these commands that can cause deadlock apply to not only explicitly
     indicated system catalog tables above but also to any other [user] catalog
     table.
    </para>
   </sect2>
  </sect1>
 </chapter>