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559 lines
23 KiB
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559 lines
23 KiB
Plaintext
<!-- doc/src/sgml/logicaldecoding.sgml -->
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<chapter id="logicaldecoding">
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<title>Logical Decoding</title>
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<indexterm zone="logicaldecoding">
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<primary>Logical Decoding</primary>
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</indexterm>
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<para>
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PostgreSQL provides infrastructure to stream the modifications performed
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via SQL to external consumers. This functionality can be used to for a
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variety of purposes, including replication solutions and auditing.
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</para>
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<para>
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Changes are sent out in streams identified by logical replication slots.
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Each stream outputs each change exactly once.
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</para>
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<para>
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The format in which those changes are streamed is determined by the output
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plugin used. An example plugin is provided, and additional plugins can be
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written to extend the choice of available formats without modifying any
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core code.
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Every output plugin has access to each individual new row produced
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by <command>INSERT</command> and the new row version created
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by <command>UPDATE</command>. Availability of old row versions for
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<command>UPDATE</command> and <command>DELETE</command> depends on
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the configured
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<link linkend="SQL-CREATETABLE-REPLICA-IDENTITY"><literal>REPLICA
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IDENTITY</literal></link>.
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</para>
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<para>
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Changes can be consumed either using the streaming replication protocol
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(see <xref linkend="protocol-replication"> and
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<xref linkend="logicaldecoding-walsender">), or by calling functions
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via SQL (see <xref linkend="logicaldecoding-sql">). It is also possible
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to write additional methods of consuming the output of a replication slot
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without modifying core code
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(see <xref linkend="logicaldecoding-writer">).
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</para>
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<sect1 id="logicaldecoding-example">
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<title>Logical Decoding Example</title>
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<para>
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The following example demonstrates the SQL interface.
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</para>
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<para>
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Before you can use logical decoding, you must set
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<xref linkend="guc-wal-level"> to <literal>logical</literal> and
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<xref linkend="guc-max-replication-slots"> to at least 1.
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Then, you should connect to the target database (in the example
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below, <literal>postgres</literal>) as a superuser.
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</para>
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<programlisting>
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postgres=# -- Create a slot named 'regression_slot' using the output plugin 'test_decoding'
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postgres=# SELECT * FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');
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slotname | xlog_position
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-----------------+---------------
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regression_slot | 0/16B1970
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(1 row)
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postgres=# SELECT * FROM pg_replication_slots;
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slot_name | plugin | slot_type | datoid | database | active | xmin | catalog_xmin | restart_lsn
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-----------------+---------------+-----------+--------+----------+--------+--------+--------------+-------------
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regression_slot | test_decoding | logical | 12052 | postgres | f | | 684 | 0/16A4408
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(1 row)
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postgres=# -- There are no changes to see yet
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postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
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location | xid | data
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----------+-----+------
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(0 rows)
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postgres=# CREATE TABLE data(id serial primary key, data text);
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CREATE TABLE
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postgres=# -- DDL isn't replicated, so all you'll see is the transaction
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postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
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location | xid | data
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-----------+-----+------------
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0/16D5D48 | 688 | BEGIN 688
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0/16E0380 | 688 | COMMIT 688
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(2 rows)
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postgres=# -- Once changes are read, they're consumed and not emitted
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postgres=# -- in a subsequent call:
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postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
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location | xid | data
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----------+-----+------
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(0 rows)
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postgres=# BEGIN;
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postgres=# INSERT INTO data(data) VALUES('1');
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postgres=# INSERT INTO data(data) VALUES('2');
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postgres=# COMMIT;
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postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
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location | xid | data
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-----------+-----+-----------------------------------------------
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0/16E0478 | 689 | BEGIN 689
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0/16E0478 | 689 | table public.data: INSERT: id[integer]:1 data[text]:'1'
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0/16E0580 | 689 | table public.data: INSERT: id[integer]:2 data[text]:'2'
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0/16E0650 | 689 | COMMIT 689
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(4 rows)
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postgres=# INSERT INTO data(data) VALUES('3');
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postgres=# -- You can also peek ahead in the change stream without consuming changes
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postgres=# SELECT * FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL);
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location | xid | data
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-----------+-----+-----------------------------------------------
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0/16E09C0 | 690 | BEGIN 690
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0/16E09C0 | 690 | table public.data: INSERT: id[integer]:3 data[text]:'3'
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0/16E0B90 | 690 | COMMIT 690
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(3 rows)
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postgres=# -- You can also peek ahead in the change stream without consuming changes
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postgres=# SELECT * FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL);
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location | xid | data
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-----------+-----+-----------------------------------------------
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0/16E09C0 | 690 | BEGIN 690
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0/16E09C0 | 690 | table public.data: INSERT: id[integer]:3 data[text]:'3'
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0/16E0B90 | 690 | COMMIT 690
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(3 rows)
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postgres=# -- options can be passed to output plugin, to influence the formatting
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postgres=# SELECT * FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL, 'include-timestamp', 'on');
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location | xid | data
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-----------+-----+-----------------------------------------------
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0/16E09C0 | 690 | BEGIN 690
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0/16E09C0 | 690 | table public.data: INSERT: id[integer]:3 data[text]:'3'
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0/16E0B90 | 690 | COMMIT 690 (at 2014-02-27 16:41:51.863092+01)
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(3 rows)
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postgres=# -- Remember to destroy a slot you no longer need to stop it consuming
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postgres=# -- server resources:
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postgres=# SELECT pg_drop_replication_slot('regression_slot');
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pg_drop_replication_slot
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-----------------------
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(1 row)
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</programlisting>
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<para>
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The following example shows usage of the walsender interface using
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the <link linkend="app-pgrecvlogical"><command>pg_recvlogical</command></link>
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shell command. It requires the replication configurations to be allowed
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(see <xref linkend="streaming-replication-authentication">)
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and <varname>max_wal_senders</varname> to be set sufficiently high for
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another connection.
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</para>
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<programlisting>
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# pg_recvlogical -d postgres --slot test --create
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# pg_recvlogical -d postgres --slot test --start -f -
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CTRL-Z
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# psql -d postgres -c "INSERT INTO data(data) VALUES('4');"
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# fg
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BEGIN 693
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table public.data: INSERT: id[integer]:4 data[text]:'4'
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COMMIT 693
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CTRL-C
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# pg_recvlogical -d postgres --slot test --drop
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</programlisting>
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</sect1>
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<sect1 id="logicaldecoding-explanation">
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<title>Logical Decoding Concepts</title>
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<sect2>
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<title>Logical Decoding</title>
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<indexterm>
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<primary>Logical Decoding</primary>
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</indexterm>
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<para>
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Logical decoding is the process of extracting all persistent changes
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to a database's tables into a coherent, easy to understand format which
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can be interpreted without detailed knowledge of the database's internal
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state.
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</para>
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<para>
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In <productname>PostgreSQL</productname>, logical decoding is implemented
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by decoding the contents of the <link linkend="wal">write-ahead
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log</link>, which describe changes on a storage level, into an
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application-specific form such as a stream of tuples or SQL statements.
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</para>
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</sect2>
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<sect2>
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<title>Replication Slots</title>
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<indexterm>
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<primary>replication slot</primary>
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<secondary>logical replication</secondary>
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</indexterm>
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<para>
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In the context of logical replication, a slot represents a stream of
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changes which can be replayed to a client in the order they were made on
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the origin server. Each slot streams a sequence of changes from a single
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database, sending each change exactly once (except when peeking forward
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in the stream).
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</para>
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<note>
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<para><productname>PostgreSQL</productname> also has streaming replication slots
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(see <xref linkend="streaming-replication">), but they are used somewhat
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differently there.
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</para>
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</note>
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<para>
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Replication slots have an identifier which is unique across all databases
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in a <productname>PostgreSQL</productname> cluster. Slots persist
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independently of the connection using them and are crash-safe.
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</para>
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<para>
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Multiple independent slots may exist for a single database. Each slot has
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its own state, allowing different consumers to receive changes from
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different points in the database change stream. For most applications, a
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separate slot will be required for each consumer.
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</para>
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<para>
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A logical replication slot knows nothing about the state of the
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receiver(s). It's even possible to have multiple different receivers using
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the same slot at different times; they'll just get the changes following
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on from when the last receiver stopped consuming them. Only one receiver
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may consume changes from a slot at any given time.
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</para>
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<note>
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<para>
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Replication slots persist across crashes and know nothing about the state
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of their consumer(s). They will prevent removal of required resources
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even when there is no connection using them. This consumes storage
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because neither required WAL nor required rows from the system catalogs
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can be removed by VACUUM as long as they are required by a replication
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slot, so if a slot is no longer required it should be dropped.
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</para>
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</note>
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</sect2>
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<sect2>
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<title>Output Plugins</title>
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<para>
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Output plugins transform the data from the write-ahead log's internal
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representation into the format the consumer of a replication slot desires.
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</para>
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</sect2>
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<sect2>
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<title>Exported Snapshots</title>
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<para>
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When a new replication slot is created using the walsender interface a
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snapshot is exported
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(see <xref linkend="functions-snapshot-synchronization">) which will show
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exactly the state of the database after which all changes will be
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included in the change stream. This can be used to create a new replica by
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using <link linkend="sql-set-transaction"><literal>SET TRANSACTION
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SNAPSHOT</literal></link> to read the state of the database at the moment
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the slot was created. This transaction can then be used to dump the
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database's state at that point in time which afterwards can be updated
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using the slot's contents without losing any changes.
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</para>
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</sect2>
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</sect1>
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<sect1 id="logicaldecoding-walsender">
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<title>Streaming Replication Protocol Interface</title>
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<para>
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The <literal>CREATE_REPLICATION_SLOT slotname LOGICAL
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options</literal>, <literal>DROP_REPLICATION_SLOT slotname</literal>
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and <literal>START_REPLICATION SLOT slotname LOGICAL options</literal>
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commands can be used to create, drop and stream changes from a replication
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slot respectively. These commands are only available over a replication
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connection; they cannot be used via SQL.
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See <xref linkend="protocol-replication">.
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</para>
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<para>
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The <command>pg_recvlogical</command> command
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(see <xref linkend="app-pgrecvlogical">) can be used to control logical
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decoding over a walsender connection.
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</para>
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</sect1>
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<sect1 id="logicaldecoding-sql">
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<title>Logical Decoding <acronym>SQL</acronym> Interface</title>
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<para>
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See <xref linkend="functions-replication"> for detailed documentation on
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the SQL-level API for interacting with logical decoding.
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</para>
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<para>
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Synchronous replication (see <xref linkend="synchronous-replication">) is
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only supported on replication slots used over the walsender interface. The
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function interface and additional, non-core interfaces do not support
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synchronous replication.
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</para>
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</sect1>
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<sect1 id="logicaldecoding-catalogs">
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<title>System catalogs related to logical decoding</title>
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<para>
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The <link linkend="catalog-pg-replication-slots"><structname>pg_replication_slots</structname></link>
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view and the
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<link linkend="monitoring-stats-views-table"><structname>pg_stat_replication</structname></link>
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view provide information about the current state of replication slots and
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walsender connections respectively. These views apply to both physical and
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logical replication.
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</para>
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</sect1>
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<sect1 id="logicaldecoding-output-plugin">
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<title>Logical Decoding Output Plugins</title>
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<para>
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An example output plugin can be found in the
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<link linkend="test-decoding">
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<filename>contrib/test_decoding</filename>
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</link>
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subdirectory of the PostgreSQL source tree.
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</para>
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<sect2 id="logicaldecoding-output-init">
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<title>Initialization Function</title>
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<indexterm zone="logicaldecoding">
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<primary>_PG_output_plugin_init</primary>
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</indexterm>
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<para>
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An output plugin is loaded by dynamically loading a shared library with
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the output plugin's name as the library basename. The normal library
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search path is used to locate the library. To provide the required output
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plugin callbacks and to indicate that the library is actually an output
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plugin it needs to provide a function named
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<function>_PG_output_plugin_init</function>. This function is passed a
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struct that needs to be filled with the callback function pointers for
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individual actions.
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<programlisting>
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typedef struct OutputPluginCallbacks
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{
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LogicalDecodeStartupCB startup_cb;
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LogicalDecodeBeginCB begin_cb;
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LogicalDecodeChangeCB change_cb;
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LogicalDecodeCommitCB commit_cb;
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LogicalDecodeShutdownCB shutdown_cb;
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} OutputPluginCallbacks;
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typedef void (*LogicalOutputPluginInit)(struct OutputPluginCallbacks *cb);
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</programlisting>
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The <function>begin_cb</function>, <function>change_cb</function>
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and <function>commit_cb</function> callbacks are required,
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while <function>startup_cb</function>
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and <function>shutdown_cb</function> are optional.
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</para>
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</sect2>
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<sect2 id="logicaldecoding-capabilities">
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<title>Capabilities</title>
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<para>
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To decode, format and output changes, output plugins can use most of the
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backend's normal infrastructure, including calling output functions. Read
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only access to relations is permitted as long as only relations are
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accessed that either have been created by <command>initdb</command> in
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the <literal>pg_catalog</literal> schema, or have been marked as user
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provided catalog tables using
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<programlisting>
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ALTER TABLE user_catalog_table SET (user_catalog_table = true);
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CREATE TABLE another_catalog_table(data text) WITH (user_catalog_table = true);
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</programlisting>
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Any actions leading to xid assignment are prohibited. That, among others,
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includes writing to tables, performing DDL changes and
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calling <literal>txid_current()</literal>.
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</para>
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</sect2>
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<sect2 id="logicaldecoding-output-plugin-callbacks">
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<title>Output Plugin Callbacks</title>
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<para>
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An output plugin gets notified about changes that are happening via
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various callbacks it needs to provide.
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</para>
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<para>
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Concurrent transactions are decoded in commit order and only changes
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belonging to a specific transaction are decoded inbetween
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the <literal>begin</literal> and <literal>commit</literal>
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callbacks. Transactions that were rolled back explicitly or implicitly
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never get
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decoded. Successful <link linkend="SQL-SAVEPOINT">SAVEPOINTs</link> are
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folded into the transaction containing them in the order they were
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executed within that transaction.
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</para>
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<note>
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<para>
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Only transactions that have already safely been flushed to disk will be
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decoded. That can lead to a COMMIT not immediately being decoded in a
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directly following <literal>pg_logical_slot_get_changes()</literal>
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when <varname>synchronous_commit</varname> is set
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to <literal>off</literal>.
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</para>
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</note>
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<sect3 id="logicaldecoding-output-plugin-startup">
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<title>Startup Callback</title>
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<para>
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The optional <function>startup_cb</function> callback is called whenever
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a replication slot is created or asked to stream changes, independent
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of the number of changes that are ready to be put out.
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<programlisting>
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typedef void (*LogicalDecodeStartupCB) (
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struct LogicalDecodingContext *ctx,
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OutputPluginOptions *options,
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bool is_init
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);
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</programlisting>
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The <literal>is_init</literal> parameter will be true when the
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replication slot is being created and false
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otherwise. <parameter>options</parameter> points to a struct of options
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that output plugins can set:
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<programlisting>
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typedef struct OutputPluginOptions
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{
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OutputPluginOutputType output_type;
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} OutputPluginOptions;
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</programlisting>
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<literal>output_type</literal> has to either be set to
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<literal>OUTPUT_PLUGIN_TEXTUAL_OUTPUT</literal>
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or <literal>OUTPUT_PLUGIN_BINARY_OUTPUT</literal>.
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</para>
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<para>
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The startup callback should validate the options present in
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<literal>ctx->output_plugin_options</literal>. If the output plugin
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needs to have a state, it can
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use <literal>ctx->output_plugin_private</literal> to store it.
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</para>
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</sect3>
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<sect3 id="logicaldecoding-output-plugin-shutdown">
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<title>Shutdown Callback</title>
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<para>
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The optional <function>shutdown_cb</function> callback is called
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whenever a formerly active replication slot is not used anymore and can
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be used to deallocate resources private to the output plugin. The slot
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isn't necessarily being dropped, streaming is just being stopped.
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<programlisting>
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typedef void (*LogicalDecodeShutdownCB) (
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struct LogicalDecodingContext *ctx
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);
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</programlisting>
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</para>
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</sect3>
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<sect3 id="logicaldecoding-output-plugin-begin">
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<title>Transaction Begin Callback</title>
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<para>
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The required <function>begin_cb</function> callback is called whenever a
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start of a commited transaction has been decoded. Aborted transactions
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and their contents never get decoded.
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<programlisting>
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typedef void (*LogicalDecodeBeginCB) (
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struct LogicalDecodingContext *,
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ReorderBufferTXN *txn
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);
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</programlisting>
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The <parameter>txn</parameter> parameter contains meta information about
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the transaction, like the timestamp at which it has been committed and
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its XID.
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</para>
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</sect3>
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<sect3 id="logicaldecoding-output-plugin-commit">
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<title>Transaction End Callback</title>
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<para>
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The required <function>commit_cb</function> callback is called whenever
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|
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 *,
|
|
ReorderBufferTXN *txn
|
|
);
|
|
</programlisting>
|
|
</para>
|
|
</sect3>
|
|
<sect3 id="logicaldecoding-output-plugin-change">
|
|
<title>Callback called for each individual change in a
|
|
transaction</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 indvidually 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>
|
|
</sect2>
|
|
<sect2 id="logicaldecoding-output-plugin-output">
|
|
<title>Functions for producing output from an output plugin</title>
|
|
<para>
|
|
To actually produce output, output plugins can write data to
|
|
the <literal>StringInfo</literal> output buffer
|
|
in <literal>ctx->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>
|
|
<para>
|
|
Logical decoding may be used to 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 walsender 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>
|
|
</sect1>
|
|
</chapter>
|