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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>31.2. Subscription</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="logical-replication-publication.html" title="31.1. Publication" /><link rel="next" href="logical-replication-row-filter.html" title="31.3. Row Filters" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">31.2. Subscription</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="logical-replication-publication.html" title="31.1. Publication">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="logical-replication.html" title="Chapter 31. Logical Replication">Up</a></td><th width="60%" align="center">Chapter 31. Logical Replication</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="logical-replication-row-filter.html" title="31.3. Row Filters">Next</a></td></tr></table><hr /></div><div class="sect1" id="LOGICAL-REPLICATION-SUBSCRIPTION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">31.2. Subscription</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="logical-replication-subscription.html#LOGICAL-REPLICATION-SUBSCRIPTION-SLOT">31.2.1. Replication Slot Management</a></span></dt><dt><span class="sect2"><a href="logical-replication-subscription.html#LOGICAL-REPLICATION-SUBSCRIPTION-EXAMPLES">31.2.2. Examples</a></span></dt></dl></div><p>
A <em class="firstterm">subscription</em> is the downstream side of logical
replication. The node where a subscription is defined is referred to as
the <em class="firstterm">subscriber</em>. A subscription defines the connection
to another database and set of publications (one or more) to which it wants
to subscribe.
</p><p>
The subscriber database behaves in the same way as any other PostgreSQL
instance and can be used as a publisher for other databases by defining its
own publications.
</p><p>
A subscriber node may have multiple subscriptions if desired. It is
possible to define multiple subscriptions between a single
publisher-subscriber pair, in which case care must be taken to ensure
that the subscribed publication objects don't overlap.
</p><p>
Each subscription will receive changes via one replication slot (see
<a class="xref" href="warm-standby.html#STREAMING-REPLICATION-SLOTS" title="27.2.6. Replication Slots">Section 27.2.6</a>). Additional replication
slots may be required for the initial data synchronization of
pre-existing table data and those will be dropped at the end of data
synchronization.
</p><p>
A logical replication subscription can be a standby for synchronous
replication (see <a class="xref" href="warm-standby.html#SYNCHRONOUS-REPLICATION" title="27.2.8. Synchronous Replication">Section 27.2.8</a>). The standby
name is by default the subscription name. An alternative name can be
specified as <code class="literal">application_name</code> in the connection
information of the subscription.
</p><p>
Subscriptions are dumped by <code class="command">pg_dump</code> if the current user
is a superuser. Otherwise a warning is written and subscriptions are
skipped, because non-superusers cannot read all subscription information
from the <code class="structname">pg_subscription</code> catalog.
</p><p>
The subscription is added using <a class="link" href="sql-createsubscription.html" title="CREATE SUBSCRIPTION"><code class="command">CREATE SUBSCRIPTION</code></a> and
can be stopped/resumed at any time using the
<a class="link" href="sql-altersubscription.html" title="ALTER SUBSCRIPTION"><code class="command">ALTER SUBSCRIPTION</code></a> command and removed using
<a class="link" href="sql-dropsubscription.html" title="DROP SUBSCRIPTION"><code class="command">DROP SUBSCRIPTION</code></a>.
</p><p>
When a subscription is dropped and recreated, the synchronization
information is lost. This means that the data has to be resynchronized
afterwards.
</p><p>
The schema definitions are not replicated, and the published tables must
exist on the subscriber. Only regular tables may be
the target of replication. For example, you can't replicate to a view.
</p><p>
The tables are matched between the publisher and the subscriber using the
fully qualified table name. Replication to differently-named tables on the
subscriber is not supported.
</p><p>
Columns of a table are also matched by name. The order of columns in the
subscriber table does not need to match that of the publisher. The data
types of the columns do not need to match, as long as the text
representation of the data can be converted to the target type. For
example, you can replicate from a column of type <code class="type">integer</code> to a
column of type <code class="type">bigint</code>. The target table can also have
additional columns not provided by the published table. Any such columns
will be filled with the default value as specified in the definition of the
target table.
</p><div class="sect2" id="LOGICAL-REPLICATION-SUBSCRIPTION-SLOT"><div class="titlepage"><div><div><h3 class="title">31.2.1. Replication Slot Management</h3></div></div></div><p>
As mentioned earlier, each (active) subscription receives changes from a
replication slot on the remote (publishing) side.
</p><p>
Additional table synchronization slots are normally transient, created
internally to perform initial table synchronization and dropped
automatically when they are no longer needed. These table synchronization
slots have generated names: <span class="quote">“<span class="quote"><code class="literal">pg_%u_sync_%u_%llu</code></span>”</span>
(parameters: Subscription <em class="parameter"><code>oid</code></em>,
Table <em class="parameter"><code>relid</code></em>, system identifier <em class="parameter"><code>sysid</code></em>)
</p><p>
Normally, the remote replication slot is created automatically when the
subscription is created using <code class="command">CREATE SUBSCRIPTION</code> and it
is dropped automatically when the subscription is dropped using
<code class="command">DROP SUBSCRIPTION</code>. In some situations, however, it can
be useful or necessary to manipulate the subscription and the underlying
replication slot separately. Here are some scenarios:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
When creating a subscription, the replication slot already exists. In
that case, the subscription can be created using
the <code class="literal">create_slot = false</code> option to associate with the
existing slot.
</p></li><li class="listitem"><p>
When creating a subscription, the remote host is not reachable or in an
unclear state. In that case, the subscription can be created using
the <code class="literal">connect = false</code> option. The remote host will then not
be contacted at all. This is what <span class="application">pg_dump</span>
uses. The remote replication slot will then have to be created
manually before the subscription can be activated.
</p></li><li class="listitem"><p>
When dropping a subscription, the replication slot should be kept.
This could be useful when the subscriber database is being moved to a
different host and will be activated from there. In that case,
disassociate the slot from the subscription using <code class="command">ALTER
SUBSCRIPTION</code> before attempting to drop the subscription.
</p></li><li class="listitem"><p>
When dropping a subscription, the remote host is not reachable. In
that case, disassociate the slot from the subscription
using <code class="command">ALTER SUBSCRIPTION</code> before attempting to drop
the subscription. If the remote database instance no longer exists, no
further action is then necessary. If, however, the remote database
instance is just unreachable, the replication slot (and any still
remaining table synchronization slots) should then be
dropped manually; otherwise it/they would continue to reserve WAL and might
eventually cause the disk to fill up. Such cases should be carefully
investigated.
</p></li></ul></div><p>
</p></div><div class="sect2" id="LOGICAL-REPLICATION-SUBSCRIPTION-EXAMPLES"><div class="titlepage"><div><div><h3 class="title">31.2.2. Examples</h3></div></div></div><p>
Create some test tables on the publisher.
</p><pre class="programlisting">
test_pub=# CREATE TABLE t1(a int, b text, PRIMARY KEY(a));
CREATE TABLE
test_pub=# CREATE TABLE t2(c int, d text, PRIMARY KEY(c));
CREATE TABLE
test_pub=# CREATE TABLE t3(e int, f text, PRIMARY KEY(e));
CREATE TABLE
</pre><p>
Create the same tables on the subscriber.
</p><pre class="programlisting">
test_sub=# CREATE TABLE t1(a int, b text, PRIMARY KEY(a));
CREATE TABLE
test_sub=# CREATE TABLE t2(c int, d text, PRIMARY KEY(c));
CREATE TABLE
test_sub=# CREATE TABLE t3(e int, f text, PRIMARY KEY(e));
CREATE TABLE
</pre><p>
Insert data to the tables at the publisher side.
</p><pre class="programlisting">
test_pub=# INSERT INTO t1 VALUES (1, 'one'), (2, 'two'), (3, 'three');
INSERT 0 3
test_pub=# INSERT INTO t2 VALUES (1, 'A'), (2, 'B'), (3, 'C');
INSERT 0 3
test_pub=# INSERT INTO t3 VALUES (1, 'i'), (2, 'ii'), (3, 'iii');
INSERT 0 3
</pre><p>
Create publications for the tables. The publications <code class="literal">pub2</code>
and <code class="literal">pub3a</code> disallow some <code class="literal">publish</code>
operations. The publication <code class="literal">pub3b</code> has a row filter (see
<a class="xref" href="logical-replication-row-filter.html" title="31.3. Row Filters">Section 31.3</a>).
</p><pre class="programlisting">
test_pub=# CREATE PUBLICATION pub1 FOR TABLE t1;
CREATE PUBLICATION
test_pub=# CREATE PUBLICATION pub2 FOR TABLE t2 WITH (publish = 'truncate');
CREATE PUBLICATION
test_pub=# CREATE PUBLICATION pub3a FOR TABLE t3 WITH (publish = 'truncate');
CREATE PUBLICATION
test_pub=# CREATE PUBLICATION pub3b FOR TABLE t3 WHERE (e > 5);
CREATE PUBLICATION
</pre><p>
Create subscriptions for the publications. The subscription
<code class="literal">sub3</code> subscribes to both <code class="literal">pub3a</code> and
<code class="literal">pub3b</code>. All subscriptions will copy initial data by default.
</p><pre class="programlisting">
test_sub=# CREATE SUBSCRIPTION sub1
test_sub-# CONNECTION 'host=localhost dbname=test_pub application_name=sub1'
test_sub-# PUBLICATION pub1;
CREATE SUBSCRIPTION
test_sub=# CREATE SUBSCRIPTION sub2
test_sub-# CONNECTION 'host=localhost dbname=test_pub application_name=sub2'
test_sub-# PUBLICATION pub2;
CREATE SUBSCRIPTION
test_sub=# CREATE SUBSCRIPTION sub3
test_sub-# CONNECTION 'host=localhost dbname=test_pub application_name=sub3'
test_sub-# PUBLICATION pub3a, pub3b;
CREATE SUBSCRIPTION
</pre><p>
Observe that initial table data is copied, regardless of the
<code class="literal">publish</code> operation of the publication.
</p><pre class="programlisting">
test_sub=# SELECT * FROM t1;
a | b
---+-------
1 | one
2 | two
3 | three
(3 rows)
test_sub=# SELECT * FROM t2;
c | d
---+---
1 | A
2 | B
3 | C
(3 rows)
</pre><p>
Furthermore, because the initial data copy ignores the <code class="literal">publish</code>
operation, and because publication <code class="literal">pub3a</code> has no row filter,
it means the copied table <code class="literal">t3</code> contains all rows even when
they do not match the row filter of publication <code class="literal">pub3b</code>.
</p><pre class="programlisting">
test_sub=# SELECT * FROM t3;
e | f
---+-----
1 | i
2 | ii
3 | iii
(3 rows)
</pre><p>
Insert more data to the tables at the publisher side.
</p><pre class="programlisting">
test_pub=# INSERT INTO t1 VALUES (4, 'four'), (5, 'five'), (6, 'six');
INSERT 0 3
test_pub=# INSERT INTO t2 VALUES (4, 'D'), (5, 'E'), (6, 'F');
INSERT 0 3
test_pub=# INSERT INTO t3 VALUES (4, 'iv'), (5, 'v'), (6, 'vi');
INSERT 0 3
</pre><p>
Now the publisher side data looks like:
</p><pre class="programlisting">
test_pub=# SELECT * FROM t1;
a | b
---+-------
1 | one
2 | two
3 | three
4 | four
5 | five
6 | six
(6 rows)
test_pub=# SELECT * FROM t2;
c | d
---+---
1 | A
2 | B
3 | C
4 | D
5 | E
6 | F
(6 rows)
test_pub=# SELECT * FROM t3;
e | f
---+-----
1 | i
2 | ii
3 | iii
4 | iv
5 | v
6 | vi
(6 rows)
</pre><p>
Observe that during normal replication the appropriate
<code class="literal">publish</code> operations are used. This means publications
<code class="literal">pub2</code> and <code class="literal">pub3a</code> will not replicate the
<code class="literal">INSERT</code>. Also, publication <code class="literal">pub3b</code> will
only replicate data that matches the row filter of <code class="literal">pub3b</code>.
Now the subscriber side data looks like:
</p><pre class="programlisting">
test_sub=# SELECT * FROM t1;
a | b
---+-------
1 | one
2 | two
3 | three
4 | four
5 | five
6 | six
(6 rows)
test_sub=# SELECT * FROM t2;
c | d
---+---
1 | A
2 | B
3 | C
(3 rows)
test_sub=# SELECT * FROM t3;
e | f
---+-----
1 | i
2 | ii
3 | iii
6 | vi
(4 rows)
</pre></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="logical-replication-publication.html" title="31.1. Publication">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="logical-replication.html" title="Chapter 31. Logical Replication">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="logical-replication-row-filter.html" title="31.3. Row Filters">Next</a></td></tr><tr><td width="40%" align="left" valign="top">31.1. Publication </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 31.3. Row Filters</td></tr></table></div></body></html>
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