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  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>

  2. <!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>30.2. 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 V1.79.1" /><link rel="prev" href="logical-replication-publication.html" title="30.1. Publication" /><link rel="next" href="logical-replication-conflicts.html" title="30.3. Conflicts" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">30.2. Subscription</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="logical-replication-publication.html" title="30.1. Publication">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="logical-replication.html" title="Chapter 30. Logical Replication">Up</a></td><th width="60%" align="center">Chapter 30. Logical Replication</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="logical-replication-conflicts.html" title="30.3. Conflicts">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="LOGICAL-REPLICATION-SUBSCRIPTION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">30.2. 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">30.2.1. Replication Slot Management</a></span></dt></dl></div><p>

  3.    A <em class="firstterm">subscription</em> is the downstream side of logical

  4.    replication.  The node where a subscription is defined is referred to as

  5.    the <em class="firstterm">subscriber</em>.  A subscription defines the connection

  6.    to another database and set of publications (one or more) to which it wants

  7.    to subscribe.

  8.   </p><p>

  9.    The subscriber database behaves in the same way as any other PostgreSQL

  10.    instance and can be used as a publisher for other databases by defining its

  11.    own publications.

  12.   </p><p>

  13.    A subscriber node may have multiple subscriptions if desired.  It is

  14.    possible to define multiple subscriptions between a single

  15.    publisher-subscriber pair, in which case care must be taken to ensure

  16.    that the subscribed publication objects don't overlap.

  17.   </p><p>

  18.    Each subscription will receive changes via one replication slot (see

  19.    <a class="xref" href="warm-standby.html#STREAMING-REPLICATION-SLOTS" title="26.2.6. Replication Slots">Section 26.2.6</a>).  Additional temporary

  20.    replication slots may be required for the initial data synchronization

  21.    of pre-existing table data.

  22.   </p><p>

  23.    A logical replication subscription can be a standby for synchronous

  24.    replication (see <a class="xref" href="warm-standby.html#SYNCHRONOUS-REPLICATION" title="26.2.8. Synchronous Replication">Section 26.2.8</a>).  The standby

  25.    name is by default the subscription name.  An alternative name can be

  26.    specified as <code class="literal">application_name</code> in the connection

  27.    information of the subscription.

  28.   </p><p>

  29.    Subscriptions are dumped by <code class="command">pg_dump</code> if the current user

  30.    is a superuser.  Otherwise a warning is written and subscriptions are

  31.    skipped, because non-superusers cannot read all subscription information

  32.    from the <code class="structname">pg_subscription</code> catalog.

  33.   </p><p>

  34.    The subscription is added using <a class="xref" href="sql-createsubscription.html" title="CREATE SUBSCRIPTION"><span class="refentrytitle">CREATE SUBSCRIPTION</span></a> and

  35.    can be stopped/resumed at any time using the

  36.    <a class="xref" href="sql-altersubscription.html" title="ALTER SUBSCRIPTION"><span class="refentrytitle">ALTER SUBSCRIPTION</span></a> command and removed using

  37.    <a class="xref" href="sql-dropsubscription.html" title="DROP SUBSCRIPTION"><span class="refentrytitle">DROP SUBSCRIPTION</span></a>.

  38.   </p><p>

  39.    When a subscription is dropped and recreated, the synchronization

  40.    information is lost.  This means that the data has to be resynchronized

  41.    afterwards.

  42.   </p><p>

  43.    The schema definitions are not replicated, and the published tables must

  44.    exist on the subscriber.  Only regular tables may be

  45.    the target of replication.  For example, you can't replicate to a view.

  46.   </p><p>

  47.    The tables are matched between the publisher and the subscriber using the

  48.    fully qualified table name.  Replication to differently-named tables on the

  49.    subscriber is not supported.

  50.   </p><p>

  51.    Columns of a table are also matched by name.  The order of columns in the

  52.    subscriber table does not need to match that of the publisher.  The data

  53.    types of the columns do not need to match, as long as the text

  54.    representation of the data can be converted to the target type.  For

  55.    example, you can replicate from a column of type <code class="type">integer</code> to a

  56.    column of type <code class="type">bigint</code>.  The target table can also have

  57.    additional columns not provided by the published table.  Any such columns

  58.    will be filled with the default value as specified in the definition of the

  59.    target table.

  60.   </p><div class="sect2" id="LOGICAL-REPLICATION-SUBSCRIPTION-SLOT"><div class="titlepage"><div><div><h3 class="title">30.2.1. Replication Slot Management</h3></div></div></div><p>

  61.     As mentioned earlier, each (active) subscription receives changes from a

  62.     replication slot on the remote (publishing) side.  Normally, the remote

  63.     replication slot is created automatically when the subscription is created

  64.     using <code class="command">CREATE SUBSCRIPTION</code> and it is dropped

  65.     automatically when the subscription is dropped using <code class="command">DROP

  66.     SUBSCRIPTION</code>.  In some situations, however, it can be useful or

  67.     necessary to manipulate the subscription and the underlying replication

  68.     slot separately.  Here are some scenarios:

  69. 

  70.     </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>

  71.        When creating a subscription, the replication slot already exists.  In

  72.        that case, the subscription can be created using

  73.        the <code class="literal">create_slot = false</code> option to associate with the

  74.        existing slot.

  75.       </p></li><li class="listitem"><p>

  76.        When creating a subscription, the remote host is not reachable or in an

  77.        unclear state.  In that case, the subscription can be created using

  78.        the <code class="literal">connect = false</code> option.  The remote host will then not

  79.        be contacted at all.  This is what <span class="application">pg_dump</span>

  80.        uses.  The remote replication slot will then have to be created

  81.        manually before the subscription can be activated.

  82.       </p></li><li class="listitem"><p>

  83.        When dropping a subscription, the replication slot should be kept.

  84.        This could be useful when the subscriber database is being moved to a

  85.        different host and will be activated from there.  In that case,

  86.        disassociate the slot from the subscription using <code class="command">ALTER

  87.        SUBSCRIPTION</code> before attempting to drop the subscription.

  88.       </p></li><li class="listitem"><p>

  89.        When dropping a subscription, the remote host is not reachable.  In

  90.        that case, disassociate the slot from the subscription

  91.        using <code class="command">ALTER SUBSCRIPTION</code> before attempting to drop

  92.        the subscription.  If the remote database instance no longer exists, no

  93.        further action is then necessary.  If, however, the remote database

  94.        instance is just unreachable, the replication slot should then be

  95.        dropped manually; otherwise it would continue to reserve WAL and might

  96.        eventually cause the disk to fill up.  Such cases should be carefully

  97.        investigated.

  98.       </p></li></ul></div><p>

  99.    </p></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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="logical-replication.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="logical-replication-conflicts.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">30.1. Publication </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 30.3. Conflicts</td></tr></table></div></body></html>
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