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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>19.6. Replication</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="runtime-config-wal.html" title="19.5. Write Ahead Log" /><link rel="next" href="runtime-config-query.html" title="19.7. Query Planning" /></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">19.6. Replication</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="runtime-config-wal.html" title="19.5. Write Ahead Log">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="runtime-config.html" title="Chapter 19. Server Configuration">Up</a></td><th width="60%" align="center">Chapter 19. Server Configuration</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="runtime-config-query.html" title="19.7. Query Planning">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="RUNTIME-CONFIG-REPLICATION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">19.6. Replication</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="runtime-config-replication.html#RUNTIME-CONFIG-REPLICATION-SENDER">19.6.1. Sending Servers</a></span></dt><dt><span class="sect2"><a href="runtime-config-replication.html#RUNTIME-CONFIG-REPLICATION-MASTER">19.6.2. Master Server</a></span></dt><dt><span class="sect2"><a href="runtime-config-replication.html#RUNTIME-CONFIG-REPLICATION-STANDBY">19.6.3. Standby Servers</a></span></dt><dt><span class="sect2"><a href="runtime-config-replication.html#RUNTIME-CONFIG-REPLICATION-SUBSCRIBER">19.6.4. Subscribers</a></span></dt></dl></div><p>
These settings control the behavior of the built-in
<em class="firstterm">streaming replication</em> feature (see
<a class="xref" href="warm-standby.html#STREAMING-REPLICATION" title="26.2.5. Streaming Replication">Section 26.2.5</a>). Servers will be either a
master or a standby server. Masters can send data, while standbys
are always receivers of replicated data. When cascading replication
(see <a class="xref" href="warm-standby.html#CASCADING-REPLICATION" title="26.2.7. Cascading Replication">Section 26.2.7</a>) is used, standby servers
can also be senders, as well as receivers.
Parameters are mainly for sending and standby servers, though some
parameters have meaning only on the master server. Settings may vary
across the cluster without problems if that is required.
</p><div class="sect2" id="RUNTIME-CONFIG-REPLICATION-SENDER"><div class="titlepage"><div><div><h3 class="title">19.6.1. Sending Servers</h3></div></div></div><p>
These parameters can be set on any server that is
to send replication data to one or more standby servers.
The master is always a sending server, so these parameters must
always be set on the master.
The role and meaning of these parameters does not change after a
standby becomes the master.
</p><div class="variablelist"><dl class="variablelist"><dt id="GUC-MAX-WAL-SENDERS"><span class="term"><code class="varname">max_wal_senders</code> (<code class="type">integer</code>)
<a id="id-1.6.6.9.3.3.1.1.3" class="indexterm"></a>
</span></dt><dd><p>
Specifies the maximum number of concurrent connections from
standby servers or streaming base backup clients (i.e., the
maximum number of simultaneously running WAL sender
processes). The default is 10. The value 0 means replication is
disabled. WAL sender processes count towards the total number
of connections, so this parameter's value must be less than
<a class="xref" href="runtime-config-connection.html#GUC-MAX-CONNECTIONS">max_connections</a> minus
<a class="xref" href="runtime-config-connection.html#GUC-SUPERUSER-RESERVED-CONNECTIONS">superuser_reserved_connections</a>.
Abrupt streaming client disconnection might leave an orphaned
connection slot behind until
a timeout is reached, so this parameter should be set slightly
higher than the maximum number of expected clients so disconnected
clients can immediately reconnect. This parameter can only
be set at server start.
Also, <code class="varname">wal_level</code> must be set to
<code class="literal">replica</code> or higher to allow connections from standby
servers.
</p></dd><dt id="GUC-MAX-REPLICATION-SLOTS"><span class="term"><code class="varname">max_replication_slots</code> (<code class="type">integer</code>)
<a id="id-1.6.6.9.3.3.2.1.3" class="indexterm"></a>
</span></dt><dd><p>
Specifies the maximum number of replication slots
(see <a class="xref" href="warm-standby.html#STREAMING-REPLICATION-SLOTS" title="26.2.6. Replication Slots">Section 26.2.6</a>) that the server
can support. The default is 10. This parameter can only be set at
server start.
Setting it to a lower value than the number of currently
existing replication slots will prevent the server from starting.
Also, <code class="varname">wal_level</code> must be set
to <code class="literal">replica</code> or higher to allow replication slots to
be used.
</p></dd><dt id="GUC-WAL-KEEP-SEGMENTS"><span class="term"><code class="varname">wal_keep_segments</code> (<code class="type">integer</code>)
<a id="id-1.6.6.9.3.3.3.1.3" class="indexterm"></a>
</span></dt><dd><p>
Specifies the minimum number of past log file segments kept in the
<code class="filename">pg_wal</code>
directory, in case a standby server needs to fetch them for streaming
replication. Each segment is normally 16 megabytes. If a standby
server connected to the sending server falls behind by more than
<code class="varname">wal_keep_segments</code> segments, the sending server might remove
a WAL segment still needed by the standby, in which case the
replication connection will be terminated. Downstream connections
will also eventually fail as a result. (However, the standby
server can recover by fetching the segment from archive, if WAL
archiving is in use.)
</p><p>
This sets only the minimum number of segments retained in
<code class="filename">pg_wal</code>; the system might need to retain more segments
for WAL archival or to recover from a checkpoint. If
<code class="varname">wal_keep_segments</code> is zero (the default), the system
doesn't keep any extra segments for standby purposes, so the number
of old WAL segments available to standby servers is a function of
the location of the previous checkpoint and status of WAL
archiving.
This parameter can only be set in the
<code class="filename">postgresql.conf</code> file or on the server command line.
</p></dd><dt id="GUC-WAL-SENDER-TIMEOUT"><span class="term"><code class="varname">wal_sender_timeout</code> (<code class="type">integer</code>)
<a id="id-1.6.6.9.3.3.4.1.3" class="indexterm"></a>
</span></dt><dd><p>
Terminate replication connections that are inactive longer
than the specified number of milliseconds. This is useful for
the sending server to detect a standby crash or network outage.
A value of zero disables the timeout mechanism. This parameter
can only be set in
the <code class="filename">postgresql.conf</code> file or on the server command line.
The default value is 60 seconds.
</p></dd><dt id="GUC-TRACK-COMMIT-TIMESTAMP"><span class="term"><code class="varname">track_commit_timestamp</code> (<code class="type">boolean</code>)
<a id="id-1.6.6.9.3.3.5.1.3" class="indexterm"></a>
</span></dt><dd><p>
Record commit time of transactions. This parameter
can only be set in <code class="filename">postgresql.conf</code> file or on the server
command line. The default value is <code class="literal">off</code>.
</p></dd></dl></div></div><div class="sect2" id="RUNTIME-CONFIG-REPLICATION-MASTER"><div class="titlepage"><div><div><h3 class="title">19.6.2. Master Server</h3></div></div></div><p>
These parameters can be set on the master/primary server that is
to send replication data to one or more standby servers.
Note that in addition to these parameters,
<a class="xref" href="runtime-config-wal.html#GUC-WAL-LEVEL">wal_level</a> must be set appropriately on the master
server, and optionally WAL archiving can be enabled as
well (see <a class="xref" href="runtime-config-wal.html#RUNTIME-CONFIG-WAL-ARCHIVING" title="19.5.3. Archiving">Section 19.5.3</a>).
The values of these parameters on standby servers are irrelevant,
although you may wish to set them there in preparation for the
possibility of a standby becoming the master.
</p><div class="variablelist"><dl class="variablelist"><dt id="GUC-SYNCHRONOUS-STANDBY-NAMES"><span class="term"><code class="varname">synchronous_standby_names</code> (<code class="type">string</code>)
<a id="id-1.6.6.9.4.3.1.1.3" class="indexterm"></a>
</span></dt><dd><p>
Specifies a list of standby servers that can support
<em class="firstterm">synchronous replication</em>, as described in
<a class="xref" href="warm-standby.html#SYNCHRONOUS-REPLICATION" title="26.2.8. Synchronous Replication">Section 26.2.8</a>.
There will be one or more active synchronous standbys;
transactions waiting for commit will be allowed to proceed after
these standby servers confirm receipt of their data.
The synchronous standbys will be those whose names appear
in this list, and
that are both currently connected and streaming data in real-time
(as shown by a state of <code class="literal">streaming</code> in the <a class="link" href="monitoring-stats.html#PG-STAT-REPLICATION-VIEW" title="Table 28.5. pg_stat_replication View"><code class="literal">pg_stat_replication</code></a>
view).
Specifying more than one synchronous standby can allow for very high
availability and protection against data loss.
</p><p>
The name of a standby server for this purpose is the
<code class="varname">application_name</code> setting of the standby, as set in the
standby's connection information. In case of a physical replication
standby, this should be set in the <code class="varname">primary_conninfo</code>
setting in <code class="filename">recovery.conf</code>; the default
is <code class="literal">walreceiver</code>. For logical replication, this can
be set in the connection information of the subscription, and it
defaults to the subscription name. For other replication stream
consumers, consult their documentation.
</p><p>
This parameter specifies a list of standby servers using
either of the following syntaxes:
</p><pre class="synopsis">
[FIRST] <em class="replaceable"><code>num_sync</code></em> ( <em class="replaceable"><code>standby_name</code></em> [, ...] )
ANY <em class="replaceable"><code>num_sync</code></em> ( <em class="replaceable"><code>standby_name</code></em> [, ...] )
<em class="replaceable"><code>standby_name</code></em> [, ...]
</pre><p>
where <em class="replaceable"><code>num_sync</code></em> is
the number of synchronous standbys that transactions need to
wait for replies from,
and <em class="replaceable"><code>standby_name</code></em>
is the name of a standby server.
<code class="literal">FIRST</code> and <code class="literal">ANY</code> specify the method to choose
synchronous standbys from the listed servers.
</p><p>
The keyword <code class="literal">FIRST</code>, coupled with
<em class="replaceable"><code>num_sync</code></em>, specifies a
priority-based synchronous replication and makes transaction commits
wait until their WAL records are replicated to
<em class="replaceable"><code>num_sync</code></em> synchronous
standbys chosen based on their priorities. For example, a setting of
<code class="literal">FIRST 3 (s1, s2, s3, s4)</code> will cause each commit to wait for
replies from three higher-priority standbys chosen from standby servers
<code class="literal">s1</code>, <code class="literal">s2</code>, <code class="literal">s3</code> and <code class="literal">s4</code>.
The standbys whose names appear earlier in the list are given higher
priority and will be considered as synchronous. Other standby servers
appearing later in this list represent potential synchronous standbys.
If any of the current synchronous standbys disconnects for whatever
reason, it will be replaced immediately with the next-highest-priority
standby. The keyword <code class="literal">FIRST</code> is optional.
</p><p>
The keyword <code class="literal">ANY</code>, coupled with
<em class="replaceable"><code>num_sync</code></em>, specifies a
quorum-based synchronous replication and makes transaction commits
wait until their WAL records are replicated to <span class="emphasis"><em>at least</em></span>
<em class="replaceable"><code>num_sync</code></em> listed standbys.
For example, a setting of <code class="literal">ANY 3 (s1, s2, s3, s4)</code> will cause
each commit to proceed as soon as at least any three standbys of
<code class="literal">s1</code>, <code class="literal">s2</code>, <code class="literal">s3</code> and <code class="literal">s4</code>
reply.
</p><p>
<code class="literal">FIRST</code> and <code class="literal">ANY</code> are case-insensitive. If these
keywords are used as the name of a standby server,
its <em class="replaceable"><code>standby_name</code></em> must
be double-quoted.
</p><p>
The third syntax was used before <span class="productname">PostgreSQL</span>
version 9.6 and is still supported. It's the same as the first syntax
with <code class="literal">FIRST</code> and
<em class="replaceable"><code>num_sync</code></em> equal to 1.
For example, <code class="literal">FIRST 1 (s1, s2)</code> and <code class="literal">s1, s2</code> have
the same meaning: either <code class="literal">s1</code> or <code class="literal">s2</code> is chosen
as a synchronous standby.
</p><p>
The special entry <code class="literal">*</code> matches any standby name.
</p><p>
There is no mechanism to enforce uniqueness of standby names. In case
of duplicates one of the matching standbys will be considered as
higher priority, though exactly which one is indeterminate.
</p><div class="note"><h3 class="title">Note</h3><p>
Each <em class="replaceable"><code>standby_name</code></em>
should have the form of a valid SQL identifier, unless it
is <code class="literal">*</code>. You can use double-quoting if necessary. But note
that <em class="replaceable"><code>standby_name</code></em>s are
compared to standby application names case-insensitively, whether
double-quoted or not.
</p></div><p>
If no synchronous standby names are specified here, then synchronous
replication is not enabled and transaction commits will not wait for
replication. This is the default configuration. Even when
synchronous replication is enabled, individual transactions can be
configured not to wait for replication by setting the
<a class="xref" href="runtime-config-wal.html#GUC-SYNCHRONOUS-COMMIT">synchronous_commit</a> parameter to
<code class="literal">local</code> or <code class="literal">off</code>.
</p><p>
This parameter can only be set in the <code class="filename">postgresql.conf</code>
file or on the server command line.
</p></dd><dt id="GUC-VACUUM-DEFER-CLEANUP-AGE"><span class="term"><code class="varname">vacuum_defer_cleanup_age</code> (<code class="type">integer</code>)
<a id="id-1.6.6.9.4.3.2.1.3" class="indexterm"></a>
</span></dt><dd><p>
Specifies the number of transactions by which <code class="command">VACUUM</code> and
<acronym class="acronym">HOT</acronym> updates will defer cleanup of dead row versions. The
default is zero transactions, meaning that dead row versions can be
removed as soon as possible, that is, as soon as they are no longer
visible to any open transaction. You may wish to set this to a
non-zero value on a primary server that is supporting hot standby
servers, as described in <a class="xref" href="hot-standby.html" title="26.5. Hot Standby">Section 26.5</a>. This allows
more time for queries on the standby to complete without incurring
conflicts due to early cleanup of rows. However, since the value
is measured in terms of number of write transactions occurring on the
primary server, it is difficult to predict just how much additional
grace time will be made available to standby queries.
This parameter can only be set in the <code class="filename">postgresql.conf</code>
file or on the server command line.
</p><p>
You should also consider setting <code class="varname">hot_standby_feedback</code>
on standby server(s) as an alternative to using this parameter.
</p><p>
This does not prevent cleanup of dead rows which have reached the age
specified by <code class="varname">old_snapshot_threshold</code>.
</p></dd></dl></div></div><div class="sect2" id="RUNTIME-CONFIG-REPLICATION-STANDBY"><div class="titlepage"><div><div><h3 class="title">19.6.3. Standby Servers</h3></div></div></div><p>
These settings control the behavior of a standby server that is
to receive replication data. Their values on the master server
are irrelevant.
</p><div class="variablelist"><dl class="variablelist"><dt id="GUC-HOT-STANDBY"><span class="term"><code class="varname">hot_standby</code> (<code class="type">boolean</code>)
<a id="id-1.6.6.9.5.3.1.1.3" class="indexterm"></a>
</span></dt><dd><p>
Specifies whether or not you can connect and run queries during
recovery, as described in <a class="xref" href="hot-standby.html" title="26.5. Hot Standby">Section 26.5</a>.
The default value is <code class="literal">on</code>.
This parameter can only be set at server start. It only has effect
during archive recovery or in standby mode.
</p></dd><dt id="GUC-MAX-STANDBY-ARCHIVE-DELAY"><span class="term"><code class="varname">max_standby_archive_delay</code> (<code class="type">integer</code>)
<a id="id-1.6.6.9.5.3.2.1.3" class="indexterm"></a>
</span></dt><dd><p>
When Hot Standby is active, this parameter determines how long the
standby server should wait before canceling standby queries that
conflict with about-to-be-applied WAL entries, as described in
<a class="xref" href="hot-standby.html#HOT-STANDBY-CONFLICT" title="26.5.2. Handling Query Conflicts">Section 26.5.2</a>.
<code class="varname">max_standby_archive_delay</code> applies when WAL data is
being read from WAL archive (and is therefore not current).
The default is 30 seconds. Units are milliseconds if not specified.
A value of -1 allows the standby to wait forever for conflicting
queries to complete.
This parameter can only be set in the <code class="filename">postgresql.conf</code>
file or on the server command line.
</p><p>
Note that <code class="varname">max_standby_archive_delay</code> is not the same as the
maximum length of time a query can run before cancellation; rather it
is the maximum total time allowed to apply any one WAL segment's data.
Thus, if one query has resulted in significant delay earlier in the
WAL segment, subsequent conflicting queries will have much less grace
time.
</p></dd><dt id="GUC-MAX-STANDBY-STREAMING-DELAY"><span class="term"><code class="varname">max_standby_streaming_delay</code> (<code class="type">integer</code>)
<a id="id-1.6.6.9.5.3.3.1.3" class="indexterm"></a>
</span></dt><dd><p>
When Hot Standby is active, this parameter determines how long the
standby server should wait before canceling standby queries that
conflict with about-to-be-applied WAL entries, as described in
<a class="xref" href="hot-standby.html#HOT-STANDBY-CONFLICT" title="26.5.2. Handling Query Conflicts">Section 26.5.2</a>.
<code class="varname">max_standby_streaming_delay</code> applies when WAL data is
being received via streaming replication.
The default is 30 seconds. Units are milliseconds if not specified.
A value of -1 allows the standby to wait forever for conflicting
queries to complete.
This parameter can only be set in the <code class="filename">postgresql.conf</code>
file or on the server command line.
</p><p>
Note that <code class="varname">max_standby_streaming_delay</code> is not the same as
the maximum length of time a query can run before cancellation; rather
it is the maximum total time allowed to apply WAL data once it has
been received from the primary server. Thus, if one query has
resulted in significant delay, subsequent conflicting queries will
have much less grace time until the standby server has caught up
again.
</p></dd><dt id="GUC-WAL-RECEIVER-STATUS-INTERVAL"><span class="term"><code class="varname">wal_receiver_status_interval</code> (<code class="type">integer</code>)
<a id="id-1.6.6.9.5.3.4.1.3" class="indexterm"></a>
</span></dt><dd><p>
Specifies the minimum frequency for the WAL receiver
process on the standby to send information about replication progress
to the primary or upstream standby, where it can be seen using the
<a class="link" href="monitoring-stats.html#PG-STAT-REPLICATION-VIEW" title="Table 28.5. pg_stat_replication View"><code class="literal">pg_stat_replication</code></a>
view. The standby will report
the last write-ahead log location it has written, the last position it
has flushed to disk, and the last position it has applied.
This parameter's
value is the maximum interval, in seconds, between reports. Updates are
sent each time the write or flush positions change, or at least as
often as specified by this parameter. Thus, the apply position may
lag slightly behind the true position. Setting this parameter to zero
disables status updates completely. This parameter can only be set in
the <code class="filename">postgresql.conf</code> file or on the server command line.
The default value is 10 seconds.
</p></dd><dt id="GUC-HOT-STANDBY-FEEDBACK"><span class="term"><code class="varname">hot_standby_feedback</code> (<code class="type">boolean</code>)
<a id="id-1.6.6.9.5.3.5.1.3" class="indexterm"></a>
</span></dt><dd><p>
Specifies whether or not a hot standby will send feedback to the primary
or upstream standby
about queries currently executing on the standby. This parameter can
be used to eliminate query cancels caused by cleanup records, but
can cause database bloat on the primary for some workloads.
Feedback messages will not be sent more frequently than once per
<code class="varname">wal_receiver_status_interval</code>. The default value is
<code class="literal">off</code>. This parameter can only be set in the
<code class="filename">postgresql.conf</code> file or on the server command line.
</p><p>
If cascaded replication is in use the feedback is passed upstream
until it eventually reaches the primary. Standbys make no other use
of feedback they receive other than to pass upstream.
</p><p>
This setting does not override the behavior of
<code class="varname">old_snapshot_threshold</code> on the primary; a snapshot on the
standby which exceeds the primary's age threshold can become invalid,
resulting in cancellation of transactions on the standby. This is
because <code class="varname">old_snapshot_threshold</code> is intended to provide an
absolute limit on the time which dead rows can contribute to bloat,
which would otherwise be violated because of the configuration of a
standby.
</p></dd><dt id="GUC-WAL-RECEIVER-TIMEOUT"><span class="term"><code class="varname">wal_receiver_timeout</code> (<code class="type">integer</code>)
<a id="id-1.6.6.9.5.3.6.1.3" class="indexterm"></a>
</span></dt><dd><p>
Terminate replication connections that are inactive longer
than the specified number of milliseconds. This is useful for
the receiving standby server to detect a primary node crash or network
outage.
A value of zero disables the timeout mechanism. This parameter
can only be set in
the <code class="filename">postgresql.conf</code> file or on the server command line.
The default value is 60 seconds.
</p></dd><dt id="GUC-WAL-RETRIEVE-RETRY-INTERVAL"><span class="term"><code class="varname">wal_retrieve_retry_interval</code> (<code class="type">integer</code>)
<a id="id-1.6.6.9.5.3.7.1.3" class="indexterm"></a>
</span></dt><dd><p>
Specify how long the standby server should wait when WAL data is not
available from any sources (streaming replication,
local <code class="filename">pg_wal</code> or WAL archive) before retrying to
retrieve WAL data. This parameter can only be set in the
<code class="filename">postgresql.conf</code> file or on the server command line.
The default value is 5 seconds. Units are milliseconds if not specified.
</p><p>
This parameter is useful in configurations where a node in recovery
needs to control the amount of time to wait for new WAL data to be
available. For example, in archive recovery, it is possible to
make the recovery more responsive in the detection of a new WAL
log file by reducing the value of this parameter. On a system with
low WAL activity, increasing it reduces the amount of requests necessary
to access WAL archives, something useful for example in cloud
environments where the amount of times an infrastructure is accessed
is taken into account.
</p></dd></dl></div></div><div class="sect2" id="RUNTIME-CONFIG-REPLICATION-SUBSCRIBER"><div class="titlepage"><div><div><h3 class="title">19.6.4. Subscribers</h3></div></div></div><p>
These settings control the behavior of a logical replication subscriber.
Their values on the publisher are irrelevant.
</p><p>
Note that <code class="varname">wal_receiver_timeout</code>,
<code class="varname">wal_receiver_status_interval</code> and
<code class="varname">wal_retrieve_retry_interval</code> configuration parameters
affect the logical replication workers as well.
</p><div class="variablelist"><dl class="variablelist"><dt id="GUC-MAX-LOGICAL-REPLICATION-WORKERS"><span class="term"><code class="varname">max_logical_replication_workers</code> (<code class="type">int</code>)
<a id="id-1.6.6.9.6.4.1.1.3" class="indexterm"></a>
</span></dt><dd><p>
Specifies maximum number of logical replication workers. This includes
both apply workers and table synchronization workers.
</p><p>
Logical replication workers are taken from the pool defined by
<code class="varname">max_worker_processes</code>.
</p><p>
The default value is 4.
</p></dd><dt id="GUC-MAX-SYNC-WORKERS-PER-SUBSCRIPTION"><span class="term"><code class="varname">max_sync_workers_per_subscription</code> (<code class="type">integer</code>)
<a id="id-1.6.6.9.6.4.2.1.3" class="indexterm"></a>
</span></dt><dd><p>
Maximum number of synchronization workers per subscription. This
parameter controls the amount of parallelism of the initial data copy
during the subscription initialization or when new tables are added.
</p><p>
Currently, there can be only one synchronization worker per table.
</p><p>
The synchronization workers are taken from the pool defined by
<code class="varname">max_logical_replication_workers</code>.
</p><p>
The default value is 2.
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