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CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="RUNTIME-CONFIG-REPLICATION"
>18.6. Replication</A
></H1
><P
> These settings control the behavior of the built-in
<I
CLASS="FIRSTTERM"
>streaming replication</I
> feature (see
<A
HREF="warm-standby.html#STREAMING-REPLICATION"
>Section 25.2.5</A
>). Servers will be either a
Master or a Standby server. Masters can send data, while Standby(s)
are always receivers of replicated data. When cascading replication
(see <A
HREF="warm-standby.html#CASCADING-REPLICATION"
>Section 25.2.6</A
>) is used, Standby server(s)
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"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-REPLICATION-SENDER"
>18.6.1. Sending Server(s)</A
></H2
><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
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-MAX-WAL-SENDERS"
></A
><TT
CLASS="VARNAME"
>max_wal_senders</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</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 zero, meaning replication is
disabled. WAL sender processes count towards the total number
of connections, so the parameter cannot be set higher than
<A
HREF="runtime-config-connection.html#GUC-MAX-CONNECTIONS"
>max_connections</A
>. This parameter can only
be set at server start. <TT
CLASS="VARNAME"
>wal_level</TT
> must be set
to <TT
CLASS="LITERAL"
>archive</TT
> or <TT
CLASS="LITERAL"
>hot_standby</TT
> to allow
connections from standby servers.
</P
></DD
><DT
><A
NAME="GUC-WAL-KEEP-SEGMENTS"
></A
><TT
CLASS="VARNAME"
>wal_keep_segments</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> Specifies the minimum number of past log file segments kept in the
<TT
CLASS="FILENAME"
>pg_xlog</TT
>
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
<TT
CLASS="VARNAME"
>wal_keep_segments</TT
> 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
<TT
CLASS="FILENAME"
>pg_xlog</TT
>; the system might need to retain more segments
for WAL archival or to recover from a checkpoint. If
<TT
CLASS="VARNAME"
>wal_keep_segments</TT
> 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
<TT
CLASS="FILENAME"
>postgresql.conf</TT
> file or on the server command line.
</P
></DD
><DT
><A
NAME="GUC-REPLICATION-TIMEOUT"
></A
><TT
CLASS="VARNAME"
>replication_timeout</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</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 <TT
CLASS="FILENAME"
>postgresql.conf</TT
> file or on the server command line.
The default value is 60 seconds.
</P
><P
> To prevent connections from being terminated prematurely,
<A
HREF="runtime-config-replication.html#GUC-WAL-RECEIVER-STATUS-INTERVAL"
>wal_receiver_status_interval</A
>
must be enabled on the standby, and its value must be less than the
value of <TT
CLASS="VARNAME"
>replication_timeout</TT
>.
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-REPLICATION-MASTER"
>18.6.2. Master Server</A
></H2
><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
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
HREF="runtime-config-wal.html#RUNTIME-CONFIG-WAL-ARCHIVING"
>Section 18.5.3</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
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-SYNCHRONOUS-STANDBY-NAMES"
></A
><TT
CLASS="VARNAME"
>synchronous_standby_names</TT
> (<TT
CLASS="TYPE"
>string</TT
>)</DT
><DD
><P
> Specifies a comma-separated list of standby names that can support
<I
CLASS="FIRSTTERM"
>synchronous replication</I
>, as described in
<A
HREF="warm-standby.html#SYNCHRONOUS-REPLICATION"
>Section 25.2.7</A
>.
At any one time there will be at most one active synchronous standby;
transactions waiting for commit will be allowed to proceed after
this standby server confirms receipt of their data.
The synchronous standby will be the first standby named in this list
that is both currently connected and streaming data in real-time
(as shown by a state of <TT
CLASS="LITERAL"
>streaming</TT
> in the
<A
HREF="monitoring-stats.html#MONITORING-STATS-VIEWS-TABLE"
> <TT
CLASS="LITERAL"
>pg_stat_replication</TT
></A
> view).
Other standby servers appearing later in this list represent potential
synchronous standbys.
If the current synchronous standby disconnects for whatever reason,
it will be replaced immediately with the next-highest-priority standby.
Specifying more than one standby name can allow very high availability.
</P
><P
> The name of a standby server for this purpose is the
<TT
CLASS="VARNAME"
>application_name</TT
> setting of the standby, as set in the
<TT
CLASS="VARNAME"
>primary_conninfo</TT
> of the standby's walreceiver. There is
no mechanism to enforce uniqueness. In case of duplicates one of the
matching standbys will be chosen to be the synchronous standby, though
exactly which one is indeterminate.
The special entry <TT
CLASS="LITERAL"
>*</TT
> matches any
<TT
CLASS="VARNAME"
>application_name</TT
>, including the default application name
of <TT
CLASS="LITERAL"
>walreceiver</TT
>.
</P
><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
HREF="runtime-config-wal.html#GUC-SYNCHRONOUS-COMMIT"
>synchronous_commit</A
> parameter to
<TT
CLASS="LITERAL"
>local</TT
> or <TT
CLASS="LITERAL"
>off</TT
>.
</P
><P
> This parameter can only be set in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
file or on the server command line.
</P
></DD
><DT
><A
NAME="GUC-VACUUM-DEFER-CLEANUP-AGE"
></A
><TT
CLASS="VARNAME"
>vacuum_defer_cleanup_age</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</DT
><DD
><P
> Specifies the number of transactions by which <TT
CLASS="COMMAND"
>VACUUM</TT
> 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
HREF="hot-standby.html"
>Section 25.5</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 <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
file or on the server command line.
</P
><P
> You should also consider setting <TT
CLASS="VARNAME"
>hot_standby_feedback</TT
>
on standby server(s) as an alternative to using this parameter.
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-REPLICATION-STANDBY"
>18.6.3. Standby Servers</A
></H2
><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
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-HOT-STANDBY"
></A
><TT
CLASS="VARNAME"
>hot_standby</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
> Specifies whether or not you can connect and run queries during
recovery, as described in <A
HREF="hot-standby.html"
>Section 25.5</A
>.
The default value is <TT
CLASS="LITERAL"
>off</TT
>.
This parameter can only be set at server start. It only has effect
during archive recovery or in standby mode.
</P
></DD
><DT
><A
NAME="GUC-MAX-STANDBY-ARCHIVE-DELAY"
></A
><TT
CLASS="VARNAME"
>max_standby_archive_delay</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</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
HREF="hot-standby.html#HOT-STANDBY-CONFLICT"
>Section 25.5.2</A
>.
<TT
CLASS="VARNAME"
>max_standby_archive_delay</TT
> 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 <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
file or on the server command line.
</P
><P
> Note that <TT
CLASS="VARNAME"
>max_standby_archive_delay</TT
> 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
><A
NAME="GUC-MAX-STANDBY-STREAMING-DELAY"
></A
><TT
CLASS="VARNAME"
>max_standby_streaming_delay</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</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
HREF="hot-standby.html#HOT-STANDBY-CONFLICT"
>Section 25.5.2</A
>.
<TT
CLASS="VARNAME"
>max_standby_streaming_delay</TT
> 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 <TT
CLASS="FILENAME"
>postgresql.conf</TT
>
file or on the server command line.
</P
><P
> Note that <TT
CLASS="VARNAME"
>max_standby_streaming_delay</TT
> 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
><A
NAME="GUC-WAL-RECEIVER-STATUS-INTERVAL"
></A
><TT
CLASS="VARNAME"
>wal_receiver_status_interval</TT
> (<TT
CLASS="TYPE"
>integer</TT
>)</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
HREF="monitoring-stats.html#MONITORING-STATS-VIEWS-TABLE"
> <TT
CLASS="LITERAL"
>pg_stat_replication</TT
></A
> view. The standby will report
the last transaction log position 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 <TT
CLASS="FILENAME"
>postgresql.conf</TT
> file or on the server command line.
The default value is 10 seconds.
</P
><P
> When <A
HREF="runtime-config-replication.html#GUC-REPLICATION-TIMEOUT"
>replication_timeout</A
> is enabled on a sending server,
<TT
CLASS="VARNAME"
>wal_receiver_status_interval</TT
> must be enabled, and its value
must be less than the value of <TT
CLASS="VARNAME"
>replication_timeout</TT
>.
</P
></DD
><DT
><A
NAME="GUC-HOT-STANDBY-FEEDBACK"
></A
><TT
CLASS="VARNAME"
>hot_standby_feedback</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</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
<TT
CLASS="VARNAME"
>wal_receiver_status_interval</TT
>. The default value is
<TT
CLASS="LITERAL"
>off</TT
>. This parameter can only be set in the
<TT
CLASS="FILENAME"
>postgresql.conf</TT
> 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
></DD
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