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repmgr/doc/cloning-standbys.xml
Ian Barwick b74f965f54 standby clone: rename --recovery-conf-only to --replication-conf-only 
A more generic option name to cover pre- and post-Pg12 replication
configuration methods.

--recovery-conf-only is retained as an alias for backwards
compatibility.
2019-10-18 14:44:57 +09:00

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<chapter id="cloning-standbys" xreflabel="cloning standbys">
<title>Cloning standbys</title>
<sect1 id="cloning-from-barman" xreflabel="Cloning from Barman">
<title>Cloning a standby from Barman</title>
<indexterm>
<primary>cloning</primary>
<secondary>from Barman</secondary>
</indexterm>
<indexterm>
<primary>Barman</primary>
<secondary>cloning a standby</secondary>
</indexterm>
<para>
<xref linkend="repmgr-standby-clone"/> can use
<ulink url="https://www.2ndquadrant.com/">2ndQuadrant</ulink>'s
<ulink url="https://www.pgbarman.org/">Barman</ulink> application
to clone a standby (and also as a fallback source for WAL files).
</para>
<tip>
<simpara>
Barman (aka PgBarman) should be considered as an integral part of any
PostgreSQL replication cluster. For more details see:
<ulink url="https://www.pgbarman.org/">https://www.pgbarman.org/</ulink>.
</simpara>
</tip>
<para>
Barman support provides the following advantages:
<itemizedlist spacing="compact" mark="bullet">
<listitem>
<para>
the primary node does not need to perform a new backup every time a
new standby is cloned
</para>
</listitem>
<listitem>
<para>
a standby node can be disconnected for longer periods without losing
the ability to catch up, and without causing accumulation of WAL
files on the primary node
</para>
</listitem>
<listitem>
<para>
WAL management on the primary becomes much easier as there's no need
to use replication slots, and <varname>wal_keep_segments</varname>
does not need to be set.
</para>
</listitem>
</itemizedlist>
</para>
<note>
<para>
Currently &repmgr;'s support for cloning from Barman is implemented by using
<productname>rsync</productname> to clone from the Barman server.
</para>
<para>
It is therefore not able to make use of Barman's parallel restore facility, which
is executed on the Barman server and clones to the target server.
</para>
<para>
Barman's parallel restore facility can be used by executing it manually on
the Barman server and configuring replication on the resulting cloned
standby using
<command><link linkend="repmgr-standby-clone">repmgr standby clone --replication-conf-only</link></command>.
</para>
</note>
<sect2 id="cloning-from-barman-prerequisites">
<title>Prerequisites for cloning from Barman</title>
<para>
In order to enable Barman support for <command>repmgr standby clone</command>, following
prerequisites must be met:
<itemizedlist spacing="compact" mark="bullet">
<listitem>
<para>
the Barman catalogue must include at least one valid backup for this server;
</para>
</listitem>
<listitem>
<para>
the <varname>barman_host</varname> setting in <filename>repmgr.conf</filename> is set to the SSH
hostname of the Barman server;
</para>
</listitem>
<listitem>
<para>
the <varname>barman_server</varname> setting in <filename>repmgr.conf</filename> is the same as the
server configured in Barman.
</para>
</listitem>
</itemizedlist>
</para>
<para>
For example, assuming Barman is located on the host &quot;<literal>barmansrv</literal>&quot;
under the &quot;<literal>barman</literal>&quot; user account,
<filename>repmgr.conf</filename> should contain the following entries:
<programlisting>
barman_host='barman@barmansrv'
barman_server='somedb'</programlisting>
</para>
<note>
<para>
To use a non-default Barman configuration file on the Barman server,
specify this in <filename>repmgr.conf</filename> with <filename>barman_config</filename>:
<programlisting>
barman_config=/path/to/barman.conf</programlisting>
</para>
</note>
<para>
We also recommend configuring the <varname>restore_command</varname> setting in <filename>repmgr.conf</filename>
to use the <command>barman-wal-restore</command> script
(see section <xref linkend="cloning-from-barman-restore-command"/> below).
</para>
<tip>
<simpara>
If you have a non-default SSH configuration on the Barman
server, e.g. using a port other than 22, then you can set those
parameters in a dedicated Host section in <filename>~/.ssh/config</filename>
corresponding to the value of <varname>barman_host</varname> in
<filename>repmgr.conf</filename>. See the <literal>Host</literal>
section in <command>man 5 ssh_config</command> for more details.
</simpara>
</tip>
<para>
It's now possible to clone a standby from Barman, e.g.:
<programlisting>
$ repmgr -f /etc/repmgr.conf -h node1 -U repmgr -d repmgr standby clone
NOTICE: destination directory "/var/lib/postgresql/data" provided
INFO: connecting to Barman server to verify backup for "test_cluster"
INFO: checking and correcting permissions on existing directory "/var/lib/postgresql/data"
INFO: creating directory "/var/lib/postgresql/data/repmgr"...
INFO: connecting to Barman server to fetch server parameters
INFO: connecting to source node
DETAIL: current installation size is 30 MB
NOTICE: retrieving backup from Barman...
(...)
NOTICE: standby clone (from Barman) complete
NOTICE: you can now start your PostgreSQL server
HINT: for example: pg_ctl -D /var/lib/postgresql/data start</programlisting>
</para>
<note>
<simpara>
Barman support is automatically enabled if <varname>barman_server</varname>
is set. Normally it is good practice to use Barman, for instance
when fetching a base backup while cloning a standby; in any case,
Barman mode can be disabled using the <literal>--without-barman</literal>
command line option.
</simpara>
</note>
</sect2>
<sect2 id="cloning-from-barman-restore-command" xreflabel="Using Barman as a WAL file source">
<title>Using Barman as a WAL file source</title>
<indexterm>
<primary>Barman</primary>
<secondary>fetching archived WAL</secondary>
</indexterm>
<para>
As a fallback in case streaming replication is interrupted, PostgreSQL can optionally
retrieve WAL files from an archive, such as that provided by Barman. This is done by
setting <varname>restore_command</varname> in <filename>recovery.conf</filename> to
a valid shell command which can retrieve a specified WAL file from the archive.
</para>
<para>
<command>barman-wal-restore</command> is a Python script provided as part of the <literal>barman-cli</literal>
package (Barman 2.0 ~ 2.7) or as part of the core Barman distribution (Barman 2.8 and later).
</para>
<para>
To use <command>barman-wal-restore</command> with &repmgr;,
assuming Barman is located on the host &quot;<literal>barmansrv</literal>&quot;
under the &quot;<literal>barman</literal>&quot; user account,
and that <command>barman-wal-restore</command> is located as an executable at
<filename>/usr/bin/barman-wal-restore</filename>,
<filename>repmgr.conf</filename> should include the following lines:
<programlisting>
barman_host='barman@barmansrv'
barman_server='somedb'
restore_command='/usr/bin/barman-wal-restore barmansrv somedb %f %p'</programlisting>
</para>
<note>
<simpara>
<command>barman-wal-restore</command> supports command line switches to
control parallelism (<literal>--parallel=N</literal>) and compression
(<literal>--bzip2</literal>, <literal>--gzip</literal>).
</simpara>
</note>
</sect2>
</sect1>
<sect1 id="cloning-replication-slots" xreflabel="Cloning and replication slots">
<title>Cloning and replication slots</title>
<indexterm>
<primary>cloning</primary>
<secondary>replication slots</secondary>
</indexterm>
<indexterm>
<primary>replication slots</primary>
<secondary>cloning</secondary>
</indexterm>
<para>
Replication slots were introduced with PostgreSQL 9.4 and are designed to ensure
that any standby connected to the primary using a replication slot will always
be able to retrieve the required WAL files. This removes the need to manually
manage WAL file retention by estimating the number of WAL files that need to
be maintained on the primary using <varname>wal_keep_segments</varname>.
Do however be aware that if a standby is disconnected, WAL will continue to
accumulate on the primary until either the standby reconnects or the replication
slot is dropped.
</para>
<para>
To enable &repmgr; to use replication slots, set the boolean parameter
<varname>use_replication_slots</varname> in <filename>repmgr.conf</filename>:
<programlisting>
use_replication_slots=true</programlisting>
</para>
<para>
Replication slots must be enabled in <filename>postgresql.conf</filename> by
setting the parameter <varname>max_replication_slots</varname> to at least the
number of expected standbys (changes to this parameter require a server restart).
</para>
<para>
When cloning a standby, &repmgr; will automatically generate an appropriate
slot name, which is stored in the <literal>repmgr.nodes</literal> table, and create the slot
on the upstream node:
<programlisting>
repmgr=# SELECT node_id, upstream_node_id, active, node_name, type, priority, slot_name
FROM repmgr.nodes ORDER BY node_id;
node_id | upstream_node_id | active | node_name | type | priority | slot_name
---------+------------------+--------+-----------+---------+----------+---------------
1 | | t | node1 | primary | 100 | repmgr_slot_1
2 | 1 | t | node2 | standby | 100 | repmgr_slot_2
3 | 1 | t | node3 | standby | 100 | repmgr_slot_3
(3 rows)</programlisting>
<programlisting>
repmgr=# SELECT slot_name, slot_type, active, active_pid FROM pg_replication_slots ;
slot_name | slot_type | active | active_pid
---------------+-----------+--------+------------
repmgr_slot_2 | physical | t | 23658
repmgr_slot_3 | physical | t | 23687
(2 rows)</programlisting>
</para>
<para>
Note that a slot name will be created by default for the primary but not
actually used unless the primary is converted to a standby using e.g.
<command>repmgr standby switchover</command>.
</para>
<para>
Further information on replication slots in the PostgreSQL documentation:
<ulink url="https://www.postgresql.org/docs/current/interactive/warm-standby.html#STREAMING-REPLICATION-SLOTS">https://www.postgresql.org/docs/current/interactive/warm-standby.html#STREAMING-REPLICATION-SLOTS</ulink>
</para>
<tip>
<simpara>
While replication slots can be useful for streaming replication, it's
recommended to monitor for inactive slots as these will cause WAL files to
build up indefinitely, possibly leading to server failure.
</simpara>
<simpara>
As an alternative we recommend using 2ndQuadrant's <ulink url="https://www.pgbarman.org/">Barman</ulink>,
which offloads WAL management to a separate server, removing the requirement to use a replication
slot for each individual standby to reserve WAL. See section <xref linkend="cloning-from-barman"/>
for more details on using &repmgr; together with Barman.
</simpara>
</tip>
</sect1>
<sect1 id="cloning-cascading" xreflabel="Cloning and cascading replication">
<title>Cloning and cascading replication</title>
<indexterm>
<primary>cloning</primary>
<secondary>cascading replication</secondary>
</indexterm>
<para>
Cascading replication, introduced with PostgreSQL 9.2, enables a standby server
to replicate from another standby server rather than directly from the primary,
meaning replication changes "cascade" down through a hierarchy of servers. This
can be used to reduce load on the primary and minimize bandwith usage between
sites. For more details, see the
<ulink url="https://www.postgresql.org/docs/current/warm-standby.html#CASCADING-REPLICATION">
PostgreSQL cascading replication documentation</ulink>.
</para>
<para>
&repmgr; supports cascading replication. When cloning a standby,
set the command-line parameter <literal>--upstream-node-id</literal> to the
<varname>node_id</varname> of the server the standby should connect to, and
&repmgr; will create <filename>recovery.conf</filename> to point to it. Note
that if <literal>--upstream-node-id</literal> is not explicitly provided,
&repmgr; will set the standby's <filename>recovery.conf</filename> to
point to the primary node.
</para>
<para>
To demonstrate cascading replication, first ensure you have a primary and standby
set up as shown in the <xref linkend="quickstart"/>.
Then create an additional standby server with <filename>repmgr.conf</filename> looking
like this:
<programlisting>
node_id=3
node_name=node3
conninfo='host=node3 user=repmgr dbname=repmgr'
data_directory='/var/lib/postgresql/data'</programlisting>
</para>
<para>
Clone this standby (using the connection parameters for the existing standby),
ensuring <literal>--upstream-node-id</literal> is provide with the <varname>node_id</varname>
of the previously created standby (if following the example, this will be <literal>2</literal>):
<programlisting>
$ repmgr -h node2 -U repmgr -d repmgr -f /etc/repmgr.conf standby clone --upstream-node-id=2
NOTICE: using configuration file "/etc/repmgr.conf"
NOTICE: destination directory "/var/lib/postgresql/data" provided
INFO: connecting to upstream node
INFO: connected to source node, checking its state
NOTICE: checking for available walsenders on upstream node (2 required)
INFO: sufficient walsenders available on upstream node (2 required)
INFO: successfully connected to source node
DETAIL: current installation size is 29 MB
INFO: creating directory "/var/lib/postgresql/data"...
NOTICE: starting backup (using pg_basebackup)...
HINT: this may take some time; consider using the -c/--fast-checkpoint option
INFO: executing: 'pg_basebackup -l "repmgr base backup" -D /var/lib/postgresql/data -h node2 -U repmgr -X stream '
NOTICE: standby clone (using pg_basebackup) complete
NOTICE: you can now start your PostgreSQL server
HINT: for example: pg_ctl -D /var/lib/postgresql/data start</programlisting>
then register it (note that <literal>--upstream-node-id</literal> must be provided here
too):
<programlisting>
$ repmgr -f /etc/repmgr.conf standby register --upstream-node-id=2
NOTICE: standby node "node2" (ID: 2) successfully registered
</programlisting>
</para>
<para>
After starting the standby, the cluster will look like this, showing that <literal>node3</literal>
is attached to <literal>node2</literal>, not the primary (<literal>node1</literal>).
<programlisting>
$ repmgr -f /etc/repmgr.conf cluster show
ID | Name | Role | Status | Upstream | Location | Connection string
----+-------+---------+-----------+----------+----------+--------------------------------------
1 | node1 | primary | * running | | default | host=node1 dbname=repmgr user=repmgr
2 | node2 | standby | running | node1 | default | host=node2 dbname=repmgr user=repmgr
3 | node3 | standby | running | node2 | default | host=node3 dbname=repmgr user=repmgr
</programlisting>
</para>
<tip>
<simpara>
Under some circumstances when setting up a cascading replication
cluster, you may wish to clone a downstream standby whose upstream node
does not yet exist. In this case you can clone from the primary (or
another upstream node); provide the parameter <literal>--upstream-conninfo</literal>
to explictly set the upstream's <varname>primary_conninfo</varname> string
in <filename>recovery.conf</filename>.
</simpara>
</tip>
</sect1>
<sect1 id="cloning-advanced" xreflabel="Advanced cloning options">
<title>Advanced cloning options</title>
<indexterm>
<primary>cloning</primary>
<secondary>advanced options</secondary>
</indexterm>
<sect2 id="cloning-advanced-pg-basebackup-options" xreflabel="pg_basebackup options when cloning a standby">
<title>pg_basebackup options when cloning a standby</title>
<para>
As &repmgr; uses <command>pg_basebackup</command> to clone a standby, it's possible to
provide additional parameters for <command>pg_basebackup</command> to customise the
cloning process.
</para>
<para>
By default, <command>pg_basebackup</command> performs a checkpoint before beginning the backup
process. However, a normal checkpoint may take some time to complete;
a fast checkpoint can be forced with <command><link linkend="repmgr-standby-clone">repmgr standby clone</link></command>'s
<literal>-c/--fast-checkpoint</literal> option.
Note that this may impact performance of the server being cloned from (typically the primary)
so should be used with care.
</para>
<tip>
<simpara>
If <application>Barman</application> is set up for the cluster, it's possible to
clone the standby directly from Barman, without any impact on the server the standby
is being cloned from. For more details see <xref linkend="cloning-from-barman"/>.
</simpara>
</tip>
<para>
Other options can be passed to <command>pg_basebackup</command> by including them
in the <filename>repmgr.conf</filename> setting <varname>pg_basebackup_options</varname>.
</para>
<para>
Not that by default, &repmgr; executes <command>pg_basebackup</command> with <option>-X/--wal-method</option>
(PostgreSQL 9.6 and earlier: <option>-X/--xlog-method</option>) set to <literal>stream</literal>.
From PostgreSQL 9.6, if replication slots are in use, it will also create a replication slot before
running the base backup, and execute <command>pg_basebackup</command> with the
<option>-S/--slot</option> option set to the name of the previously created replication slot.
</para>
<para>
These parameters can set by the user in <varname>pg_basebackup_options</varname>, in which case they
will override the &repmgr; default values. However normally there's no reason to do this.
</para>
<para>
If using a separate directory to store WAL files, provide the option <literal>--waldir</literal>
(<literal>--xlogdir</literal> in PostgreSQL 9.6 and earlier) with the absolute path to the
WAL directory. Any WALs generated during the cloning process will be copied here, and
a symlink will automatically be created from the main data directory.
</para>
<para>
See the <ulink url="https://www.postgresql.org/docs/current/app-pgbasebackup.html">PostgreSQL pg_basebackup documentation</ulink>
for more details of available options.
</para>
</sect2>
<sect2 id="cloning-advanced-managing-passwords" xreflabel="Managing passwords">
<title>Managing passwords</title>
<indexterm>
<primary>cloning</primary>
<secondary>using passwords</secondary>
</indexterm>
<para>
If replication connections to a standby's upstream server are password-protected,
the standby must be able to provide the password so it can begin streaming replication.
</para>
<para>
The recommended way to do this is to store the password in the <literal>postgres</literal> system
user's <filename>~/.pgpass</filename> file. It's also possible to store the password in the
environment variable <varname>PGPASSWORD</varname>, however this is not recommended for
security reasons. For more details see the
<ulink url="https://www.postgresql.org/docs/current/libpq-pgpass.html">PostgreSQL password file documentation</ulink>.
</para>
<note>
<para>
If using a <filename>pgpass</filename> file, an entry for the replication user (by default the
user who connects to the <literal>repmgr</literal> database) <emphasis>must</emphasis>
be provided, with database name set to <literal>replication</literal>, e.g.:
<programlisting>
node1:5432:replication:repmgr:12345</programlisting>
</para>
</note>
<para>
If, for whatever reason, you wish to include the password in <filename>recovery.conf</filename>,
set <varname>use_primary_conninfo_password</varname> to <literal>true</literal> in
<filename>repmgr.conf</filename>. This will read a password set in <varname>PGPASSWORD</varname>
(but not <filename>~/.pgpass</filename>) and place it into the <varname>primary_conninfo</varname>
string in <filename>recovery.conf</filename>. Note that <varname>PGPASSWORD</varname>
will need to be set during any action which causes <filename>recovery.conf</filename> to be
rewritten, e.g. <xref linkend="repmgr-standby-follow"/>.
</para>
<para>
It is of course also possible to include the password value in the <varname>conninfo</varname>
string for each node, but this is obviously a security risk and should be avoided.
</para>
<para>
From PostgreSQL 9.6, <application>libpq</application> supports the <varname>passfile</varname>
parameter in connection strings, which can be used to specify a password file other than
the default <filename>~/.pgpass</filename>.
</para>
<para>
To have &repmgr; write a custom password file in <varname>primary_conninfo</varname>,
specify its location in <varname>passfile</varname> in <filename>repmgr.conf</filename>.
</para>
</sect2>
<sect2 id="cloning-advanced-replication-user" xreflabel="Separate replication user">
<title>Separate replication user</title>
<para>
In some circumstances it might be desirable to create a dedicated replication-only
user (in addition to the user who manages the &repmgr; metadata). In this case,
the replication user should be set in <filename>repmgr.conf</filename> via the parameter
<varname>replication_user</varname>; &repmgr; will use this value when making
replication connections and generating <filename>recovery.conf</filename>. This
value will also be stored in the parameter <literal>repmgr.nodes</literal>
table for each node; it no longer needs to be explicitly specified when
cloning a node or executing <xref linkend="repmgr-standby-follow"/>.
</para>
</sect2>
</sect1>
</chapter>
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