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Update High Availability docs. Clarify terms master/primary standby/slave,

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move two paragraphs that apply to log shipping in general from the
"Alternative method for log shipping" section to the earlier sections.
Add varname tags where missing. Some small wording changes.
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Heikki Linnakangas 2010-05-28 14:03:31 +00:00
parent f39d57b83c
commit fe76f93d11
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doc/src/sgml/high-availability.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/high-availability.sgml,v 1.68 2010/05/13 14:16:41 mha Exp $ --> <!-- $PostgreSQL: pgsql/doc/src/sgml/high-availability.sgml,v 1.69 2010/05/28 14:03:31 heikki Exp $ -->
<chapter id="high-availability"> <chapter id="high-availability">
<title>High Availability, Load Balancing, and Replication</title> <title>High Availability, Load Balancing, and Replication</title>
@ -38,10 +38,12 @@
<para> <para>
Some solutions deal with synchronization by allowing only one Some solutions deal with synchronization by allowing only one
server to modify the data. Servers that can modify data are server to modify the data. Servers that can modify data are
called read/write or "master" servers. Servers that can reply called read/write, <firstterm>master</> or <firstterm>primary</> servers.
to read-only queries are called "slave" servers. Servers that Servers that track changes in the master are called <firstterm>standby</>
cannot be accessed until they are changed to master servers are or <firstterm>slave</> servers. A standby server that cannot be connected
called "standby" servers. to until it is promoted to a master server is called a <firstterm>warm
standby</> server, and one that can accept connections and serves read-only
queries is called a <firstterm>hot standby</> server.
</para> </para>
<para> <para>
@ -144,16 +146,16 @@ protocol to make nodes agree on a serializable transactional order.
<para> <para>
Warm and hot standby servers can be kept current by reading a Warm and hot standby servers can be kept current by reading a
stream of write-ahead log (<acronym>WAL</>) stream of write-ahead log (<acronym>WAL</>)
records. If the main server fails, the warm standby contains records. If the main server fails, the standby contains
almost all of the data of the main server, and can be quickly almost all of the data of the main server, and can be quickly
made the new master database server. This is asynchronous and made the new master database server. This is asynchronous and
can only be done for the entire database server. can only be done for the entire database server.
</para> </para>
<para> <para>
A PITR standby server can be kept more up-to-date using streaming A PITR standby server can be implemented using file-based log shipping
replication.; see <xref linkend="streaming-replication">. For (<xref linkend="warm-standby">) or streaming replication (see
warm standby information, see <xref linkend="warm-standby">, and <xref linkend="streaming-replication">), or a combination of both. For
for hot standby, see <xref linkend="hot-standby">. information on hot standby, see <xref linkend="hot-standby">.
</para> </para>
</listitem> </listitem>
</varlistentry> </varlistentry>
@ -613,10 +615,9 @@ protocol to make nodes agree on a serializable transactional order.
<para> <para>
Standby mode is exited and the server switches to normal operation, Standby mode is exited and the server switches to normal operation,
when a trigger file is found (trigger_file). Before failover, it will when a trigger file is found (<varname>trigger_file</>). Before failover,
restore any WAL available in the archive or in pg_xlog, but won't try any WAL immediately available in the archive or in pg_xlog will be
to connect to the master or wait for files to become available in the restored, but no attempt is made to connect to the master.
archive.
</para> </para>
</sect2> </sect2>
@ -710,6 +711,15 @@ trigger_file = '/path/to/trigger_file'
replication, make sure you set <varname>max_wal_senders</> high enough in replication, make sure you set <varname>max_wal_senders</> high enough in
the primary to allow them to be connected simultaneously. the primary to allow them to be connected simultaneously.
</para> </para>
<para>
If you're using a WAL archive, its size can be minimized using
the <varname>restartpoint_command</> option to remove files that are no
no longer required by the standby server. Note however, that if you're
using the archive for backup purposes, you need to retain files needed
to recover from at least the latest base backup, even if they're no
longer needed by the standby.
</para>
</sect2> </sect2>
<sect2 id="streaming-replication"> <sect2 id="streaming-replication">
@ -737,15 +747,14 @@ trigger_file = '/path/to/trigger_file'
</para> </para>
<para> <para>
Streaming replication relies on file-based continuous archiving for If you use streaming replication without file-based continuous
making the base backup and for allowing the standby to catch up if it is archiving, you have to set <varname>wal_keep_segments</> in the master
disconnected from the primary for long enough for the primary to to a value high enough to ensure that old WAL segments are not recycled
delete old WAL files still required by the standby. It is possible too early, while the standby might still need them to catch up. If the
to use streaming replication without WAL archiving, but if a standby standby falls behind too much, it needs to be reinitialized from a new
falls behind too much, the primary will delete old WAL files still base backup. If you set up a WAL archive that's accessible from the
needed by the standby, and the standby will have to be manually restored standby, wal_keep_segments is not required as the standby can always
from a base backup. You can control how long the primary retains old WAL use the archive to catch up.
segments using the <varname>wal_keep_segments</> setting.
</para> </para>
<para> <para>
@ -881,6 +890,14 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
rigorous testing. rigorous testing.
</para> </para>
<para>
<productname>PostgreSQL</productname> does not provide the system
software required to identify a failure on the primary and notify
the standby database server. Many such tools exist and are well
integrated with the operating system facilities required for
successful failover, such as IP address migration.
</para>
<para> <para>
Once failover to the standby occurs, there is only a Once failover to the standby occurs, there is only a
single server in operation. This is known as a degenerate state. single server in operation. This is known as a degenerate state.
@ -921,7 +938,7 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
<para> <para>
An alternative to the built-in standby mode described in the previous An alternative to the built-in standby mode described in the previous
sections is to use a restore_command that polls the archive location. sections is to use a <varname>restore_command</> that polls the archive location.
This was the only option available in versions 8.4 and below. In this This was the only option available in versions 8.4 and below. In this
setup, set <varname>standby_mode</> off, because you are implementing setup, set <varname>standby_mode</> off, because you are implementing
the polling required for standby operation yourself. See the polling required for standby operation yourself. See
@ -932,9 +949,9 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass'
<para> <para>
Note that in this mode, the server will apply WAL one file at a Note that in this mode, the server will apply WAL one file at a
time, so if you use the standby server for queries (see Hot Standby), time, so if you use the standby server for queries (see Hot Standby),
there is a bigger delay between an action in the master and when the there is a delay between an action in the master and when the
action becomes visible in the standby, corresponding the time it takes action becomes visible in the standby, corresponding the time it takes
to fill up the WAL file. archive_timeout can be used to make that delay to fill up the WAL file. <varname>archive_timeout</> can be used to make that delay
shorter. Also note that you can't combine streaming replication with shorter. Also note that you can't combine streaming replication with
this method. this method.
</para> </para>
@ -991,14 +1008,6 @@ if (!triggered)
configurations and environments. configurations and environments.
</para> </para>
<para>
<productname>PostgreSQL</productname> does not provide the system
software required to identify a failure on the primary and notify
the standby database server. Many such tools exist and are well
integrated with the operating system facilities required for
successful failover, such as IP address migration.
</para>
<para> <para>
The method for triggering failover is an important part of planning The method for triggering failover is an important part of planning
and design. One potential option is the <varname>restore_command</> and design. One potential option is the <varname>restore_command</>
@ -1015,20 +1024,12 @@ if (!triggered)
creation of a trigger file is ideal, if this can be arranged. creation of a trigger file is ideal, if this can be arranged.
</para> </para>
<para>
The size of the WAL archive can be minimized by using the <literal>%r</>
option of the <varname>restore_command</>. This option specifies the
last archive file name that needs to be kept to allow the recovery to
restart correctly. This can be used to truncate the archive once
files are no longer required, assuming the archive is writable from the
standby server.
</para>
<sect2 id="warm-standby-config"> <sect2 id="warm-standby-config">
<title>Implementation</title> <title>Implementation</title>
<para> <para>
The short procedure for configuring a standby server is as follows. For The short procedure for configuring a standby server using this alternative
method is as follows. For
full details of each step, refer to previous sections as noted. full details of each step, refer to previous sections as noted.
<orderedlist> <orderedlist>
<listitem> <listitem>
@ -1086,9 +1087,10 @@ if (!triggered)
<title>Record-based Log Shipping</title> <title>Record-based Log Shipping</title>
<para> <para>
<productname>PostgreSQL</productname> directly supports file-based It is also possible to implement record-based log shipping using this
log shipping as described above. It is also possible to implement alternative method, though this requires custom development, and changes
record-based log shipping, though this requires custom development. will still only becomes visible to hot standby queries after a full WAL
file has been shipped.
</para> </para>
<para> <para>
@ -1573,9 +1575,9 @@ if (!triggered)
<para> <para>
Three-way deadlocks are possible between <literal>AccessExclusiveLocks</> arriving from Three-way deadlocks are possible between <literal>AccessExclusiveLocks</> arriving from
the primary, cleanup WAL records that require buffer cleanup locks, and the primary, cleanup WAL records that require buffer cleanup locks, and
user requests that are waiting behind replayed <literal>AccessExclusiveLocks</>. Deadlocks user requests that are waiting behind replayed <literal>AccessExclusiveLocks</>.
are resolved immediately, should they occur, though they are thought to be Deadlocks are resolved automatically after <varname>deadlock_timeout</>
rare in practice. seconds, though they are thought to be rare in practice.
</para> </para>
<para> <para>
@ -1684,8 +1686,10 @@ LOG: database system is ready to accept read only connections
themselves. Users will still write large sort temporary files and themselves. Users will still write large sort temporary files and
re-generate relcache info files, so no part of the database re-generate relcache info files, so no part of the database
is truly read-only during hot standby mode. is truly read-only during hot standby mode.
Note also that writes to remote databases will still be possible, Note also that writes to remote databases using
even though the transaction is read-only locally. <application>dblink</application> module, and other operations outside the
database using PL functions will still be possible, even though the
transaction is read-only locally.
</para> </para>
<para> <para>
@ -1940,7 +1944,7 @@ LOG: database system is ready to accept read only connections
</indexterm> </indexterm>
<para> <para>
In a warm standby configuration, it is possible to offload the expense of In a standby configuration, it is possible to offload the expense of
taking periodic base backups from the primary server; instead base backups taking periodic base backups from the primary server; instead base backups
can be made by backing can be made by backing
up a standby server's files. This concept is generally known as up a standby server's files. This concept is generally known as