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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="WARM-STANDBY"
>24.4. Warm Standby Servers for High Availability</A
></H1
><A
NAME="AEN28747"
></A
><A
NAME="AEN28749"
></A
><A
NAME="AEN28751"
></A
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></A
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><P
> Continuous archiving can be used to create a <I
CLASS="FIRSTTERM"
>high
availability</I
> (HA) cluster configuration with one or more
<I
CLASS="FIRSTTERM"
>standby servers</I
> ready to take over operations if the
primary server fails. This capability is widely referred to as
<I
CLASS="FIRSTTERM"
>warm standby</I
> or <I
CLASS="FIRSTTERM"
>log shipping</I
>.
</P
><P
> The primary and standby server work together to provide this capability,
though the servers are only loosely coupled. The primary server operates
in continuous archiving mode, while each standby server operates in
continuous recovery mode, reading the WAL files from the primary. No
changes to the database tables are required to enable this capability,
so it offers low administration overhead in comparison with some other
replication approaches. This configuration also has relatively low
performance impact on the primary server.
</P
><P
> Directly moving WAL records from one database server to another
is typically described as log shipping. <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
implements file-based log shipping, which means that WAL records are
transferred one file (WAL segment) at a time. WAL files (16MB) can be
shipped easily and cheaply over any distance, whether it be to an
adjacent system, another system on the same site or another system on
the far side of the globe. The bandwidth required for this technique
varies according to the transaction rate of the primary server.
Record-based log shipping is also possible with custom-developed
procedures, as discussed in <A
HREF="warm-standby.html#WARM-STANDBY-RECORD"
>Section 24.4.4</A
>.
</P
><P
> It should be noted that the log shipping is asynchronous, i.e. the WAL
records are shipped after transaction commit. As a result there is a
window for data loss should the primary server suffer a catastrophic
failure: transactions not yet shipped will be lost. The length of the
window of data loss can be limited by use of the
<TT
CLASS="VARNAME"
>archive_timeout</TT
> parameter, which can be set as low
as a few seconds if required. However such low settings will
substantially increase the bandwidth requirements for file shipping.
If you need a window of less than a minute or so, it's probably better
to look into record-based log shipping.
</P
><P
> The standby server is not available for access, since it is continually
performing recovery processing. Recovery performance is sufficiently
good that the standby will typically be only moments away from full
availability once it has been activated. As a result, we refer to this
capability as a warm standby configuration that offers high
availability. Restoring a server from an archived base backup and
rollforward will take considerably longer, so that technique only
offers a solution for disaster recovery, not high availability.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="WARM-STANDBY-PLANNING"
>24.4.1. Planning</A
></H2
><P
> It is usually wise to create the primary and standby servers
so that they are as similar as possible, at least from the
perspective of the database server. In particular, the path names
associated with tablespaces will be passed across as-is, so both
primary and standby servers must have the same mount paths for
tablespaces if that feature is used. Keep in mind that if
<A
HREF="sql-createtablespace.html"
><I
>CREATE TABLESPACE</I
></A
>
is executed on the primary, any new mount point needed for it must
be created on both the primary and all standby servers before the command
is executed. Hardware need not be exactly the same, but experience shows
that maintaining two identical systems is easier than maintaining two
dissimilar ones over the lifetime of the application and system.
In any case the hardware architecture must be the same — shipping
from, say, a 32-bit to a 64-bit system will not work.
</P
><P
> In general, log shipping between servers running different major
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> release
levels will not be possible. It is the policy of the PostgreSQL Global
Development Group not to make changes to disk formats during minor release
upgrades, so it is likely that running different minor release levels
on primary and standby servers will work successfully. However, no
formal support for that is offered and you are advised to keep primary
and standby servers at the same release level as much as possible.
When updating to a new minor release, the safest policy is to update
the standby servers first — a new minor release is more likely
to be able to read WAL files from a previous minor release than vice
versa.
</P
><P
> There is no special mode required to enable a standby server. The
operations that occur on both primary and standby servers are entirely
normal continuous archiving and recovery tasks. The only point of
contact between the two database servers is the archive of WAL files
that both share: primary writing to the archive, standby reading from
the archive. Care must be taken to ensure that WAL archives for separate
primary servers do not become mixed together or confused. The archive
need not be large, if it is only required for the standby operation.
</P
><P
> The magic that makes the two loosely coupled servers work together is
simply a <TT
CLASS="VARNAME"
>restore_command</TT
> used on the standby that waits
for the next WAL file to become available from the primary. The
<TT
CLASS="VARNAME"
>restore_command</TT
> is specified in the
<TT
CLASS="FILENAME"
>recovery.conf</TT
> file on the standby server. Normal recovery
processing would request a file from the WAL archive, reporting failure
if the file was unavailable. For standby processing it is normal for
the next file to be unavailable, so we must be patient and wait for
it to appear. A waiting <TT
CLASS="VARNAME"
>restore_command</TT
> can be written as
a custom script that loops after polling for the existence of the next
WAL file. There must also be some way to trigger failover, which should
interrupt the <TT
CLASS="VARNAME"
>restore_command</TT
>, break the loop and return
a file-not-found error to the standby server. This ends recovery and
the standby will then come up as a normal server.
</P
><P
> Pseudocode for a suitable <TT
CLASS="VARNAME"
>restore_command</TT
> is:
</P><PRE
CLASS="PROGRAMLISTING"
>triggered = false;
while (!NextWALFileReady() && !triggered)
{
sleep(100000L); /* wait for ~0.1 sec */
if (CheckForExternalTrigger())
triggered = true;
}
if (!triggered)
CopyWALFileForRecovery();</PRE
><P>
</P
><P
> A working example of a waiting <TT
CLASS="VARNAME"
>restore_command</TT
> is provided
as a <TT
CLASS="FILENAME"
>contrib</TT
> module named <SPAN
CLASS="APPLICATION"
>pg_standby</SPAN
>. This
example can be extended as needed to support specific configurations or
environments.
</P
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> does not provide the system
software required to identify a failure on the primary and notify
the standby system and then the standby database server. Many such
tools exist and are well integrated with other aspects required for
successful failover, such as IP address migration.
</P
><P
> The means for triggering failover is an important part of planning and
design. The <TT
CLASS="VARNAME"
>restore_command</TT
> is executed in full once
for each WAL file. The process running the <TT
CLASS="VARNAME"
>restore_command</TT
>
is therefore created and dies for each file, so there is no daemon
or server process and so we cannot use signals and a signal
handler. A more permanent notification is required to trigger the
failover. It is possible to use a simple timeout facility,
especially if used in conjunction with a known
<TT
CLASS="VARNAME"
>archive_timeout</TT
> setting on the primary. This is
somewhat error prone since a network problem or busy primary server might
be sufficient to initiate failover. A notification mechanism such
as the explicit creation of a trigger file is less error prone, if
this can be arranged.
</P
><P
> The size of the WAL archive can be minimized by using the <TT
CLASS="LITERAL"
>%r</TT
>
option of the <TT
CLASS="VARNAME"
>restore_command</TT
>. 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, if the archive is writable from the
standby server.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="WARM-STANDBY-CONFIG"
>24.4.2. Implementation</A
></H2
><P
> The short procedure for configuring a standby server is as follows. For
full details of each step, refer to previous sections as noted.
<P
></P
></P><OL
TYPE="1"
><LI
><P
> Set up primary and standby systems as near identically as
possible, including two identical copies of
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> at the same release level.
</P
></LI
><LI
><P
> Set up continuous archiving from the primary to a WAL archive located
in a directory on the standby server. Ensure that
<A
HREF="runtime-config-wal.html#GUC-ARCHIVE-MODE"
>archive_mode</A
>,
<A
HREF="runtime-config-wal.html#GUC-ARCHIVE-COMMAND"
>archive_command</A
> and
<A
HREF="runtime-config-wal.html#GUC-ARCHIVE-TIMEOUT"
>archive_timeout</A
>
are set appropriately on the primary
(see <A
HREF="continuous-archiving.html#BACKUP-ARCHIVING-WAL"
>Section 24.3.1</A
>).
</P
></LI
><LI
><P
> Make a base backup of the primary server (see <A
HREF="continuous-archiving.html#BACKUP-BASE-BACKUP"
>Section 24.3.2</A
>), and load this data onto the standby.
</P
></LI
><LI
><P
> Begin recovery on the standby server from the local WAL
archive, using a <TT
CLASS="FILENAME"
>recovery.conf</TT
> that specifies a
<TT
CLASS="VARNAME"
>restore_command</TT
> that waits as described
previously (see <A
HREF="continuous-archiving.html#BACKUP-PITR-RECOVERY"
>Section 24.3.3</A
>).
</P
></LI
></OL
><P>
</P
><P
> Recovery treats the WAL archive as read-only, so once a WAL file has
been copied to the standby system it can be copied to tape at the same
time as it is being read by the standby database server.
Thus, running a standby server for high availability can be performed at
the same time as files are stored for longer term disaster recovery
purposes.
</P
><P
> For testing purposes, it is possible to run both primary and standby
servers on the same system. This does not provide any worthwhile
improvement in server robustness, nor would it be described as HA.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="WARM-STANDBY-FAILOVER"
>24.4.3. Failover</A
></H2
><P
> If the primary server fails then the standby server should begin
failover procedures.
</P
><P
> If the standby server fails then no failover need take place. If the
standby server can be restarted, even some time later, then the recovery
process can also be immediately restarted, taking advantage of
restartable recovery. If the standby server cannot be restarted, then a
full new standby server instance should be created.
</P
><P
> If the primary server fails and then immediately restarts, you must have
a mechanism for informing it that it is no longer the primary. This is
sometimes known as STONITH (Shoot the Other Node In The Head), which is
necessary to avoid situations where both systems think they are the
primary, which will lead to confusion and ultimately data loss.
</P
><P
> Many failover systems use just two systems, the primary and the standby,
connected by some kind of heartbeat mechanism to continually verify the
connectivity between the two and the viability of the primary. It is
also possible to use a third system (called a witness server) to prevent
some cases of inappropriate failover, but the additional complexity
might not be worthwhile unless it is set up with sufficient care and
rigorous testing.
</P
><P
> Once failover to the standby occurs, we have only a
single server in operation. This is known as a degenerate state.
The former standby is now the primary, but the former primary is down
and might stay down. To return to normal operation we must
fully recreate a standby server,
either on the former primary system when it comes up, or on a third,
possibly new, system. Once complete the primary and standby can be
considered to have switched roles. Some people choose to use a third
server to provide backup to the new primary until the new standby
server is recreated,
though clearly this complicates the system configuration and
operational processes.
</P
><P
> So, switching from primary to standby server can be fast but requires
some time to re-prepare the failover cluster. Regular switching from
primary to standby is useful, since it allows regular downtime on
each system for maintenance. This also serves as a test of the
failover mechanism to ensure that it will really work when you need it.
Written administration procedures are advised.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="WARM-STANDBY-RECORD"
>24.4.4. Record-based Log Shipping</A
></H2
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> directly supports file-based
log shipping as described above. It is also possible to implement
record-based log shipping, though this requires custom development.
</P
><P
> An external program can call the <CODE
CLASS="FUNCTION"
>pg_xlogfile_name_offset()</CODE
>
function (see <A
HREF="functions-admin.html"
>Section 9.23</A
>)
to find out the file name and the exact byte offset within it of
the current end of WAL. It can then access the WAL file directly
and copy the data from the last known end of WAL through the current end
over to the standby server(s). With this approach, the window for data
loss is the polling cycle time of the copying program, which can be very
small, but there is no wasted bandwidth from forcing partially-used
segment files to be archived. Note that the standby servers'
<TT
CLASS="VARNAME"
>restore_command</TT
> scripts still deal in whole WAL files,
so the incrementally copied data is not ordinarily made available to
the standby servers. It is of use only when the primary dies —
then the last partial WAL file is fed to the standby before allowing
it to come up. So correct implementation of this process requires
cooperation of the <TT
CLASS="VARNAME"
>restore_command</TT
> script with the data
copying program.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="BACKUP-INCREMENTAL-UPDATED"
>24.4.5. Incrementally Updated Backups</A
></H2
><A
NAME="AEN28844"
></A
><A
NAME="AEN28846"
></A
><P
> In a warm standby configuration, it is possible to offload the expense of
taking periodic base backups from the primary server; instead base backups
can be made by backing
up a standby server's files. This concept is generally known as
incrementally updated backups, log change accumulation, or more simply,
change accumulation.
</P
><P
> If we take a backup of the standby server's data directory while it is processing
logs shipped from the primary, we will be able to reload that data and
restart the standby's recovery process from the last restart point.
We no longer need to keep WAL files from before the restart point.
If we need to recover, it will be faster to recover from the incrementally
updated backup than from the original base backup.
</P
><P
> Since the standby server is not <SPAN
CLASS="QUOTE"
>"live"</SPAN
>, it is not possible to
use <CODE
CLASS="FUNCTION"
>pg_start_backup()</CODE
> and <CODE
CLASS="FUNCTION"
>pg_stop_backup()</CODE
>
to manage the backup process; it will be up to you to determine how
far back you need to keep WAL segment files to have a recoverable
backup. You can do this by running <SPAN
CLASS="APPLICATION"
>pg_controldata</SPAN
>
on the standby server to inspect the control file and determine the
current checkpoint WAL location, or by using the
<TT
CLASS="VARNAME"
>log_restartpoints</TT
> option to print values to the server log.
</P
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>
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