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<refsect1 id='failover'>
<title>FAILOVER</title>
<para>
The failover feature allows back ends to automatically switch to
a different server if the current server fails.
</para>
<refsect2 id='failover_syntax'>
<title>Failover Syntax</title>
<para>
The list of servers is given as a comma-separated list; any
number of spaces is allowed around the comma. The servers are
listed in order of preference. The list can contain any number
of servers.
</para>
<para>
For each failover-enabled config option, two variants exist:
<emphasis>primary</emphasis> and <emphasis>backup</emphasis>.
The idea is that servers in the primary list are preferred and
backup servers are only searched if no primary servers can be
reached. If a backup server is selected, a timeout of 31 seconds
is set. After this timeout SSSD will periodically try to reconnect
to one of the primary servers. If it succeeds, it will replace
the current active (backup) server.
</para>
</refsect2>
<refsect2 id='failover_mechanism'>
<title>The Failover Mechanism</title>
<para>
The failover mechanism distinguishes between a machine and a
service. The back end first tries to resolve the hostname of a
given machine; if this resolution attempt fails, the machine is
considered offline. No further attempts are made to connect
to this machine for any other service. If the resolution
attempt succeeds, the back end tries to connect to a service
on this machine. If the service connection attempt fails,
then only this particular service is considered offline and
the back end automatically switches over to the next service.
The machine is still considered online and might still be tried
for another service.
</para>
<para>
Further connection attempts are made to machines or services
marked as offline after a specified period of time; this is
currently hard coded to 30 seconds.
</para>
<para>
If there are no more machines to try, the back end as a whole
switches to offline mode, and then attempts to reconnect
every 30 seconds.
</para>
</refsect2>
</refsect1>
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