<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>