src/man/include/failover.xml - master - githubmirror/sssd

src/man/include/failover.xml
132 строки6 KB
Pavel Březina
man: fix typo in failover description
02 окт 2020, 13:18
02 окт 2020, 13:18a62a13a
<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>
    <refsect2 id='failover_tuning'>
        <title>Failover time outs and tuning</title>
        <para>
            Resolving a server to connect to can be as simple as running
            a single DNS query or can involve several steps, such as finding
            the correct site or trying out multiple host names in case some
            of the configured servers are not reachable. The more complex
            scenarios can take some time and SSSD needs to balance between
            providing enough time to finish the resolution process but on
            the other hand, not trying for too long before falling back
            to offline mode. If the SSSD debug logs show that the server
            resolution is timing out before a live server is contacted,
            you can consider changing the time outs.
        </para>
        <para>
            This section lists the available tunables. Please refer to their
            description in the
            <citerefentry>
                <refentrytitle>sssd.conf</refentrytitle><manvolnum>5</manvolnum>
            </citerefentry>,
            manual page.
            <variablelist>
                <varlistentry>
                    <term>
                        dns_resolver_server_timeout
                    </term>
                    <listitem>
                        <para>
                            Time in milliseconds that sets how long would SSSD
                            talk to a single DNS server before trying next one.
                        </para>
                        <para>
                            Default: 1000
                        </para>
                    </listitem>
                </varlistentry>
                <varlistentry>
                    <term>
                        dns_resolver_op_timeout
                    </term>
                    <listitem>
                        <para>
                            Time in seconds to tell how long would SSSD try
                            to resolve single DNS query (e.g. resolution of a
                            hostname or an SRV record) before trying the next
                            hostname or discovery domain.
                        </para>
                        <para>
                            Default: 3
                        </para>
                    </listitem>
                </varlistentry>
                <varlistentry>
                    <term>
                        dns_resolver_timeout
                    </term>
                    <listitem>
                        <para>
                            How long would SSSD try to resolve a failover
                            service. This service resolution internally might
                            include several steps, such as resolving DNS SRV
                            queries or locating the site.
                        </para>
                        <para>
                            Default: 6
                        </para>
                    </listitem>
                </varlistentry>
            </variablelist>
        </para>
        <para>
            For LDAP-based providers, the resolve operation is performed
            as part of an LDAP connection operation. Therefore, also the
            <quote>ldap_opt_timeout</quote> timeout should be set to
            a larger value than <quote>dns_resolver_timeout</quote>
            which in turn should be set to a larger value than
            <quote>dns_resolver_op_timeout</quote> which should be larger
            than <quote>dns_resolver_server_timeout</quote>.
        </para>
    </refsect2>
</refsect1>