openstack-manuals/doc/admin-guide-cloud/compute/section_compute-configure-service-groups.xml

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]><section xmlns="http://docbook.org/ns/docbook"
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  version="5.0"
  xml:id="configuring-compute-service-groups">
  <title>Configure Compute service groups</title>
  <para>To effectively manage and utilize compute nodes, the Compute service must know their
        statuses. For example, when a user launches a new VM, the Compute scheduler sends the
        request to a live node; the Compute service queries the ServiceGroup API to get information
        about whether a node is alive.</para>
  <para>When a compute worker (running the <systemitem class="service">nova-compute</systemitem>
        daemon) starts, it calls the <systemitem>join</systemitem> API to join the compute group.
        Any interested service (for example, the scheduler) can query the group's membership and the
        status of its nodes. Internally, the <systemitem>ServiceGroup</systemitem> client driver
        automatically updates the compute worker status.</para>
  <para>The database, ZooKeeper, and Memcache drivers are available.</para>
  <section xml:id="database-servicegroup-driver">
   <title>Database ServiceGroup driver</title>
   <para>By default, Compute uses the database driver to track node liveness. In a compute worker,
            this driver periodically sends a <command>db update</command> command to the database,
            saying <quote>I'm OK</quote> with a timestamp. Compute uses a pre-defined timeout
                (<literal>service_down_time</literal>) to determine whether a node is dead.</para>
   <para>The driver has limitations, which can be an issue depending on your setup. The more compute
            worker nodes that you have, the more pressure you put on the database. By default, the
            timeout is 60 seconds so it might take some time to detect node failures. You could
            reduce the timeout value, but you must also make the database update more frequently,
            which again increases the database workload.</para>
   <para>The database contains data that is both transient (whether the node is alive) and persistent
       (for example, entries for VM owners). With the ServiceGroup abstraction, Compute can treat
        each type separately.</para>
  </section>
  <section xml:id="zookeeper-servicegroup-driver">
   <title>ZooKeeper ServiceGroup driver</title>
   <para>The ZooKeeper ServiceGroup driver works by using ZooKeeper
   ephemeral nodes. ZooKeeper, in contrast to databases, is a
   distributed system. Its load is divided among several servers.
   At a compute worker node, after establishing a ZooKeeper session,
   the driver creates an ephemeral znode in the group directory. Ephemeral
   znodes have the same lifespan as the session. If the worker node
   or the <systemitem class="service">nova-compute</systemitem> daemon crashes, or a network
   partition is in place between the worker and the ZooKeeper server quorums,
   the ephemeral znodes are removed automatically. The driver
   gets the group membership by running the <command>ls</command> command in the group directory.</para>
   <para>To use the ZooKeeper driver, you must install ZooKeeper servers and client libraries.
            Setting up ZooKeeper servers is outside the scope of this guide (for more information,
            see <link xlink:href="http://zookeeper.apache.org/"
                >Apache Zookeeper</link>).</para>
     <para>To use ZooKeeper, you must install client-side Python libraries on every nova node:
                <literal>python-zookeeper</literal> &ndash; the official Zookeeper Python binding
            and <literal>evzookeeper</literal> &ndash; the library to make the binding work with the
            eventlet threading model.</para>
        <para>The following example assumes the ZooKeeper server addresses and ports are
                <literal>192.168.2.1:2181</literal>, <literal>192.168.2.2:2181</literal>, and
                <literal>192.168.2.3:2181</literal>.</para>
        <para>The following values in the <filename>/etc/nova/nova.conf</filename> file (on every
            node) are required for the <systemitem>ZooKeeper</systemitem> driver:</para>
<programlisting language="ini"># Driver for the ServiceGroup service
servicegroup_driver="zk"

[zookeeper]
address="192.168.2.1:2181,192.168.2.2:2181,192.168.2.3:2181"</programlisting>
   <para>To customize the Compute Service groups, use the following configuration option
       settings:</para>
       <xi:include href="../../common/tables/nova-zookeeper.xml"/>
  </section>
    <section xml:id="memcache-servicegroup-driver">
      <title>Memcache ServiceGroup driver</title>
        <para>The <systemitem>memcache</systemitem> ServiceGroup driver uses memcached, which is a
            distributed memory object caching system that is often used to increase site
            performance. For more details, see <link xlink:href="http://memcached.org/"
                >memcached.org</link>.</para>
        <para>To use the <systemitem>memcache</systemitem> driver, you must install
                <systemitem>memcached</systemitem>. However, because
                <systemitem>memcached</systemitem> is often used for both OpenStack Object Storage
            and OpenStack dashboard, it might already be installed. If
                <systemitem>memcached</systemitem> is not installed, refer to the <link
                xlink:href="http://docs.openstack.org/icehouse"
                    ><citetitle>OpenStack Installation Guide</citetitle></link> for more
            information.</para>
        <para>The following values in the <filename>/etc/nova/nova.conf</filename> file (on every
            node) are required for the <systemitem>memcache</systemitem> driver:</para>
        <programlisting language="ini"># Driver for the ServiceGroup service
servicegroup_driver="mc"

# Memcached servers. Use either a list of memcached servers to use for caching (list value),
# or "&lt;None>" for in-process caching (default).
memcached_servers=&lt;None>

# Timeout; maximum time since last check-in for up service (integer value).
# Helps to define whether a node is dead
service_down_time=60</programlisting>
  </section>
 </section>