Merge "Update docco about dispatchers available for Telemetry"
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<section xml:id="section_telemetry-storing-data">
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<title>Storing samples</title>
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<para>The Telemetry module has a separate service that is responsible for persisting the data
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that is coming from the pollsters or received as notifications. The data is stored in
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a database back end, the list of supported databases can be found in
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<xref linkend="section_telemetry-supported-dbs"/>.
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that is coming from the pollsters or received as notifications. The data can be stored in
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a file or a database back end, for which the list of supported databases can be found in
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<xref linkend="section_telemetry-supported-dbs"/>. The data can also be sent to an external
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data store by using an HTTP dispatcher.
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</para>
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<para>The <systemitem class="service">ceilometer-collector</systemitem> service receives the
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samples as metering messages from the message bus of the configured AMQP service. It stores
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these samples without any modification in the configured back end. The service has to run on
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a host machine from which it has access to the database.</para>
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these samples without any modification in the configured file or database back end, or in
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the external data store as dispatched by the HTTP dispatcher. The service has to run on a
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host machine from which it has access to the configured dispatcher.</para>
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<note>
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<para>Multiple dispatchers can be configured for Telemetry at one time.</para>
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</note>
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<para>Multiple <systemitem class="service">ceilometer-collector</systemitem> process can be
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run at a time. It is also supported to start multiple worker threads per collector process.
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The <parameter>collector_workers</parameter> configuration option has to be modified in the
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<link xlink:href=
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"http://docs.openstack.org/trunk/config-reference/content/ch_configuring-openstack-telemetry.html">
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collector section</link> of the <filename>ceilometer.conf</filename> configuration file.</para>
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collector section</link> of the <filename>ceilometer.conf</filename>
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configuration file.</para>
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<note>
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<para>Using multiple workers per collector process is not recommended to be used with
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PostgreSQL as database back end.</para>
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</note>
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<para>By default the time to live value (ttl) for samples is set to -1, which means that they
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are kept in the database forever. This can be changed by modifying the <parameter>time_to_live
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</parameter> parameter in <filename>ceilometer.conf</filename>. The value has to be specified
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in seconds and it means that every sample that based on its timestamp is older, than the
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specified value will be deleted from the database.</para>
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<para>When the samples are deleted, there are cases, when users and resources remain in the
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database without any corresponding sample. There is a command line script, that deletes
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these useless entries, which is called <systemitem class="service">ceilometer-expirer</systemitem>.
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This script should be run periodically, for instance in a cron job, to ensure that the
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database is cleaned up properly.</para>
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<para>The level of support differs in case of the configured back end:</para>
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<table rules="all">
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<caption>Time-to-live support for database back ends</caption>
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<col width="24%"/>
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<col width="38%"/>
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<col width="38%"/>
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<thead>
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<tr>
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<td>Database</td>
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<td>ttl value support</td>
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<td><systemitem class="service">ceilometer-expirer</systemitem>
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capabilities</td>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>MongoDB</td>
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<td>MongoDB has a built-in mechanism for deleting samples that are older
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than the configured ttl value.</td>
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<td>In case of this database, only the lingering dead resource,
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user and project entries will be deleted by
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<systemitem class="service">ceilometer-expirer</systemitem>.
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</td>
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</tr>
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<tr>
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<td>SQL-based back ends</td>
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<td>The library (SQLAlchemy) that is used for accessing SQL-based back ends does
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not support using the ttl value.</td>
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<td><systemitem class="service">ceilometer-expirer</systemitem> has to be
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used for deleting both the samples and the remaining entires in other
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database tables. The script will delete samples based on the
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<parameter>time_to_live</parameter> value that is set in the
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configuration file.</td>
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</tr>
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<tr>
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<td>HBase</td>
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<td>HBase does not support this functionality currently, therefore the ttl value
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in the configuration file is ignored.</td>
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<td>The samples are not deleted by using
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<systemitem class="service">ceilometer-expirer</systemitem>,
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this functionality is not supported.</td>
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</tr>
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<tr>
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<td>DB2</td>
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<td>Same case as MongoDB.</td>
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<td>Same case as MongoDB.</td>
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</tr>
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</tbody>
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</table>
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<simplesect>
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<title>Database dispatcher</title>
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<para>When the database dispatcher is configured as data store, you have the option to set
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a <parameter>time_to_live</parameter> parameter (ttl) for samples. By default the time to
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live value for samples is set to -1, which means that they are kept in the database forever.
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</para>
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<para>The time to live value is specified in seconds. Each sample has a time stamp, and the
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<literal>ttl</literal> value indicates that a sample is deleted from the database when
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the number of seconds has elapsed since that sample reading was stamped. For example,
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if the sampling occurs every 60 seconds, and the time to live is set to 600, only ten
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samples are stored in the database.</para>
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<para>During the deletion of samples <literal>resources</literal> and <literal>users</literal>
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can remain in the database without a corresponding sample when the time to live has expired,
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you may need to delete the entries related to the expired samples. The command-line script,
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which you can use for this purpose is
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<systemitem class="service">ceilometer-expirer</systemitem>. You can run it in a cron job,
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which helps keeping your database in a consistent state.</para>
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<para>The level of support differs in case of the configured back end:</para>
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<table rules="all">
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<caption>Time-to-live support for database back ends</caption>
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<col width="24%"/>
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<col width="38%"/>
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<col width="38%"/>
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<thead>
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<tr>
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<td>Database</td>
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<td>ttl value support</td>
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<td><systemitem class="service">ceilometer-expirer</systemitem>
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capabilities</td>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td>MongoDB</td>
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<td>MongoDB has a built-in mechanism for deleting samples that are older
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than the configured ttl value.</td>
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<td>In case of this database, only the lingering dead resource,
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user and project entries will be deleted by
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<systemitem class="service">ceilometer-expirer</systemitem>.
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</td>
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</tr>
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<tr>
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<td>SQL-based back ends</td>
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<td>The library (SQLAlchemy) that is used for accessing SQL-based back ends does
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not support using the ttl value.</td>
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<td><systemitem class="service">ceilometer-expirer</systemitem> has to be
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used for deleting both the samples and the remaining entires in other
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database tables. The script will delete samples based on the
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<parameter>time_to_live</parameter> value that is set in the
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configuration file.</td>
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</tr>
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<tr>
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<td>HBase</td>
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<td>HBase does not support this functionality currently, therefore the ttl value
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in the configuration file is ignored.</td>
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<td>The samples are not deleted by using
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<systemitem class="service">ceilometer-expirer</systemitem>,
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this functionality is not supported.</td>
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</tr>
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<tr>
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<td>DB2</td>
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<td>Same case as MongoDB.</td>
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<td>Same case as MongoDB.</td>
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</tr>
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</tbody>
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</table>
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</simplesect>
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<simplesect>
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<title>HTTP dispatcher</title>
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<para>The Telemetry module supports sending samples to an external HTTP target. The
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samples are sent without any modification. To set this option as data storage, the
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<option>dispatcher</option> has to be changed to <literal>http</literal> in the
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<filename>ceilometer.conf</filename> configuration file. For the list
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of options that you need to set, see the see the
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<link xlink:href="http://docs.openstack.org/trunk/config-reference/content/ch_configuring-openstack-telemetry.html">
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<literal>dispatcher_http</literal> section</link>
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in the <citetitle>OpenStack Configuration Reference</citetitle>.</para>
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</simplesect>
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<simplesect>
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<title>File dispatcher</title>
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<para>You can store samples in a file by setting the <option>dispatcher</option>
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option in <filename>ceilometer.conf</filename> o <literal>file</literal>. For the list
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of configuration options, see the
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<link xlink:href="http://docs.openstack.org/trunk/config-reference/content/ch_configuring-openstack-telemetry.html">
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<literal>dispatcher_file</literal> section</link>
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in the <citetitle>OpenStack Configuration Reference</citetitle>.</para>
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</simplesect>
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</section>
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</section>
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