php-doc-en/reference/mysqlnd_ms/functions/mysqlnd-ms-set-qos.xml

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<refentry xml:id="function.mysqlnd-ms-set-qos" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
 <refnamediv>
  <refname>mysqlnd_ms_set_qos</refname>
  <refpurpose>Sets the quality of service needed from the cluster</refpurpose>
 </refnamediv>

 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>bool</type>
   <methodname>mysqlnd_ms_set_qos</methodname>
   <methodparam><type>mixed</type><parameter>connection</parameter></methodparam>
   <methodparam><type>int</type><parameter>service_level</parameter></methodparam>
   <methodparam choice="opt"><type>int</type><parameter>service_level_option</parameter></methodparam>
   <methodparam choice="opt"><type>mixed</type><parameter>option_value</parameter></methodparam>
  </methodsynopsis>
  <para>
   Sets the quality of service needed from the cluster. A database cluster delivers
   a certain quality of service to the user depending on its architecture. A major
   aspect of the quality of service is the consistency level the cluster can offer.
   An asynchronous MySQL replication cluster defaults to eventual consistency for
   slave reads: a slave may serve stale data, current data, or it may have not the
   requested data at all, because it is not synchronous to the master. In a
   MySQL replication cluster, only master accesses can give strong consistency, which
   promises that all clients see each others changes.
  </para>
  <para>
   PECL/mysqlnd_ms hides the complexity of choosing appropriate nodes to achieve
   a certain level of service from the cluster. The
   "Quality of Service" filter implements the necessary logic. The filter can either
   be configured in the plugins configuration file, or at runtime using
   <function>mysqlnd_ms_set_qos</function>.
  </para>
  <para>
   Similar results can be achieved with PECL mysqlnd_ms &lt; 1.2.0, if using
   SQL hints to force the use of a certain type of node or using the
   <literal>master_on_write</literal> plugin configuration option. The first
   requires more code and causes more work on the application side.
   The latter is less refined than using the quality of service filter.
   Settings made through the function call can be reversed, as shown in the example
   below. The example temporarily switches to a higher service level
   (session consistency, read your writes) and returns
   back to the clusters default after it has performed all operations that require the
   better service. This way, read load on the master can be minimized compared to
   using <literal>master_on_write</literal>, which would continue using the master
   after the first write.
  </para>
  <para>
   Since 1.5.0 calls will fail when done in the middle of a transaction if
   <link linkend="ini.mysqlnd-ms-plugin-config-v2.trx-stickiness">transaction stickiness</link>
   is enabled and transaction boundaries have been detected.
   properly.
  </para>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <variablelist>
   <varlistentry>
    <term><parameter>connection</parameter></term>
    <listitem>
     <para>
      A PECL/mysqlnd_ms connection handle to a MySQL server of the type
      <link linkend="ref.pdo-mysql">PDO_MYSQL</link>, <link linkend="book.mysqli">mysqli</link> or
      <link linkend="book.mysql">ext/mysql</link> for which a service level is to be set.
      The connection handle is obtained when opening a connection with
      a host name that matches a mysqlnd_ms configuration file
      entry using any of the above three MySQL driver extensions.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><parameter>service_level</parameter></term>
    <listitem>
     <para>
      The requested service level: <constant>MYSQLND_MS_QOS_CONSISTENCY_EVENTUAL</constant>,
      <constant>MYSQLND_MS_QOS_CONSISTENCY_SESSION</constant> or
      <constant>MYSQLND_MS_QOS_CONSISTENCY_STRONG</constant>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><parameter>service_level_option</parameter></term>
    <listitem>
     <para>
      An option to parameterize the requested service level.
      The option can either be <constant>MYSQLND_MS_QOS_OPTION_GTID</constant>
      or <constant>MYSQLND_MS_QOS_OPTION_AGE</constant>.
     </para>
     <para>
       The option <constant>MYSQLND_MS_QOS_OPTION_GTID</constant> can be used
       to refine the service level <constant>MYSQLND_MS_QOS_CONSISTENCY_SESSION</constant>.
       It must be combined with a fourth function parameter, the
       <parameter>option_value</parameter>. The <parameter>option_value</parameter>
       shall be a global transaction ID obtained from
       <function>mysqlnd_ms_get_last_gtid</function>. If set, the
       plugin considers both master servers and asynchronous slaves for session
       consistency (read your writes). Otherwise, only masters are
       used to achieve session consistency. A slave is considered up-to-date and
       checked if it has already replicated the global transaction ID from
       <parameter>option_value</parameter>. Please note, searching appropriate slaves
       is an expensive and slow operation. Use the feature sparsely, if the
       master cannot handle the read load alone.
     </para>
     <para>
       The <constant>MYSQLND_MS_QOS_OPTION_AGE</constant> option can be combined
       with the <constant>MYSQLND_MS_QOS_CONSISTENCY_EVENTUAL</constant>
       service level, to filter out asynchronous slaves that lag more seconds behind
       the master than <parameter>option_value</parameter>. If set, the plugin
       will only consider slaves for reading if
       <literal>SHOW SLAVE STATUS</literal> reports
       <literal>Slave_IO_Running=Yes</literal>,
       <literal>Slave_SQL_Running=Yes</literal> and
       <literal>Seconds_Behind_Master &lt;= option_value</literal>. Please note,
       searching appropriate slaves is an expensive and slow operation.
       Use the feature sparsely in version 1.2.0. Future versions may improve the
       algorithm used to identify candidates. Please, see the MySQL reference
       manual about the precision, accuracy and limitations of the MySQL administrative
       command <literal>SHOW SLAVE STATUS</literal>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term><parameter>option_value</parameter></term>
    <listitem>
     <para>
      Parameter value for the service level option. See also the
      <parameter>service_level_option</parameter> parameter.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   Returns &true; if the connections service level
   has been switched to the requested. Otherwise, returns &false;
  </para>
 </refsect1>

 <refsect1>
  &reftitle.notes;
  <note>
   <para>
    <function>mysqlnd_ms_set_qos</function> requires
    PHP &gt;= 5.4.0 and PECL mysqlnd_ms &gt;= 1.2.0. Internally, it is using
    a <literal>mysqlnd</literal> library C functionality not available with PHP 5.3.
   </para>
  </note>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title><function>mysqlnd_ms_set_qos</function> example</title>
    <programlisting role="php">
<![CDATA[
<?php
/* Open mysqlnd_ms connection using mysqli, PDO_MySQL or mysql extension */
$mysqli = new mysqli("myapp", "username", "password", "database");
if (!$mysqli)
  /* Of course, your error handling is nicer... */
  die(sprintf("[%d] %s\n", mysqli_connect_errno(), mysqli_connect_error()));

/* Session consistency: read your writes */
$ret = mysqlnd_ms_set_qos($mysqli, MYSQLND_MS_QOS_CONSISTENCY_SESSION);
if (!$ret)
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

/* Will use master and return fresh data, client can see his last write */
if (!$res = $mysqli->query("SELECT item, price FROM orders WHERE order_id = 1"))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));

/* Back to default: use of all slaves and masters permitted, stale data can happen */
if (!mysqlnd_ms_set_qos($mysqli, MYSQLND_MS_QOS_CONSISTENCY_EVENTUAL))
  die(sprintf("[%d] %s\n", $mysqli->errno, $mysqli->error));
?>
]]>
    </programlisting>
   </example>
  </para>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member>
     <function>mysqlnd_ms_get_last_gtid</function>
    </member>
    <member>
     <link linkend="mysqlnd-ms.qos-consistency">Service level and consistency concept</link>
    </member>
    <member>
     <link linkend="mysqlnd-ms.filter">Filter concept</link>
    </member>
   </simplelist>
  </para>
 </refsect1>

</refentry>

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