taf.xml

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<chapter xml:id="oci8.taf" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
 <title>OCI8 Transparent Application Failover (TAF) Support</title>
 <section>
  <para>
   TAF is an Oracle Database feature that provides high availability.
   It enables PHP OCI8 applications to automatically reconnect to a
   preconfigured database when database connectivity fails due to
   instance or network failure.
  </para>
  <para>
   In a configured Oracle Database system, TAF occurs when the PHP
   application detects that the database instance is down or
   unreachable. It establishes a connection to another node in an
   Oracle <link xlink:href="&url.oracle.taf.rac;">RAC</link>
   configuration, a hot standby database, or the same database
   instance
   itself. See <link xlink:href="&url.oracle.taf.ociguide;">Oracle
   Call Interface Programmer's Guide</link> for more information about
   OCI TAF.
  </para>
  <para>
   An application callback can be registered
   with <function>oci_register_taf_callback</function>. During
   failover, normal application processing stops and the registered
   callback is invoked.  The callback notifies the application of the
   failover events. If the failover succeeds, normal processing will
   be resumed. If the failover aborts, any following database
   operations in the application will fail due to no connection being
   available.
  </para>
  <para>
   When a connection fails over to another database, the callback can
   reset any necessary connection state, for example replaying any
   necessary ALTER SESSION commands if the database service did not have
   -failover_restore enabled.
  </para>
  <para>
   An application callback can be removed by calling <function>oci_unregister_taf_callback</function>.
  </para>
 </section>
 <section>
  <title>Configuring Transparent Application Failover</title>
  <para>
   TAF can be configured on the PHP OCI8 side or in the database
   configuration. If both are configured, database-side settings
   take precedence.
  </para>
  <para>
   Configure TAF in PHP OCI8 (the client side) by including the
   FAILOVER_MODE parameter in the CONNECT_DATA portion of a
   connect descriptor. See Configuring Transparent Application
   Failover in <link xlink:href="&url.oracle.taf.clientconfig;">
   Oracle Database Net Services Administrator's Guide</link> for
   more information about client side configuration of TAF.
  </para>
  <para>
   An example tnsnames.ora to configure TAF reconnecting to the
   same database instance:
  </para>
  <para>
    <informalexample>
     <screen>
<![CDATA[
    ORCL =
      (DESCRIPTION =
        (ADDRESS = (PROTOCOL = TCP)(HOST = 127.0.0.1)(PORT = 1521))
        (CONNECT_DATA =
          (SERVICE_NAME = orclpdb1)
          (FAILOVER_MODE =
            (TYPE = SELECT)
            (METHOD = BASIC)
            (RETRIES = 20)
            (DELAY = 15))))
 ]]>
     </screen>
   </informalexample>
  </para>
  <para>
   Alternatively configure TAF on the database side by
   modifying the target service with
   <link xlink:href="&url.oracle.taf.srvctl;">srvctl</link>
   (for RAC) or the
   <link xlink:href="&url.oracle.taf.dbmsservice;">
   DBMS_SERVICE.MODIFY_SERVICE</link> packaged procedure
   (for single instance databases).
  </para>
 </section>
 <section>
  <title>Using TAF Callbacks in OCI8</title>
  <para>
   A TAF callback is an application function that can be
   registered to be called during failover. It is called
   several times while re-establishing the application's connection.
  </para>
  <para>
   Callback first occurs when a loss of connection is detected.
   This allows the application to act accordingly for the
   upcoming delay of the failover. If the failover is successful,
   the callback is invoked after connection is re-established
   and usable. At this time, the application can resynchronize
   session settings and take actions such as informing the user
   that failover occurred. If failover is unsuccessful, a
   callback occurs to inform the application that a failover did
   not take place and the connection is unusable.
  </para>
  <para>
   The interface of a TAF user-defined callback function is as
   follows:
  </para>
  <methodsynopsis>
   <type>int</type><methodname>userCallbackFn</methodname>
   <methodparam><type>resource</type><parameter>connection</parameter></methodparam>
   <methodparam><type>int</type><parameter>event</parameter></methodparam>
   <methodparam><type>int</type><parameter>type</parameter></methodparam>
  </methodsynopsis>
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>connection</parameter></term>
     <listitem>
      <para>
       The Oracle connection on which the TAF callback was
       registered via <function>oci_register_taf_callback</function>.
       The connection is not valid until the
       failover completes successfully.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>event</parameter></term>
     <listitem>
      <para>
       The failover event indicates the current status of
       the failover.
      </para>
      <para>
       <itemizedlist>
        <listitem>
         <para>
          <constant>OCI_FO_BEGIN</constant> indicates that
          failover has detected a lost connection and failover
          is starting.
         </para>
        </listitem>
        <listitem>
         <para>
          <constant>OCI_FO_END</constant> indicates successful
          completion of failover.
         </para>
        </listitem>
        <listitem>
         <para>
          <constant>OCI_FO_ABORT</constant> indicates that
          failover was unsuccessful, and there is no option
          of retrying.
         </para>
        </listitem>
        <listitem>
         <para>
          <constant>OCI_FO_ERROR</constant> also indicates
          that failover was unsuccessful, but it gives the
          application the opportunity to handle the error
          and return OCI_FO_RETRY to retry failover.
         </para>
        </listitem>
        <listitem>
         <para>
           <constant>OCI_FO_REAUTH</constant> indicates
           that an Oracle user has been re-authenticated.
         </para>
        </listitem>
       </itemizedlist>
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>type</parameter></term>
     <listitem>
      <para>
       The failover type. This lets the callback know what
       type of failover the application has requested. The
       usual values are as follows:
      </para>
      <para>
       <itemizedlist>
        <listitem>
         <para>
          <constant>OCI_FO_SESSION</constant> indicates that
          the user has requested only session failover. For
          example, if a user's connection is lost, then a
          new session is automatically created for the user
          on the backup. This type of failover does not
          attempt to recover SELECTs.
         </para>
        </listitem>
        <listitem>
         <para>
          <constant>OCI_FO_SELECT</constant> indicates that
          the user has requested SELECT failover as well.
          It allows users with open cursors to continue
          fetching from them after failure.
         </para>
        </listitem>
       </itemizedlist>
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>return value</parameter></term>
     <listitem>
      <para>
       <itemizedlist>
        <listitem>
         <para>
          <literal>0</literal> indicates the failover
          steps should continue normally.
         </para>
        </listitem>
        <listitem>
         <para>
          <constant>OCI_FO_RETRY</constant> indicates that
          the failover should be attempted again by Oracle.
          In case of an error while failing over to a new
          connection, TAF is able to retry the failover.
          Typically, the application code should sleep for
          a while before returning OCI_FO_RETRY.
         </para>
        </listitem>
       </itemizedlist>
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
  <example>
   <para>
    The following example registers a TAF callback
   </para>
   <programlisting role="php">
<![CDATA[
<?php

// Define userspace callback
class MyClass {
    public static $retry_count;
    public static function TAFCallback($conn, $event, $type)
    {
        switch ($event) {
            case OCI_FO_BEGIN:
                printf(" Failing Over ... Please stand by\n");
                printf(" Failover type was found to be %s \n",
                       (($type==OCI_FO_SESSION) ? "SESSION"
                        :(($type==OCI_FO_SELECT) ? "SELECT" : "UNKNOWN!")));
                self::$retry_count = 0;
                break;
            case OCI_FO_ABORT:
                // The application cannot continue using the database
                printf(" Failover aborted. Failover will not take place.\n");
                break;
            case OCI_FO_END:
                // Failover completes successfully. Inform users a failover occurs.
                printf(" Failover ended ... resuming services\n");
                break;
            case OCI_FO_REAUTH:
                printf(" Failed over user ... resuming services\n");
                // Replay any ALTER SESSION commands associated with this connection
                // eg. oci_parse($conn, ‘ALTER SESSION …’) ;
                break;
            case OCI_FO_ERROR:
                // Stop retrying if we have already attempted for 20 times.
                if (self::$retry_count >= 20)
                    return 0;
                printf(" Failover error received. Sleeping...\n");
                sleep(10);
                self::$retry_count++;
                return OCI_FO_RETRY; // retry failover
                break;
            default:
                printf("Bad Failover Event: %d.\n", $event);
                break;
        }
        return 0;
    }
}

$fn_name = 'MyClass::TAFCallback';

$conn = oci_connect('hr', 'welcome', 'orcl');
$sysconn = oci_connect('system', 'oracle', 'orcl');

// Use a privileged connection to construct a SQL statement that will initiate failover
$sql = <<< 'END'
select unique 'alter system disconnect session '''||sid||','||serial#||''''
from v$session_connect_info
where sid = sys_context('USERENV', 'SID')
END;

$s = oci_parse($conn, $sql);
oci_execute($s);
$r = oci_fetch_array($s);
$disconnectssql = $r[0];

oci_register_taf_callback($conn, $fn_name); // Register TAFCallback to Oracle TAF

print "Parsing user query\n";
$sql = "select systimestamp from dual";
$stmt = oci_parse($conn, $sql);

// For example, if a connection loss occurs at this point, oci_execute() will
// detect the loss and failover begins. During failover, oci_execute() will
// invoke the TAF callback function several times. If the failover is successful,
// a new connection is transparently created and oci_execute() will continue as
// usual. The connection session settings can be reset in the TAF callback
// function. If the failover is aborted, oci_execute() will return an error
// because a valid connection is not available.

// Disconnect the user, which initiates failover
print "Disconnecting the user\n";
$discsql = oci_parse($sysconn, $disconnectssql);
oci_execute($discsql);

print "Executing user query\n";
$e = oci_execute($stmt);
if (!$e) {
    $m = oci_error($stmt);
    trigger_error("Could not execute statement: ". $m['message'], E_USER_ERROR);
}
$row = oci_fetch_array($stmt);
print $row[0] . "\n";

// do other SQL statements with the new connection, if it is valid
// $stmt = oci_parse($conn,  . . .);

?>
]]>
   </programlisting>
  </example>
 </section>

 <section>
  &reftitle.seealso;
  <simplelist>
   <member><function>oci_register_taf_callback</function></member>
   <member><function>oci_unregister_taf_callback</function></member>
  </simplelist>
 </section>


</chapter>

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