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<title>OCI8 Transparent Application Failover (TAF) Support</title>
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</div><hr /><div id="oci8.taf" class="chapter">
<h1>OCI8 Transparent Application Failover (TAF) Support</h1>
<div class="section">
<p class="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.
</p>
<p class="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 <a href="http://docs.oracle.com/database/122/RACAD/ensuring-application-continuity.htm#RACAD-GUID-DEF850F6-27E9-428E-B8FC-530230D78AD2" class="link external">» RAC</a>
configuration, a hot standby database, or the same database
instance
itself. See <a href="http://docs.oracle.com/database/122/LNOCI/oci-programming-advanced-topics.htm#LNOCI16675" class="link external">» Oracle
Call Interface Programmer's Guide</a> for more information about
OCI TAF.
</p>
<p class="para">
An application callback can be registered
with <span class="function"><a href="function.oci-register-taf-callback.html" class="function">oci_register_taf_callback()</a></span>. 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.
</p>
<p class="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.
</p>
<p class="para">
An application callback can be removed by calling <span class="function"><a href="function.oci-unregister-taf-callback.html" class="function">oci_unregister_taf_callback()</a></span>.
</p>
</div>
<div class="section">
<h2 class="title">Configuring Transparent Application Failover</h2>
<p class="para">
TAF can be configured on the PHP OCI8 side or in the database
configuration. If both are configured, database-side settings
take precedence.
</p>
<p class="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 <a href="http://docs.oracle.com/database/122/NETAG/enabling-advanced-features.htm#NETAG338" class="link external">»
Oracle Database Net Services Administrator's Guide</a> for
more information about client side configuration of TAF.
</p>
<p class="para">
An example tnsnames.ora to configure TAF reconnecting to the
same database instance:
</p>
<p class="para">
<div class="informalexample">
<div class="example-contents screen">
<div class="cdata"><pre>
ORCL =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(HOST = 127.0.0.1)(PORT = 1521))
(CONNECT_DATA =
(SERVICE_NAME = orcl)
(FAILOVER_MODE =
(TYPE = SELECT)
(METHOD = BASIC)
(RETRIES = 20)
(DELAY = 15))))
</pre></div>
</div>
</div>
</p>
<p class="para">
Alternatively configure TAF on the database side by
modifying the target service with
<a href="http://docs.oracle.com/database/122/RACAD/workload-management-with-dynamic-database-services.htm#RACAD076" class="link external">» srvctl</a>
(for RAC) or the
<a href="http://docs.oracle.com/database/122/ARPLS/DBMS_SERVICE.htm#ARPLS092" class="link external">»
DBMS_SERVICE.MODIFY_SERVICE</a> packaged procedure
(for single instance databases).
</p>
</div>
<div class="section">
<h2 class="title">Using TAF Callbacks in OCI8</h2>
<p class="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.
</p>
<p class="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.
</p>
<p class="para">
The interface of a TAF user-defined callback function is as
follows:
</p>
<div class="methodsynopsis dc-description">
<span class="type">int</span> <span class="methodname"><strong>userCallbackFn</strong></span>
( <span class="methodparam"><span class="type">resource</span> <code class="parameter">$connection</code></span>
, <span class="methodparam"><span class="type">int</span> <code class="parameter">$event</code></span>
, <span class="methodparam"><span class="type">int</span> <code class="parameter">$type</code></span>
)</div>
<p class="para">
<dl>
<dt>
<code class="parameter">connection</code></dt>
<dd>
<p class="para">
The Oracle connection on which the TAF callback was
registered via <span class="function"><a href="function.oci-register-taf-callback.html" class="function">oci_register_taf_callback()</a></span>.
The connection is not valid until the
failover completes successfully.
</p>
</dd>
<dt>
<code class="parameter">event</code></dt>
<dd>
<p class="para">
The failover event indicates the current status of
the failover.
</p>
<p class="para">
<ul class="itemizedlist">
<li class="listitem">
<p class="para">
<strong><code>OCI_FO_BEGIN</code></strong> indicates that
failover has detected a lost connection and failover
is starting.
</p>
</li>
<li class="listitem">
<p class="para">
<strong><code>OCI_FO_END</code></strong> indicates successful
completion of failover.
</p>
</li>
<li class="listitem">
<p class="para">
<strong><code>OCI_FO_ABORT</code></strong> indicates that
failover was unsuccessful, and there is no option
of retrying.
</p>
</li>
<li class="listitem">
<p class="para">
<strong><code>OCI_FO_ERROR</code></strong> 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.
</p>
</li>
<li class="listitem">
<p class="para">
<strong><code>OCI_FO_REAUTH</code></strong> indicates
that an Oracle user has been re-authenticated.
</p>
</li>
</ul>
</p>
</dd>
<dt>
<code class="parameter">type</code></dt>
<dd>
<p class="para">
The failover type. This lets the callback know what
type of failover the application has requested. The
usual values are as follows:
</p>
<p class="para">
<ul class="itemizedlist">
<li class="listitem">
<p class="para">
<strong><code>OCI_FO_SESSION</code></strong> 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.
</p>
</li>
<li class="listitem">
<p class="para">
<strong><code>OCI_FO_SELECT</code></strong> indicates that
the user has requested SELECT failover as well.
It allows users with open cursors to continue
fetching from them after failure.
</p>
</li>
</ul>
</p>
</dd>
<dt>
<code class="parameter">return value</code></dt>
<dd>
<p class="para">
<ul class="itemizedlist">
<li class="listitem">
<p class="para">
<strong><code>0</code></strong> indicates the failover
steps should continue normally.
</p>
</li>
<li class="listitem">
<p class="para">
<strong><code>OCI_FO_RETRY</code></strong> 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.
</p>
</li>
</ul>
</p>
</dd>
</dl>
</p>
<div class="example" id="example-2423">
<div class="example-contents"><p>
The following example registers a TAF callback
</p></div>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /><br /></span><span style="color: #FF8000">// Define userspace callback<br /></span><span style="color: #007700">class </span><span style="color: #0000BB">MyClass </span><span style="color: #007700">{<br /> public static </span><span style="color: #0000BB">$retry_count</span><span style="color: #007700">;<br /> public static function </span><span style="color: #0000BB">TAFCallback</span><span style="color: #007700">(</span><span style="color: #0000BB">$conn</span><span style="color: #007700">, </span><span style="color: #0000BB">$event</span><span style="color: #007700">, </span><span style="color: #0000BB">$type</span><span style="color: #007700">)<br /> {<br /> switch (</span><span style="color: #0000BB">$event</span><span style="color: #007700">) {<br /> case </span><span style="color: #0000BB">OCI_FO_BEGIN</span><span style="color: #007700">:<br /> </span><span style="color: #0000BB">printf</span><span style="color: #007700">(</span><span style="color: #DD0000">" Failing Over ... Please stand by"</span><span style="color: #007700">);<br /> </span><span style="color: #0000BB">printf</span><span style="color: #007700">(</span><span style="color: #DD0000">" Failover type was found to be %s \n"</span><span style="color: #007700">,<br /> ((</span><span style="color: #0000BB">$type</span><span style="color: #007700">==</span><span style="color: #0000BB">OCI_FO_SESSION</span><span style="color: #007700">) ? </span><span style="color: #DD0000">"SESSION"<br /> </span><span style="color: #007700">:(</span><span style="color: #0000BB">$type</span><span style="color: #007700">==</span><span style="color: #0000BB">OCI_FO_SELECT</span><span style="color: #007700">) ? </span><span style="color: #DD0000">"SELECT"<br /> </span><span style="color: #007700">: </span><span style="color: #DD0000">"UNKNOWN!"</span><span style="color: #007700">));<br /> </span><span style="color: #0000BB">self</span><span style="color: #007700">::</span><span style="color: #0000BB">$retry_count </span><span style="color: #007700">= </span><span style="color: #0000BB">0</span><span style="color: #007700">;<br /> break;<br /> case </span><span style="color: #0000BB">OCI_FO_ABORT</span><span style="color: #007700">:<br /> </span><span style="color: #FF8000">// The application cannot continue using the database<br /> </span><span style="color: #0000BB">printf</span><span style="color: #007700">(</span><span style="color: #DD0000">" Failover aborted. Failover will not take place.\n"</span><span style="color: #007700">);<br /> break;<br /> case </span><span style="color: #0000BB">OCI_FO_END</span><span style="color: #007700">:<br /> </span><span style="color: #FF8000">// Failover completes successfully. Inform users a failover occurs.<br /> </span><span style="color: #0000BB">printf</span><span style="color: #007700">(</span><span style="color: #DD0000">" Failover ended ... resuming services\n"</span><span style="color: #007700">);<br /> break;<br /> case </span><span style="color: #0000BB">OCI_FO_REAUTH</span><span style="color: #007700">:<br /> </span><span style="color: #0000BB">printf</span><span style="color: #007700">(</span><span style="color: #DD0000">" Failed over user ... resuming services\n"</span><span style="color: #007700">);<br /> </span><span style="color: #FF8000">// Replay any ALTER SESSION commands associated with this connection<br /> // eg. oci_parse($conn, ‘ALTER SESSION …’) ;<br /> </span><span style="color: #007700">break;<br /> case </span><span style="color: #0000BB">OCI_FO_ERROR</span><span style="color: #007700">:<br /> </span><span style="color: #FF8000">// Stop retrying if we have already attempted for 20 times.<br /> </span><span style="color: #007700">if (</span><span style="color: #0000BB">self</span><span style="color: #007700">::</span><span style="color: #0000BB">$retry_count </span><span style="color: #007700">>= </span><span style="color: #0000BB">20</span><span style="color: #007700">)<br /> return </span><span style="color: #0000BB">0</span><span style="color: #007700">;<br /> </span><span style="color: #0000BB">printf</span><span style="color: #007700">(</span><span style="color: #DD0000">" Failover error received. Sleeping...\n"</span><span style="color: #007700">);<br /> </span><span style="color: #0000BB">sleep</span><span style="color: #007700">(</span><span style="color: #0000BB">10</span><span style="color: #007700">);<br /> </span><span style="color: #0000BB">self</span><span style="color: #007700">::</span><span style="color: #0000BB">$retry_count</span><span style="color: #007700">++;<br /> return </span><span style="color: #0000BB">OCI_FO_RETRY</span><span style="color: #007700">; </span><span style="color: #FF8000">// retry failover<br /> </span><span style="color: #007700">break;<br /> default:<br /> </span><span style="color: #0000BB">printf</span><span style="color: #007700">(</span><span style="color: #DD0000">"Bad Failover Event: %d.\n"</span><span style="color: #007700">, </span><span style="color: #0000BB">$event</span><span style="color: #007700">);<br /> break;<br /> }<br /> return </span><span style="color: #0000BB">0</span><span style="color: #007700">;<br /> }<br />}<br /><br /></span><span style="color: #0000BB">$conn </span><span style="color: #007700">= </span><span style="color: #0000BB">oci_connect</span><span style="color: #007700">(</span><span style="color: #DD0000">'hr'</span><span style="color: #007700">, </span><span style="color: #DD0000">'welcome'</span><span style="color: #007700">, </span><span style="color: #DD0000">'localhost/XE'</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">$fn_name </span><span style="color: #007700">= </span><span style="color: #DD0000">'MyClass::TAFCallback'</span><span style="color: #007700">;<br /><br /></span><span style="color: #0000BB">oci_register_taf_callback</span><span style="color: #007700">(</span><span style="color: #0000BB">$conn</span><span style="color: #007700">, </span><span style="color: #0000BB">$fn_name</span><span style="color: #007700">); </span><span style="color: #FF8000">// Register TAFCallback to Oracle TAF <br /><br /></span><span style="color: #0000BB">$sql </span><span style="color: #007700">= </span><span style="color: #DD0000">"SELECT col1 FROM mytab"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$stmt </span><span style="color: #007700">= </span><span style="color: #0000BB">oci_parse</span><span style="color: #007700">(</span><span style="color: #0000BB">$conn</span><span style="color: #007700">, </span><span style="color: #0000BB">$sql</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">oci_define_by_name</span><span style="color: #007700">(</span><span style="color: #0000BB">$stmt</span><span style="color: #007700">, </span><span style="color: #DD0000">'COL1'</span><span style="color: #007700">, </span><span style="color: #0000BB">$col1</span><span style="color: #007700">);<br /><br /></span><span style="color: #FF8000">// For example, if a connection loss occurs at this point, oci_execute() will<br />// detect the loss and failover begins. During failover, oci_execute() will<br />// invoke the TAF callback function several times. If the failover is successful,<br />// a new connection is transparently created and oci_execute() will continue as<br />// usual. The connection session settings can be reset in the TAF callback<br />// function. If the failover is aborted, oci_execute() will return an error <br />// because a valid connection is not available.<br /><br /></span><span style="color: #0000BB">$e </span><span style="color: #007700">= </span><span style="color: #0000BB">oci_execute</span><span style="color: #007700">(</span><span style="color: #0000BB">$stmt</span><span style="color: #007700">);<br />if (</span><span style="color: #0000BB">$e </span><span style="color: #007700">== </span><span style="color: #0000BB">false</span><span style="color: #007700">)<br />{<br /> </span><span style="color: #FF8000">// do error handling, if oci_execute() fails<br /> // var_dump(oci_error($stmt));<br /></span><span style="color: #007700">}<br />while (</span><span style="color: #0000BB">oci_fetch</span><span style="color: #007700">(</span><span style="color: #0000BB">$stmt</span><span style="color: #007700">))<br />{<br /> echo </span><span style="color: #DD0000">"COL1 value is </span><span style="color: #0000BB">$col1</span><span style="color: #DD0000"><br>\n"</span><span style="color: #007700">;<br />}<br /><br /></span><span style="color: #FF8000">// do other SQL statements with the new connection, if it is valid<br />// $stmt = oci_parse($conn, . . .); <br /><br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</div>
<div class="section">
<h2 class="title">See Also</h2>
<ul class="simplelist">
<li class="member"><span class="function"><a href="function.oci-register-taf-callback.html" class="function">oci_register_taf_callback()</a></span></li>
<li class="member"><span class="function"><a href="function.oci-unregister-taf-callback.html" class="function">oci_unregister_taf_callback()</a></span></li>
</ul>
</div>
</div>
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