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<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<section xml:id="mongodb.tutorial.apm" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
<title>Application Performance Monitoring (APM)</title>
<para>
The extension contains an event subscriber API, which allows applications to
monitor commands and internal activity pertaining to the
<link xlink:href="&url.mongodb.sdam;">Server Discovery and Monitoring Specification</link>.
This tutorial will demonstrate command monitoring using the
<classname>MongoDB\Driver\Monitoring\CommandSubscriber</classname> interface.
</para>
<para>
The <classname>MongoDB\Driver\Monitoring\CommandSubscriber</classname>
interface defines three methods: <literal>commandStarted</literal>,
<literal>commandSucceeded</literal>, and <literal>commandFailed</literal>.
Each of these three methods accept a single <parameter>event</parameter>
argument of a specific class for the respective event. For example, the
<literal>commandSucceeded</literal>'s <parameter>$event</parameter> argument
is a <classname>MongoDB\Driver\Monitoring\CommandSucceededEvent</classname>
object.
</para>
<para>
In this tutorial we will implement a subscriber that creates a list of all
the query profiles and the average time they took.
</para>
<section>
<title>Subscriber Class Scaffolding</title>
<para>
We start with the framework for our subscriber:
</para>
<programlisting role="php">
<![CDATA[
<?php
class QueryTimeCollector implements \MongoDB\Driver\Monitoring\CommandSubscriber
{
public function commandStarted( \MongoDB\Driver\Monitoring\CommandStartedEvent $event ): void
{
}
public function commandSucceeded( \MongoDB\Driver\Monitoring\CommandSucceededEvent $event ): void
{
}
public function commandFailed( \MongoDB\Driver\Monitoring\CommandFailedEvent $event ): void
{
}
}
?>
]]>
</programlisting>
</section>
<section>
<title>Registering the Subscriber</title>
<para>
Once a subscriber object is instantiated, it needs to be registered with the
extensions's monitoring system. This is done by calling
<methodname>MongoDB\Driver\Monitoring\addSubscriber</methodname> or
<methodname>MongoDB\Driver\Manager::addSubscriber</methodname> to register
the subscriber globally or with a specific Manager, respectively.
</para>
<programlisting role="php">
<![CDATA[
<?php
\MongoDB\Driver\Monitoring\addSubscriber( new QueryTimeCollector() );
?>
]]>
</programlisting>
</section>
<section>
<title>Implementing the Logic</title>
<para>
With the object registered, the only thing left is to implement the logic
in the subscriber class. To correlate the two events that make up a
successfully executed command (commandStarted and commandSucceeded), each
event object exposes a <literal>requestId</literal> field.
</para>
<para>
To record the average time per query shape, we will start by checking for a
<literal>find</literal> command in the commandStarted event. We will then add
an item to the <literal>pendingCommands</literal> property indexed by its
<literal>requestId</literal> and with its value representing the query shape.
</para>
<para>
If we receive a corresponding commandSucceeded event with the same
<literal>requestId</literal>, we add the duration of the event (from
<literal>durationMicros</literal>) to the total time and increment the
operation count.
</para>
<para>
If a corresponding commandFailed event is encountered, we just remove the
entry from the <literal>pendingCommands</literal> property.
</para>
<programlisting role="php">
<![CDATA[
<?php
class QueryTimeCollector implements \MongoDB\Driver\Monitoring\CommandSubscriber
{
private $pendingCommands = [];
private $queryShapeStats = [];
/* Creates a query shape out of the filter argument. Right now it only
* takes the top level fields into account */
private function createQueryShape( array $filter )
{
return json_encode( array_keys( $filter ) );
}
public function commandStarted( \MongoDB\Driver\Monitoring\CommandStartedEvent $event ): void
{
if ( 'find' === $event->getCommandName() )
{
$queryShape = $this->createQueryShape( (array) $event->getCommand()->filter );
$this->pendingCommands[$event->getRequestId()] = $queryShape;
}
}
public function commandSucceeded( \MongoDB\Driver\Monitoring\CommandSucceededEvent $event ): void
{
$requestId = $event->getRequestId();
if ( array_key_exists( $requestId, $this->pendingCommands ) )
{
$this->queryShapeStats[$this->pendingCommands[$requestId]]['count']++;
$this->queryShapeStats[$this->pendingCommands[$requestId]]['duration'] += $event->getDurationMicros();
unset( $this->pendingCommands[$requestId] );
}
}
public function commandFailed( \MongoDB\Driver\Monitoring\CommandFailedEvent $event ): void
{
if ( array_key_exists( $event->getRequestId(), $this->pendingCommands ) )
{
unset( $this->pendingCommands[$event->getRequestId()] );
}
}
public function __destruct()
{
foreach( $this->queryShapeStats as $shape => $stats )
{
echo "Shape: ", $shape, " (", $stats['count'], ")\n ",
$stats['duration'] / $stats['count'], "µs\n\n";
}
}
}
$m = new \MongoDB\Driver\Manager( 'mongodb://localhost:27016' );
/* Add the subscriber */
\MongoDB\Driver\Monitoring\addSubscriber( new QueryTimeCollector() );
/* Do a bunch of queries */
$query = new \MongoDB\Driver\Query( [
'region_slug' => 'scotland-highlands', 'age' => [ '$gte' => 20 ]
] );
$cursor = $m->executeQuery( 'dramio.whisky', $query );
$query = new \MongoDB\Driver\Query( [
'region_slug' => 'scotland-lowlands', 'age' => [ '$gte' => 15 ]
] );
$cursor = $m->executeQuery( 'dramio.whisky', $query );
$query = new \MongoDB\Driver\Query( [ 'region_slug' => 'scotland-lowlands' ] );
$cursor = $m->executeQuery( 'dramio.whisky', $query );
?>
]]>
</programlisting>
</section>
</section>
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