Application Performance Monitoring (APM)

Since version 1.3, the MongoDB driver contains an API for performance monitoring. This API allows you to find out how long specific operations take by setting up subscribers. Each subscriber is required to implement one or more interfaces under the MongoDB\Driver\Monitoring namespace. Currently, only the MongoDB\Driver\Monitoring\CommandSubscriber interface is available.

The MongoDB\Driver\Monitoring\CommandSubscriber interface defines three methods: commandStarted, commandSucceeded, and commandFailed. Each of these three methods accept a single event argument of a specific class for the respective event. For example, the commandSucceeded's $event argument is a MongoDB\Driver\Monitoring\CommandSucceededEvent object.

In this tutorial we will implement a subscriber that creates a list of all the query profiles and the average time they took.

Subscriber Class Scaffolding

We start with the framework for our subscriber:


<?php

class QueryTimeCollector implements \MongoDB\Driver\Monitoring\CommandSubscriber
{
    public function commandStarted( \MongoDB\Driver\Monitoring\CommandStartedEvent $event )
    {
    }

    public function commandSucceeded( \MongoDB\Driver\Monitoring\CommandSucceededEvent $event )
    {
    }

    public function commandFailed( \MongoDB\Driver\Monitoring\CommandFailedEvent $event )
    {
    }
}

?>

Registering the Subscriber

Once a subscriber object is instantiated, it needs to be registered with the driver's monitoring system. This is done by calling MongoDB\Driver\Monitoring\addSubscriber().


<?php

\MongoDB\Driver\Monitoring\addSubscriber( new QueryTimeCollector() );

?>

Implementing the Logic

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 requestId field.

To record the average time per query shape, we will start by checking for a find command in the commandStarted event. We will then add an item to the pendingCommands property indexed by its requestId and with its value representing the query shape.

If we receive a corresponding commandSucceeded event with the same requestId, we add the duration of the event (from durationMicros) to the total time and increment the operation count.

If a corresponding commandFailed event is encountered, we just remove the entry from the pendingCommands property.


<?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 )
    {
        if ( array_key_exists( 'find', (array) $event->getCommand() ) )
        {
            $queryShape = $this->createQueryShape( (array) $event->getCommand()->filter );
            $this->pendingCommands[$event->getRequestId()] = $queryShape;
        }
    }

    public function commandSucceeded( \MongoDB\Driver\Monitoring\CommandSucceededEvent $event )
    {
        $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 )
    {
        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 );

?>