# Application Performance Monitoring (APM)

The MongoDB driver contains an event subscriber API, which allows applications to monitor commands and internal activity pertaining to the Server Discovery and Monitoring Specification. This tutorial will demonstrate command monitoring using the `MongoDB\Driver\Monitoring\CommandSubscriber` interface.

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 ): void
        {
        }

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

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

    ?>

## 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` or `MongoDB\Driver\Manager::addSubscriber` to register the subscriber globally or with a specific Manager, respectively.

    <?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 ): void
        {
            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 ): 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 );

    ?>