LoadMonitor.php

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<?php
interface LoadMonitor {
    public function __construct( $parent );

    public function scaleLoads( &$loads, $group = false, $wiki = false );

    public function getLagTimes( $serverIndexes, $wiki );
}

class LoadMonitorNull implements LoadMonitor {
    public function __construct( $parent ) {
    }

    public function scaleLoads( &$loads, $group = false, $wiki = false ) {
    }

    public function getLagTimes( $serverIndexes, $wiki ) {
        return array_fill_keys( $serverIndexes, 0 );
    }
}

class LoadMonitorMySQL implements LoadMonitor {
    public $parent;
    protected $cache;

    public function __construct( $parent ) {
        global $wgMemc;

        $this->parent = $parent;
        $this->cache = $wgMemc ?: wfGetMainCache();
    }

    public function scaleLoads( &$loads, $group = false, $wiki = false ) {
    }

    public function getLagTimes( $serverIndexes, $wiki ) {
        if ( count( $serverIndexes ) == 1 && reset( $serverIndexes ) == 0 ) {
            // Single server only, just return zero without caching
            return array( 0 => 0 );
        }

        $section = new ProfileSection( __METHOD__ );

        $expiry = 5;
        $requestRate = 10;

        $cache = $this->cache;
        $masterName = $this->parent->getServerName( 0 );
        $memcKey = wfMemcKey( 'lag_times', $masterName );
        $times = $cache->get( $memcKey );
        if ( is_array( $times ) ) {
            # Randomly recache with probability rising over $expiry
            $elapsed = time() - $times['timestamp'];
            $chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
            if ( mt_rand( 0, $chance ) != 0 ) {
                unset( $times['timestamp'] ); // hide from caller

                return $times;
            }
            wfIncrStats( 'lag_cache_miss_expired' );
        } else {
            wfIncrStats( 'lag_cache_miss_absent' );
        }

        # Cache key missing or expired
        if ( $cache->add( "$memcKey:lock", 1, 10 ) ) {
            # Let this process alone update the cache value
            $unlocker = new ScopedCallback( function () use ( $cache, $memcKey ) {
                $cache->delete( $memcKey );
            } );
        } elseif ( is_array( $times ) ) {
            # Could not acquire lock but an old cache exists, so use it
            unset( $times['timestamp'] ); // hide from caller

            return $times;
        }

        $times = array();
        foreach ( $serverIndexes as $i ) {
            if ( $i == 0 ) { # Master
                $times[$i] = 0;
            } elseif ( false !== ( $conn = $this->parent->getAnyOpenConnection( $i ) ) ) {
                $times[$i] = $conn->getLag();
            } elseif ( false !== ( $conn = $this->parent->openConnection( $i, $wiki ) ) ) {
                $times[$i] = $conn->getLag();
                // Close the connection to avoid sleeper connections piling up.
                // Note that the caller will pick one of these DBs and reconnect,
                // which is slightly inefficient, but this only matters for the lag
                // time cache miss cache, which is far less common that cache hits.
                $this->parent->closeConnection( $conn );
            }
        }

        # Add a timestamp key so we know when it was cached
        $times['timestamp'] = time();
        $cache->set( $memcKey, $times, $expiry + 10 );
        unset( $times['timestamp'] ); // hide from caller

        return $times;
    }
}