LoadMonitor.php

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

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

    function postConnectionBackoff( $conn, $threshold );

    function getLagTimes( $serverIndexes, $wiki );
}

class LoadMonitor_Null implements LoadMonitor {
    function __construct( $parent ) {
    }

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

    function postConnectionBackoff( $conn, $threshold ) {
    }

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

class LoadMonitor_MySQL implements LoadMonitor {

    var $parent;

    function __construct( $parent ) {
        $this->parent = $parent;
    }

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

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

        wfProfileIn( __METHOD__ );
        $expiry = 5;
        $requestRate = 10;

        global $wgMemc;
        if ( empty( $wgMemc ) ) {
            $wgMemc = wfGetMainCache();
        }

        $masterName = $this->parent->getServerName( 0 );
        $memcKey = wfMemcKey( 'lag_times', $masterName );
        $times = $wgMemc->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
                wfProfileOut( __METHOD__ );
                return $times;
            }
            wfIncrStats( 'lag_cache_miss_expired' );
        } else {
            wfIncrStats( 'lag_cache_miss_absent' );
        }

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

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

        wfProfileOut( __METHOD__ );
        return $times;
    }

    function postConnectionBackoff( $conn, $threshold ) {
        if ( !$threshold ) {
            return 0;
        }
        $status = $conn->getMysqlStatus( "Thread%" );
        if ( $status['Threads_running'] > $threshold ) {
            $server = $conn->getProperty( 'mServer' );
            wfLogDBError( "LB backoff from $server - Threads_running = {$status['Threads_running']}\n" );
            return $status['Threads_connected'];
        } else {
            return 0;
        }
    }
}