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

                # Cache key missing or expired

                $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 );

                # But don't give the timestamp to the caller
                unset($times['timestamp']);
                $lagTimes = $times;

                wfProfileOut( __METHOD__ );
                return $lagTimes;
        }

        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;
                }
        }
}