LoadBalancer.php

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<?php
class LoadBalancer {
    private $mServers;
    private $mConns;
    private $mLoads;
    private $mGroupLoads;
    private $mAllowLagged;
    private $mWaitTimeout;

    private $mParentInfo;
    private $mLoadMonitorClass;
    private $mLoadMonitor;

    private $mErrorConnection;
    private $mReadIndex;
    private $mWaitForPos;
    private $mLaggedSlaveMode;
    private $mLastError = 'Unknown error';
    private $mLagTimes;

    function __construct( $params ) {
        if ( !isset( $params['servers'] ) ) {
            throw new MWException( __CLASS__ . ': missing servers parameter' );
        }
        $this->mServers = $params['servers'];
        $this->mWaitTimeout = 10;

        $this->mReadIndex = -1;
        $this->mWriteIndex = -1;
        $this->mConns = array(
            'local' => array(),
            'foreignUsed' => array(),
            'foreignFree' => array() );
        $this->mLoads = array();
        $this->mWaitForPos = false;
        $this->mLaggedSlaveMode = false;
        $this->mErrorConnection = false;
        $this->mAllowLagged = false;

        if ( isset( $params['loadMonitor'] ) ) {
            $this->mLoadMonitorClass = $params['loadMonitor'];
        } else {
            $master = reset( $params['servers'] );
            if ( isset( $master['type'] ) && $master['type'] === 'mysql' ) {
                $this->mLoadMonitorClass = 'LoadMonitorMySQL';
            } else {
                $this->mLoadMonitorClass = 'LoadMonitorNull';
            }
        }

        foreach ( $params['servers'] as $i => $server ) {
            $this->mLoads[$i] = $server['load'];
            if ( isset( $server['groupLoads'] ) ) {
                foreach ( $server['groupLoads'] as $group => $ratio ) {
                    if ( !isset( $this->mGroupLoads[$group] ) ) {
                        $this->mGroupLoads[$group] = array();
                    }
                    $this->mGroupLoads[$group][$i] = $ratio;
                }
            }
        }
    }

    function getLoadMonitor() {
        if ( !isset( $this->mLoadMonitor ) ) {
            $class = $this->mLoadMonitorClass;
            $this->mLoadMonitor = new $class( $this );
        }

        return $this->mLoadMonitor;
    }

    function parentInfo( $x = null ) {
        return wfSetVar( $this->mParentInfo, $x );
    }

    function pickRandom( $weights ) {
        return ArrayUtils::pickRandom( $weights );
    }

    function getRandomNonLagged( $loads, $wiki = false ) {
        # Unset excessively lagged servers
        $lags = $this->getLagTimes( $wiki );
        foreach ( $lags as $i => $lag ) {
            if ( $i != 0 ) {
                if ( $lag === false ) {
                    wfDebugLog( 'replication', "Server #$i is not replicating" );
                    unset( $loads[$i] );
                } elseif ( isset( $this->mServers[$i]['max lag'] ) && $lag > $this->mServers[$i]['max lag'] ) {
                    wfDebugLog( 'replication', "Server #$i is excessively lagged ($lag seconds)" );
                    unset( $loads[$i] );
                }
            }
        }

        # Find out if all the slaves with non-zero load are lagged
        $sum = 0;
        foreach ( $loads as $load ) {
            $sum += $load;
        }
        if ( $sum == 0 ) {
            # No appropriate DB servers except maybe the master and some slaves with zero load
            # Do NOT use the master
            # Instead, this function will return false, triggering read-only mode,
            # and a lagged slave will be used instead.
            return false;
        }

        if ( count( $loads ) == 0 ) {
            return false;
        }

        #wfDebugLog( 'connect', var_export( $loads, true ) );

        # Return a random representative of the remainder
        return ArrayUtils::pickRandom( $loads );
    }

    function getReaderIndex( $group = false, $wiki = false ) {
        global $wgReadOnly, $wgDBtype;

        # @todo FIXME: For now, only go through all this for mysql databases
        if ( $wgDBtype != 'mysql' ) {
            return $this->getWriterIndex();
        }

        if ( count( $this->mServers ) == 1 ) {
            # Skip the load balancing if there's only one server
            return 0;
        } elseif ( $group === false && $this->mReadIndex >= 0 ) {
            # Shortcut if generic reader exists already
            return $this->mReadIndex;
        }

        $section = new ProfileSection( __METHOD__ );

        # Find the relevant load array
        if ( $group !== false ) {
            if ( isset( $this->mGroupLoads[$group] ) ) {
                $nonErrorLoads = $this->mGroupLoads[$group];
            } else {
                # No loads for this group, return false and the caller can use some other group
                wfDebug( __METHOD__ . ": no loads for group $group\n" );

                return false;
            }
        } else {
            $nonErrorLoads = $this->mLoads;
        }

        if ( !count( $nonErrorLoads ) ) {
            throw new MWException( "Empty server array given to LoadBalancer" );
        }

        # Scale the configured load ratios according to the dynamic load (if the load monitor supports it)
        $this->getLoadMonitor()->scaleLoads( $nonErrorLoads, $group, $wiki );

        $laggedSlaveMode = false;

        # No server found yet
        $i = false;
        # First try quickly looking through the available servers for a server that
        # meets our criteria
        $currentLoads = $nonErrorLoads;
        while ( count( $currentLoads ) ) {
            if ( $wgReadOnly || $this->mAllowLagged || $laggedSlaveMode ) {
                $i = ArrayUtils::pickRandom( $currentLoads );
            } else {
                $i = $this->getRandomNonLagged( $currentLoads, $wiki );
                if ( $i === false && count( $currentLoads ) != 0 ) {
                    # All slaves lagged. Switch to read-only mode
                    wfDebugLog( 'replication', "All slaves lagged. Switch to read-only mode" );
                    $wgReadOnly = 'The database has been automatically locked ' .
                        'while the slave database servers catch up to the master';
                    $i = ArrayUtils::pickRandom( $currentLoads );
                    $laggedSlaveMode = true;
                }
            }

            if ( $i === false ) {
                # pickRandom() returned false
                # This is permanent and means the configuration or the load monitor
                # wants us to return false.
                wfDebugLog( 'connect', __METHOD__ . ": pickRandom() returned false" );

                return false;
            }

            wfDebugLog( 'connect', __METHOD__ .
                ": Using reader #$i: {$this->mServers[$i]['host']}..." );

            $conn = $this->openConnection( $i, $wiki );
            if ( !$conn ) {
                wfDebugLog( 'connect', __METHOD__ . ": Failed connecting to $i/$wiki" );
                unset( $nonErrorLoads[$i] );
                unset( $currentLoads[$i] );
                $i = false;
                continue;
            }

            // Decrement reference counter, we are finished with this connection.
            // It will be incremented for the caller later.
            if ( $wiki !== false ) {
                $this->reuseConnection( $conn );
            }

            # Return this server
            break;
        }

        # If all servers were down, quit now
        if ( !count( $nonErrorLoads ) ) {
            wfDebugLog( 'connect', "All servers down" );
        }

        if ( $i !== false ) {
            # Slave connection successful
            # Wait for the session master pos for a short time
            if ( $this->mWaitForPos && $i > 0 ) {
                if ( !$this->doWait( $i ) ) {
                    $this->mServers[$i]['slave pos'] = $conn->getSlavePos();
                }
            }
            if ( $this->mReadIndex <= 0 && $this->mLoads[$i] > 0 && $group !== false ) {
                $this->mReadIndex = $i;
            }
        }

        return $i;
    }

    function sleep( $t ) {
        wfProfileIn( __METHOD__ );
        wfDebug( __METHOD__ . ": waiting $t us\n" );
        usleep( $t );
        wfProfileOut( __METHOD__ );

        return $t;
    }

    public function waitFor( $pos ) {
        wfProfileIn( __METHOD__ );
        $this->mWaitForPos = $pos;
        $i = $this->mReadIndex;

        if ( $i > 0 ) {
            if ( !$this->doWait( $i ) ) {
                $this->mServers[$i]['slave pos'] = $this->getAnyOpenConnection( $i )->getSlavePos();
                $this->mLaggedSlaveMode = true;
            }
        }
        wfProfileOut( __METHOD__ );
    }

    public function waitForAll( $pos, $timeout = null ) {
        wfProfileIn( __METHOD__ );
        $this->mWaitForPos = $pos;
        $serverCount = count( $this->mServers );

        $ok = true;
        for ( $i = 1; $i < $serverCount; $i++ ) {
            if ( $this->mLoads[$i] > 0 ) {
                $ok = $this->doWait( $i, true, $timeout ) && $ok;
            }
        }
        wfProfileOut( __METHOD__ );

        return $ok;
    }

    function getAnyOpenConnection( $i ) {
        foreach ( $this->mConns as $conns ) {
            if ( !empty( $conns[$i] ) ) {
                return reset( $conns[$i] );
            }
        }

        return false;
    }

    protected function doWait( $index, $open = false, $timeout = null ) {
        # Find a connection to wait on
        $conn = $this->getAnyOpenConnection( $index );
        if ( !$conn ) {
            if ( !$open ) {
                wfDebug( __METHOD__ . ": no connection open\n" );

                return false;
            } else {
                $conn = $this->openConnection( $index, '' );
                if ( !$conn ) {
                    wfDebug( __METHOD__ . ": failed to open connection\n" );

                    return false;
                }
            }
        }

        wfDebug( __METHOD__ . ": Waiting for slave #$index to catch up...\n" );
        $timeout = $timeout ?: $this->mWaitTimeout;
        $result = $conn->masterPosWait( $this->mWaitForPos, $timeout );

        if ( $result == -1 || is_null( $result ) ) {
            # Timed out waiting for slave, use master instead
            wfDebug( __METHOD__ . ": Timed out waiting for slave #$index pos {$this->mWaitForPos}\n" );

            return false;
        } else {
            wfDebug( __METHOD__ . ": Done\n" );

            return true;
        }
    }

    public function &getConnection( $i, $groups = array(), $wiki = false ) {
        wfProfileIn( __METHOD__ );

        if ( $i === null || $i === false ) {
            wfProfileOut( __METHOD__ );
            throw new MWException( 'Attempt to call ' . __METHOD__ .
                ' with invalid server index' );
        }

        if ( $wiki === wfWikiID() ) {
            $wiki = false;
        }

        # Query groups
        if ( $i == DB_MASTER ) {
            $i = $this->getWriterIndex();
        } elseif ( !is_array( $groups ) ) {
            $groupIndex = $this->getReaderIndex( $groups, $wiki );
            if ( $groupIndex !== false ) {
                $serverName = $this->getServerName( $groupIndex );
                wfDebug( __METHOD__ . ": using server $serverName for group $groups\n" );
                $i = $groupIndex;
            }
        } else {
            foreach ( $groups as $group ) {
                $groupIndex = $this->getReaderIndex( $group, $wiki );
                if ( $groupIndex !== false ) {
                    $serverName = $this->getServerName( $groupIndex );
                    wfDebug( __METHOD__ . ": using server $serverName for group $group\n" );
                    $i = $groupIndex;
                    break;
                }
            }
        }

        # Operation-based index
        if ( $i == DB_SLAVE ) {
            $this->mLastError = 'Unknown error'; // reset error string
            $i = $this->getReaderIndex( false, $wiki );
            # Couldn't find a working server in getReaderIndex()?
            if ( $i === false ) {
                $this->mLastError = 'No working slave server: ' . $this->mLastError;
                wfProfileOut( __METHOD__ );

                return $this->reportConnectionError();
            }
        }

        # Now we have an explicit index into the servers array
        $conn = $this->openConnection( $i, $wiki );
        if ( !$conn ) {
            wfProfileOut( __METHOD__ );

            return $this->reportConnectionError();
        }

        wfProfileOut( __METHOD__ );

        return $conn;
    }

    public function reuseConnection( $conn ) {
        $serverIndex = $conn->getLBInfo( 'serverIndex' );
        $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
        if ( $serverIndex === null || $refCount === null ) {
            wfDebug( __METHOD__ . ": this connection was not opened as a foreign connection\n" );

            return;
        }

        $dbName = $conn->getDBname();
        $prefix = $conn->tablePrefix();
        if ( strval( $prefix ) !== '' ) {
            $wiki = "$dbName-$prefix";
        } else {
            $wiki = $dbName;
        }
        if ( $this->mConns['foreignUsed'][$serverIndex][$wiki] !== $conn ) {
            throw new MWException( __METHOD__ . ": connection not found, has " .
                "the connection been freed already?" );
        }
        $conn->setLBInfo( 'foreignPoolRefCount', --$refCount );
        if ( $refCount <= 0 ) {
            $this->mConns['foreignFree'][$serverIndex][$wiki] = $conn;
            unset( $this->mConns['foreignUsed'][$serverIndex][$wiki] );
            wfDebug( __METHOD__ . ": freed connection $serverIndex/$wiki\n" );
        } else {
            wfDebug( __METHOD__ . ": reference count for $serverIndex/$wiki reduced to $refCount\n" );
        }
    }

    public function getConnectionRef( $db, $groups = array(), $wiki = false ) {
        return new DBConnRef( $this, $this->getConnection( $db, $groups, $wiki ) );
    }

    public function getLazyConnectionRef( $db, $groups = array(), $wiki = false ) {
        return new DBConnRef( $this, array( $db, $groups, $wiki ) );
    }

    function openConnection( $i, $wiki = false ) {
        wfProfileIn( __METHOD__ );
        if ( $wiki !== false ) {
            $conn = $this->openForeignConnection( $i, $wiki );
            wfProfileOut( __METHOD__ );

            return $conn;
        }
        if ( isset( $this->mConns['local'][$i][0] ) ) {
            $conn = $this->mConns['local'][$i][0];
        } else {
            $server = $this->mServers[$i];
            $server['serverIndex'] = $i;
            $conn = $this->reallyOpenConnection( $server, false );
            if ( $conn->isOpen() ) {
                wfDebug( "Connected to database $i at {$this->mServers[$i]['host']}\n" );
                $this->mConns['local'][$i][0] = $conn;
            } else {
                wfDebug( "Failed to connect to database $i at {$this->mServers[$i]['host']}\n" );
                $this->mErrorConnection = $conn;
                $conn = false;
            }
        }
        wfProfileOut( __METHOD__ );

        return $conn;
    }

    function openForeignConnection( $i, $wiki ) {
        wfProfileIn( __METHOD__ );
        list( $dbName, $prefix ) = wfSplitWikiID( $wiki );
        if ( isset( $this->mConns['foreignUsed'][$i][$wiki] ) ) {
            // Reuse an already-used connection
            $conn = $this->mConns['foreignUsed'][$i][$wiki];
            wfDebug( __METHOD__ . ": reusing connection $i/$wiki\n" );
        } elseif ( isset( $this->mConns['foreignFree'][$i][$wiki] ) ) {
            // Reuse a free connection for the same wiki
            $conn = $this->mConns['foreignFree'][$i][$wiki];
            unset( $this->mConns['foreignFree'][$i][$wiki] );
            $this->mConns['foreignUsed'][$i][$wiki] = $conn;
            wfDebug( __METHOD__ . ": reusing free connection $i/$wiki\n" );
        } elseif ( !empty( $this->mConns['foreignFree'][$i] ) ) {
            // Reuse a connection from another wiki
            $conn = reset( $this->mConns['foreignFree'][$i] );
            $oldWiki = key( $this->mConns['foreignFree'][$i] );

            if ( !$conn->selectDB( $dbName ) ) {
                $this->mLastError = "Error selecting database $dbName on server " .
                    $conn->getServer() . " from client host " . wfHostname() . "\n";
                $this->mErrorConnection = $conn;
                $conn = false;
            } else {
                $conn->tablePrefix( $prefix );
                unset( $this->mConns['foreignFree'][$i][$oldWiki] );
                $this->mConns['foreignUsed'][$i][$wiki] = $conn;
                wfDebug( __METHOD__ . ": reusing free connection from $oldWiki for $wiki\n" );
            }
        } else {
            // Open a new connection
            $server = $this->mServers[$i];
            $server['serverIndex'] = $i;
            $server['foreignPoolRefCount'] = 0;
            $server['foreign'] = true;
            $conn = $this->reallyOpenConnection( $server, $dbName );
            if ( !$conn->isOpen() ) {
                wfDebug( __METHOD__ . ": error opening connection for $i/$wiki\n" );
                $this->mErrorConnection = $conn;
                $conn = false;
            } else {
                $conn->tablePrefix( $prefix );
                $this->mConns['foreignUsed'][$i][$wiki] = $conn;
                wfDebug( __METHOD__ . ": opened new connection for $i/$wiki\n" );
            }
        }

        // Increment reference count
        if ( $conn ) {
            $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
            $conn->setLBInfo( 'foreignPoolRefCount', $refCount + 1 );
        }
        wfProfileOut( __METHOD__ );

        return $conn;
    }

    function isOpen( $index ) {
        if ( !is_integer( $index ) ) {
            return false;
        }

        return (bool)$this->getAnyOpenConnection( $index );
    }

    function reallyOpenConnection( $server, $dbNameOverride = false ) {
        if ( !is_array( $server ) ) {
            throw new MWException( 'You must update your load-balancing configuration. ' .
                'See DefaultSettings.php entry for $wgDBservers.' );
        }

        if ( $dbNameOverride !== false ) {
            $server['dbname'] = $dbNameOverride;
        }

        # Create object
        try {
            $db = DatabaseBase::factory( $server['type'], $server );
        } catch ( DBConnectionError $e ) {
            // FIXME: This is probably the ugliest thing I have ever done to
            // PHP. I'm half-expecting it to segfault, just out of disgust. -- TS
            $db = $e->db;
        }

        $db->setLBInfo( $server );
        if ( isset( $server['fakeSlaveLag'] ) ) {
            $db->setFakeSlaveLag( $server['fakeSlaveLag'] );
        }
        if ( isset( $server['fakeMaster'] ) ) {
            $db->setFakeMaster( true );
        }

        return $db;
    }

    private function reportConnectionError() {
        $conn = $this->mErrorConnection; // The connection which caused the error

        if ( !is_object( $conn ) ) {
            // No last connection, probably due to all servers being too busy
            wfLogDBError( "LB failure with no last connection. Connection error: {$this->mLastError}" );

            // If all servers were busy, mLastError will contain something sensible
            throw new DBConnectionError( null, $this->mLastError );
        } else {
            $server = $conn->getProperty( 'mServer' );
            wfLogDBError( "Connection error: {$this->mLastError} ({$server})" );
            $conn->reportConnectionError( "{$this->mLastError} ({$server})" ); // throws DBConnectionError
        }

        return false; /* not reached */
    }

    function getWriterIndex() {
        return 0;
    }

    function haveIndex( $i ) {
        return array_key_exists( $i, $this->mServers );
    }

    function isNonZeroLoad( $i ) {
        return array_key_exists( $i, $this->mServers ) && $this->mLoads[$i] != 0;
    }

    function getServerCount() {
        return count( $this->mServers );
    }

    function getServerName( $i ) {
        if ( isset( $this->mServers[$i]['hostName'] ) ) {
            return $this->mServers[$i]['hostName'];
        } elseif ( isset( $this->mServers[$i]['host'] ) ) {
            return $this->mServers[$i]['host'];
        } else {
            return '';
        }
    }

    function getServerInfo( $i ) {
        if ( isset( $this->mServers[$i] ) ) {
            return $this->mServers[$i];
        } else {
            return false;
        }
    }

    function setServerInfo( $i, $serverInfo ) {
        $this->mServers[$i] = $serverInfo;
    }

    function getMasterPos() {
        # If this entire request was served from a slave without opening a connection to the
        # master (however unlikely that may be), then we can fetch the position from the slave.
        $masterConn = $this->getAnyOpenConnection( 0 );
        if ( !$masterConn ) {
            $serverCount = count( $this->mServers );
            for ( $i = 1; $i < $serverCount; $i++ ) {
                $conn = $this->getAnyOpenConnection( $i );
                if ( $conn ) {
                    wfDebug( "Master pos fetched from slave\n" );

                    return $conn->getSlavePos();
                }
            }
        } else {
            wfDebug( "Master pos fetched from master\n" );

            return $masterConn->getMasterPos();
        }

        return false;
    }

    function closeAll() {
        foreach ( $this->mConns as $conns2 ) {
            foreach ( $conns2 as $conns3 ) {
                foreach ( $conns3 as $conn ) {
                    $conn->close();
                }
            }
        }
        $this->mConns = array(
            'local' => array(),
            'foreignFree' => array(),
            'foreignUsed' => array(),
        );
    }

    function closeConnection( $conn ) {
        $done = false;
        foreach ( $this->mConns as $i1 => $conns2 ) {
            foreach ( $conns2 as $i2 => $conns3 ) {
                foreach ( $conns3 as $i3 => $candidateConn ) {
                    if ( $conn === $candidateConn ) {
                        $conn->close();
                        unset( $this->mConns[$i1][$i2][$i3] );
                        $done = true;
                        break;
                    }
                }
            }
        }
        if ( !$done ) {
            $conn->close();
        }
    }

    function commitAll() {
        foreach ( $this->mConns as $conns2 ) {
            foreach ( $conns2 as $conns3 ) {
                foreach ( $conns3 as $conn ) {
                    if ( $conn->trxLevel() ) {
                        $conn->commit( __METHOD__, 'flush' );
                    }
                }
            }
        }
    }

    function commitMasterChanges() {
        // Always 0, but who knows.. :)
        $masterIndex = $this->getWriterIndex();
        foreach ( $this->mConns as $conns2 ) {
            if ( empty( $conns2[$masterIndex] ) ) {
                continue;
            }
            foreach ( $conns2[$masterIndex] as $conn ) {
                if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
                    $conn->commit( __METHOD__, 'flush' );
                }
            }
        }
    }

    function rollbackMasterChanges() {
        // Always 0, but who knows.. :)
        $masterIndex = $this->getWriterIndex();
        foreach ( $this->mConns as $conns2 ) {
            if ( empty( $conns2[$masterIndex] ) ) {
                continue;
            }
            foreach ( $conns2[$masterIndex] as $conn ) {
                if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
                    $conn->rollback( __METHOD__, 'flush' );
                }
            }
        }
    }

    function hasMasterConnection() {
        return $this->isOpen( $this->getWriterIndex() );
    }

    function hasMasterChanges() {
        // Always 0, but who knows.. :)
        $masterIndex = $this->getWriterIndex();
        foreach ( $this->mConns as $conns2 ) {
            if ( empty( $conns2[$masterIndex] ) ) {
                continue;
            }
            foreach ( $conns2[$masterIndex] as $conn ) {
                if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
                    return true;
                }
            }
        }
        return false;
    }

    function waitTimeout( $value = null ) {
        return wfSetVar( $this->mWaitTimeout, $value );
    }

    function getLaggedSlaveMode() {
        return $this->mLaggedSlaveMode;
    }

    function allowLagged( $mode = null ) {
        if ( $mode === null ) {
            return $this->mAllowLagged;
        }
        $this->mAllowLagged = $mode;

        return $this->mAllowLagged;
    }

    function pingAll() {
        $success = true;
        foreach ( $this->mConns as $conns2 ) {
            foreach ( $conns2 as $conns3 ) {
                foreach ( $conns3 as $conn ) {
                    if ( !$conn->ping() ) {
                        $success = false;
                    }
                }
            }
        }

        return $success;
    }

    function forEachOpenConnection( $callback, $params = array() ) {
        foreach ( $this->mConns as $conns2 ) {
            foreach ( $conns2 as $conns3 ) {
                foreach ( $conns3 as $conn ) {
                    $mergedParams = array_merge( array( $conn ), $params );
                    call_user_func_array( $callback, $mergedParams );
                }
            }
        }
    }

    function getMaxLag( $wiki = false ) {
        $maxLag = -1;
        $host = '';
        $maxIndex = 0;

        if ( $this->getServerCount() <= 1 ) { // no replication = no lag
            return array( $host, $maxLag, $maxIndex );
        }

        // Try to get the max lag info from the server cache
        $key = 'loadbalancer:maxlag:cluster:' . $this->mServers[0]['host'];
        $cache = ObjectCache::newAccelerator( array(), 'hash' );
        $maxLagInfo = $cache->get( $key ); // (host, lag, index)

        // Fallback to connecting to each slave and getting the lag
        if ( !$maxLagInfo ) {
            foreach ( $this->mServers as $i => $conn ) {
                if ( $i == $this->getWriterIndex() ) {
                    continue; // nothing to check
                }
                $conn = false;
                if ( $wiki === false ) {
                    $conn = $this->getAnyOpenConnection( $i );
                }
                if ( !$conn ) {
                    $conn = $this->openConnection( $i, $wiki );
                }
                if ( !$conn ) {
                    continue;
                }
                $lag = $conn->getLag();
                if ( $lag > $maxLag ) {
                    $maxLag = $lag;
                    $host = $this->mServers[$i]['host'];
                    $maxIndex = $i;
                }
            }
            $maxLagInfo = array( $host, $maxLag, $maxIndex );
            $cache->set( $key, $maxLagInfo, 5 );
        }

        return $maxLagInfo;
    }

    function getLagTimes( $wiki = false ) {
        # Try process cache
        if ( isset( $this->mLagTimes ) ) {
            return $this->mLagTimes;
        }
        if ( $this->getServerCount() == 1 ) {
            # No replication
            $this->mLagTimes = array( 0 => 0 );
        } else {
            # Send the request to the load monitor
            $this->mLagTimes = $this->getLoadMonitor()->getLagTimes(
                array_keys( $this->mServers ), $wiki );
        }

        return $this->mLagTimes;
    }

    function safeGetLag( $conn ) {
        if ( $this->getServerCount() == 1 ) {
            return 0;
        } else {
            return $conn->getLag();
        }
    }

    function clearLagTimeCache() {
        $this->mLagTimes = null;
    }
}

class DBConnRef implements IDatabase {
    protected $lb;

    protected $conn;

    protected $params;

    public function __construct( LoadBalancer $lb, $conn ) {
        $this->lb = $lb;
        if ( $conn instanceof DatabaseBase ) {
            $this->conn = $conn;
        } else {
            $this->params = $conn;
        }
    }

    public function __call( $name, $arguments ) {
        if ( $this->conn === null ) {
            list( $db, $groups, $wiki ) = $this->params;
            $this->conn = $this->lb->getConnection( $db, $groups, $wiki );
        }

        return call_user_func_array( array( $this->conn, $name ), $arguments );
    }

    function __destruct() {
        if ( $this->conn !== null ) {
            $this->lb->reuseConnection( $this->conn );
        }
    }
}