LoadMonitor.php

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<?php
use Psr\Log\LoggerInterface;
use Wikimedia\ScopedCallback;

class LoadMonitor implements ILoadMonitor {
    protected $parent;
    protected $srvCache;
    protected $mainCache;
    protected $replLogger;

    private $movingAveRatio;

    const VERSION = 1; // cache key version

    public function __construct(
        ILoadBalancer $lb, BagOStuff $srvCache, BagOStuff $cache, array $options = []
    ) {
        $this->parent = $lb;
        $this->srvCache = $srvCache;
        $this->mainCache = $cache;
        $this->replLogger = new \Psr\Log\NullLogger();

        $this->movingAveRatio = isset( $options['movingAveRatio'] )
            ? $options['movingAveRatio']
            : 0.1;
    }

    public function setLogger( LoggerInterface $logger ) {
        $this->replLogger = $logger;
    }

    public function scaleLoads( array &$weightByServer, $domain ) {
        $serverIndexes = array_keys( $weightByServer );
        $states = $this->getServerStates( $serverIndexes, $domain );
        $coefficientsByServer = $states['weightScales'];
        foreach ( $weightByServer as $i => $weight ) {
            if ( isset( $coefficientsByServer[$i] ) ) {
                $weightByServer[$i] = $weight * $coefficientsByServer[$i];
            } else { // server recently added to config?
                $host = $this->parent->getServerName( $i );
                $this->replLogger->error( __METHOD__ . ": host $host not in cache" );
            }
        }
    }

    public function getLagTimes( array $serverIndexes, $domain ) {
        $states = $this->getServerStates( $serverIndexes, $domain );

        return $states['lagTimes'];
    }

    protected function getServerStates( array $serverIndexes, $domain ) {
        $writerIndex = $this->parent->getWriterIndex();
        if ( count( $serverIndexes ) == 1 && reset( $serverIndexes ) == $writerIndex ) {
            # Single server only, just return zero without caching
            return [
                'lagTimes' => [ $writerIndex => 0 ],
                'weightScales' => [ $writerIndex => 1.0 ]
            ];
        }

        $key = $this->getCacheKey( $serverIndexes );
        # Randomize TTLs to reduce stampedes (4.0 - 5.0 sec)
        $ttl = mt_rand( 4e6, 5e6 ) / 1e6;
        # Keep keys around longer as fallbacks
        $staleTTL = 60;

        # (a) Check the local APC cache
        $value = $this->srvCache->get( $key );
        if ( $value && $value['timestamp'] > ( microtime( true ) - $ttl ) ) {
            $this->replLogger->debug( __METHOD__ . ": got lag times ($key) from local cache" );
            return $value; // cache hit
        }
        $staleValue = $value ?: false;

        # (b) Check the shared cache and backfill APC
        $value = $this->mainCache->get( $key );
        if ( $value && $value['timestamp'] > ( microtime( true ) - $ttl ) ) {
            $this->srvCache->set( $key, $value, $staleTTL );
            $this->replLogger->debug( __METHOD__ . ": got lag times ($key) from main cache" );

            return $value; // cache hit
        }
        $staleValue = $value ?: $staleValue;

        # (c) Cache key missing or expired; regenerate and backfill
        if ( $this->mainCache->lock( $key, 0, 10 ) ) {
            # Let this process alone update the cache value
            $cache = $this->mainCache;
            $unlocker = new ScopedCallback( function () use ( $cache, $key ) {
                $cache->unlock( $key );
            } );
        } elseif ( $staleValue ) {
            # Could not acquire lock but an old cache exists, so use it
            return $staleValue;
        }

        $lagTimes = [];
        $weightScales = [];
        $movAveRatio = $this->movingAveRatio;
        foreach ( $serverIndexes as $i ) {
            if ( $i == $this->parent->getWriterIndex() ) {
                $lagTimes[$i] = 0; // master always has no lag
                $weightScales[$i] = 1.0; // nominal weight
                continue;
            }

            $conn = $this->parent->getAnyOpenConnection( $i );
            if ( $conn ) {
                $close = false; // already open
            } else {
                $conn = $this->parent->openConnection( $i, $domain );
                $close = true; // new connection
            }

            $lastWeight = isset( $staleValue['weightScales'][$i] )
                ? $staleValue['weightScales'][$i]
                : 1.0;
            $coefficient = $this->getWeightScale( $i, $conn ?: null );
            $newWeight = $movAveRatio * $coefficient + ( 1 - $movAveRatio ) * $lastWeight;

            // Scale from 10% to 100% of nominal weight
            $weightScales[$i] = max( $newWeight, .10 );

            if ( !$conn ) {
                $lagTimes[$i] = false;
                $host = $this->parent->getServerName( $i );
                $this->replLogger->error( __METHOD__ . ": host $host is unreachable" );
                continue;
            }

            if ( $conn->getLBInfo( 'is static' ) ) {
                $lagTimes[$i] = 0;
            } else {
                $lagTimes[$i] = $conn->getLag();
                if ( $lagTimes[$i] === false ) {
                    $host = $this->parent->getServerName( $i );
                    $this->replLogger->error( __METHOD__ . ": host $host is not replicating?" );
                }
            }

            if ( $close ) {
                # 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
        $value = [
            'lagTimes' => $lagTimes,
            'weightScales' => $weightScales,
            'timestamp' => microtime( true )
        ];
        $this->mainCache->set( $key, $value, $staleTTL );
        $this->srvCache->set( $key, $value, $staleTTL );
        $this->replLogger->info( __METHOD__ . ": re-calculated lag times ($key)" );

        return $value;
    }

    protected function getWeightScale( $index, IDatabase $conn = null ) {
        return $conn ? 1.0 : 0.0;
    }

    private function getCacheKey( array $serverIndexes ) {
        sort( $serverIndexes );
        // Lag is per-server, not per-DB, so key on the master DB name
        return $this->srvCache->makeGlobalKey(
            'lag-times',
            self::VERSION,
            $this->parent->getServerName( $this->parent->getWriterIndex() ),
            implode( '-', $serverIndexes )
        );
    }
}