--- /dev/null
+<?php
+/**
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ * @ingroup Database
+ */
+
+namespace Wikimedia\Rdbms;
+
+use Psr\Log\LoggerInterface;
+use Psr\Log\NullLogger;
+use Wikimedia\ScopedCallback;
+use BagOStuff;
+use WANObjectCache;
+
+/**
+ * Basic DB load monitor with no external dependencies
+ * Uses memcached to cache the replication lag for a short time
+ *
+ * @ingroup Database
+ */
+class LoadMonitor implements ILoadMonitor {
+ /** @var ILoadBalancer */
+ protected $parent;
+ /** @var BagOStuff */
+ protected $srvCache;
+ /** @var WANObjectCache */
+ protected $wanCache;
+ /** @var LoggerInterface */
+ protected $replLogger;
+
+ /** @var float Moving average ratio (e.g. 0.1 for 10% weight to new weight) */
+ private $movingAveRatio;
+
+ const VERSION = 1; // cache key version
+
+ public function __construct(
+ ILoadBalancer $lb, BagOStuff $srvCache, WANObjectCache $wCache, array $options = []
+ ) {
+ $this->parent = $lb;
+ $this->srvCache = $srvCache;
+ $this->wanCache = $wCache;
+ $this->replLogger = new 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->wanCache->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->srvCache->lock( $key, 0, 10 ) ) {
+ # Let only this process update the cache value on this server
+ $sCache = $this->srvCache;
+ /** @noinspection PhpUnusedLocalVariableInspection */
+ $unlocker = new ScopedCallback( function () use ( $sCache, $key ) {
+ $sCache->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, '' );
+ $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( $newWeight, 0.10 );
+
+ if ( !$conn ) {
+ $lagTimes[$i] = false;
+ $host = $this->parent->getServerName( $i );
+ $this->replLogger->error(
+ __METHOD__ . ": host {db_server} is unreachable",
+ [ 'db_server' => $host ]
+ );
+ 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 {db_server} is not replicating?",
+ [ 'db_server' => $host ]
+ );
+ }
+ }
+
+ 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->wanCache->set( $key, $value, $staleTTL );
+ $this->srvCache->set( $key, $value, $staleTTL );
+ $this->replLogger->info( __METHOD__ . ": re-calculated lag times ($key)" );
+
+ return $value;
+ }
+
+ /**
+ * @param int $index Server index
+ * @param IDatabase|null $conn Connection handle or null on connection failure
+ * @return float
+ */
+ 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 )
+ );
+ }
+}
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