includes/db/LoadMonitor.php

   1 <?php
   2 /**
   3  * Database load monitoring.
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU General Public License as published by
   7  * the Free Software Foundation; either version 2 of the License, or
   8  * (at your option) any later version.
   9  *
  10  * This program is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  13  * GNU General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU General Public License along
  16  * with this program; if not, write to the Free Software Foundation, Inc.,
  17  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  18  * http://www.gnu.org/copyleft/gpl.html
  19  *
  20  * @file
  21  * @ingroup Database
  22  */
  23
  24 /**
  25  * An interface for database load monitoring
  26  *
  27  * @ingroup Database
  28  */
  29 interface LoadMonitor {
  30         /**
  31          * Construct a new LoadMonitor with a given LoadBalancer parent
  32          *
  33          * @param LoadBalancer $parent
  34          */
  35         public function __construct( $parent );
  36
  37         /**
  38          * Perform pre-connection load ratio adjustment.
  39          * @param array $loads
  40          * @param string|bool $group The selected query group. Default: false
  41          * @param string|bool $wiki Default: false
  42          */
  43         public function scaleLoads( &$loads, $group = false, $wiki = false );
  44
  45         /**
  46          * Return an estimate of replication lag for each server
  47          *
  48          * @param array $serverIndexes
  49          * @param string $wiki
  50          *
  51          * @return array Map of (server index => seconds)
  52          */
  53         public function getLagTimes( $serverIndexes, $wiki );
  54 }
  55
  56 class LoadMonitorNull implements LoadMonitor {
  57         public function __construct( $parent ) {
  58         }
  59
  60         public function scaleLoads( &$loads, $group = false, $wiki = false ) {
  61         }
  62
  63         public function getLagTimes( $serverIndexes, $wiki ) {
  64                 return array_fill_keys( $serverIndexes, 0 );
  65         }
  66 }
  67
  68 /**
  69  * Basic MySQL load monitor with no external dependencies
  70  * Uses memcached to cache the replication lag for a short time
  71  *
  72  * @ingroup Database
  73  */
  74 class LoadMonitorMySQL implements LoadMonitor {
  75         /** @var LoadBalancer */
  76         public $parent;
  77         /** @var BagOStuff */
  78         protected $srvCache;
  79         /** @var BagOStuff */
  80         protected $mainCache;
  81
  82         public function __construct( $parent ) {
  83                 global $wgMemc;
  84
  85                 $this->parent = $parent;
  86
  87                 $this->srvCache = ObjectCache::newAccelerator( array(), 'hash' );
  88                 $this->mainCache = $wgMemc ?: wfGetMainCache();
  89         }
  90
  91         public function scaleLoads( &$loads, $group = false, $wiki = false ) {
  92         }
  93
  94         public function getLagTimes( $serverIndexes, $wiki ) {
  95                 if ( count( $serverIndexes ) == 1 && reset( $serverIndexes ) == 0 ) {
  96                         # Single server only, just return zero without caching
  97                         return array( 0 => 0 );
  98                 }
  99
 100                 $key = $this->getLagTimeCacheKey();
 101                 # Randomize TTLs to reduce stampedes (4.0 - 5.0 sec)
 102                 $ttl = mt_rand( 4e6, 5e6 ) / 1e6;
 103                 # Keep keys around longer as fallbacks
 104                 $staleTTL = 60;
 105
 106                 # (a) Check the local APC cache
 107                 $value = $this->srvCache->get( $key );
 108                 if ( $value && $value['timestamp'] > ( microtime( true ) - $ttl ) ) {
 109                         wfDebugLog( 'replication',  __FUNCTION__ . ": got lag times ($key) from local cache" );
 110                         return $value['lagTimes']; // cache hit
 111                 }
 112                 $staleValue = $value ?: false;
 113
 114                 # (b) Check the shared cache and backfill APC
 115                 $value = $this->mainCache->get( $key );
 116                 if ( $value && $value['timestamp'] > ( microtime( true ) - $ttl ) ) {
 117                         $this->srvCache->set( $key, $value, $staleTTL );
 118                         wfDebugLog( 'replication',  __FUNCTION__ . ": got lag times ($key) from main cache" );
 119
 120                         return $value['lagTimes']; // cache hit
 121                 }
 122                 $staleValue = $value ?: $staleValue;
 123
 124                 # (c) Cache key missing or expired; regenerate and backfill
 125                 if ( $this->mainCache->lock( $key, 0, 10 ) ) {
 126                         # Let this process alone update the cache value
 127                         $cache = $this->mainCache;
 128                         $unlocker = new ScopedCallback( function () use ( $cache, $key ) {
 129                                 $cache->unlock( $key );
 130                         } );
 131                 } elseif ( $staleValue ) {
 132                         # Could not acquire lock but an old cache exists, so use it
 133                         return $value['lagTimes'];
 134                 }
 135
 136                 $lagTimes = array();
 137                 foreach ( $serverIndexes as $i ) {
 138                         if ( $i == 0 ) { # Master
 139                                 $lagTimes[$i] = 0;
 140                         } elseif ( false !== ( $conn = $this->parent->getAnyOpenConnection( $i ) ) ) {
 141                                 $lagTimes[$i] = $conn->getLag();
 142                         } elseif ( false !== ( $conn = $this->parent->openConnection( $i, $wiki ) ) ) {
 143                                 $lagTimes[$i] = $conn->getLag();
 144                                 # Close the connection to avoid sleeper connections piling up.
 145                                 # Note that the caller will pick one of these DBs and reconnect,
 146                                 # which is slightly inefficient, but this only matters for the lag
 147                                 # time cache miss cache, which is far less common that cache hits.
 148                                 $this->parent->closeConnection( $conn );
 149                         }
 150                 }
 151
 152                 # Add a timestamp key so we know when it was cached
 153                 $value = array( 'lagTimes' => $lagTimes, 'timestamp' => microtime( true ) );
 154                 $this->mainCache->set( $key, $value, $staleTTL );
 155                 $this->srvCache->set( $key, $value, $staleTTL );
 156                 wfDebugLog( 'replication',  __FUNCTION__ . ": re-calculated lag times ($key)" );
 157
 158                 return $value['lagTimes'];
 159         }
 160
 161         public function clearCaches() {
 162                 $key = $this->getLagTimeCacheKey();
 163                 $this->srvCache->delete( $key );
 164                 $this->mainCache->delete( $key );
 165         }
 166
 167         private function getLagTimeCacheKey() {
 168                 # Lag is per-server, not per-DB, so key on the master DB name
 169                 return wfForeignMemcKey( $this->parent->getServerName( 0 ), '', 'lag_times' );
 170         }
 171 }