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 $cache;
  79
  80         public function __construct( $parent ) {
  81                 global $wgMemc;
  82
  83                 $this->parent = $parent;
  84                 $this->cache = $wgMemc ?: wfGetMainCache();
  85         }
  86
  87         public function scaleLoads( &$loads, $group = false, $wiki = false ) {
  88         }
  89
  90         public function getLagTimes( $serverIndexes, $wiki ) {
  91                 if ( count( $serverIndexes ) == 1 && reset( $serverIndexes ) == 0 ) {
  92                         // Single server only, just return zero without caching
  93                         return array( 0 => 0 );
  94                 }
  95
  96                 $expiry = 5;
  97                 $requestRate = 10;
  98
  99                 $cache = $this->cache;
 100                 $masterName = $this->parent->getServerName( 0 );
 101                 $memcKey = wfMemcKey( 'lag_times', $masterName );
 102                 $times = $cache->get( $memcKey );
 103                 if ( is_array( $times ) ) {
 104                         # Randomly recache with probability rising over $expiry
 105                         $elapsed = time() - $times['timestamp'];
 106                         $chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
 107                         if ( mt_rand( 0, $chance ) != 0 ) {
 108                                 unset( $times['timestamp'] ); // hide from caller
 109
 110                                 return $times;
 111                         }
 112                         wfIncrStats( 'lag_cache_miss_expired' );
 113                 } else {
 114                         wfIncrStats( 'lag_cache_miss_absent' );
 115                 }
 116
 117                 # Cache key missing or expired
 118                 if ( $cache->add( "$memcKey:lock", 1, 10 ) ) {
 119                         # Let this process alone update the cache value
 120                         $unlocker = new ScopedCallback( function () use ( $cache, $memcKey ) {
 121                                 $cache->delete( $memcKey );
 122                         } );
 123                 } elseif ( is_array( $times ) ) {
 124                         # Could not acquire lock but an old cache exists, so use it
 125                         unset( $times['timestamp'] ); // hide from caller
 126
 127                         return $times;
 128                 }
 129
 130                 $times = array();
 131                 foreach ( $serverIndexes as $i ) {
 132                         if ( $i == 0 ) { # Master
 133                                 $times[$i] = 0;
 134                         } elseif ( false !== ( $conn = $this->parent->getAnyOpenConnection( $i ) ) ) {
 135                                 $times[$i] = $conn->getLag();
 136                         } elseif ( false !== ( $conn = $this->parent->openConnection( $i, $wiki ) ) ) {
 137                                 $times[$i] = $conn->getLag();
 138                                 // Close the connection to avoid sleeper connections piling up.
 139                                 // Note that the caller will pick one of these DBs and reconnect,
 140                                 // which is slightly inefficient, but this only matters for the lag
 141                                 // time cache miss cache, which is far less common that cache hits.
 142                                 $this->parent->closeConnection( $conn );
 143                         }
 144                 }
 145
 146                 # Add a timestamp key so we know when it was cached
 147                 $times['timestamp'] = time();
 148                 $cache->set( $memcKey, $times, $expiry + 10 );
 149                 unset( $times['timestamp'] ); // hide from caller
 150
 151                 return $times;
 152         }
 153 }