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         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 $wiki Default: false
  42          */
  43         function scaleLoads( &$loads, $group = false, $wiki = false );
  44
  45         /**
  46          * Perform post-connection backoff.
  47          *
  48          * If the connection is in overload, this should return a backoff factor
  49          * which will be used to control polling time. The number of threads
  50          * connected is a good measure.
  51          *
  52          * If there is no overload, zero can be returned.
  53          *
  54          * A threshold thread count is given, the concrete class may compare this
  55          * to the running thread count. The threshold may be false, which indicates
  56          * that the sysadmin has not configured this feature.
  57          *
  58          * @param DatabaseBase $conn
  59          * @param float $threshold
  60          */
  61         function postConnectionBackoff( $conn, $threshold );
  62
  63         /**
  64          * Return an estimate of replication lag for each server
  65          *
  66          * @param array $serverIndexes
  67          * @param string $wiki
  68          *
  69          * @return array
  70          */
  71         function getLagTimes( $serverIndexes, $wiki );
  72 }
  73
  74 class LoadMonitorNull implements LoadMonitor {
  75         function __construct( $parent ) {
  76         }
  77
  78         function scaleLoads( &$loads, $group = false, $wiki = false ) {
  79         }
  80
  81         function postConnectionBackoff( $conn, $threshold ) {
  82         }
  83
  84         /**
  85          * @param array $serverIndexes
  86          * @param string $wiki
  87          * @return array
  88          */
  89         function getLagTimes( $serverIndexes, $wiki ) {
  90                 return array_fill_keys( $serverIndexes, 0 );
  91         }
  92 }
  93
  94 /**
  95  * Basic MySQL load monitor with no external dependencies
  96  * Uses memcached to cache the replication lag for a short time
  97  *
  98  * @ingroup Database
  99  */
 100 class LoadMonitorMySQL implements LoadMonitor {
 101         /** @var LoadBalancer */
 102         public $parent;
 103
 104         /**
 105          * @param LoadBalancer $parent
 106          */
 107         function __construct( $parent ) {
 108                 $this->parent = $parent;
 109         }
 110
 111         /**
 112          * @param array $loads
 113          * @param bool $group
 114          * @param bool $wiki
 115          */
 116         function scaleLoads( &$loads, $group = false, $wiki = false ) {
 117         }
 118
 119         /**
 120          * @param array $serverIndexes
 121          * @param string $wiki
 122          * @return array
 123          */
 124         function getLagTimes( $serverIndexes, $wiki ) {
 125                 if ( count( $serverIndexes ) == 1 && reset( $serverIndexes ) == 0 ) {
 126                         // Single server only, just return zero without caching
 127                         return array( 0 => 0 );
 128                 }
 129
 130                 wfProfileIn( __METHOD__ );
 131                 $expiry = 5;
 132                 $requestRate = 10;
 133
 134                 global $wgMemc;
 135                 if ( empty( $wgMemc ) ) {
 136                         $wgMemc = wfGetMainCache();
 137                 }
 138
 139                 $masterName = $this->parent->getServerName( 0 );
 140                 $memcKey = wfMemcKey( 'lag_times', $masterName );
 141                 $times = $wgMemc->get( $memcKey );
 142                 if ( is_array( $times ) ) {
 143                         # Randomly recache with probability rising over $expiry
 144                         $elapsed = time() - $times['timestamp'];
 145                         $chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
 146                         if ( mt_rand( 0, $chance ) != 0 ) {
 147                                 unset( $times['timestamp'] ); // hide from caller
 148                                 wfProfileOut( __METHOD__ );
 149
 150                                 return $times;
 151                         }
 152                         wfIncrStats( 'lag_cache_miss_expired' );
 153                 } else {
 154                         wfIncrStats( 'lag_cache_miss_absent' );
 155                 }
 156
 157                 # Cache key missing or expired
 158                 if ( $wgMemc->add( "$memcKey:lock", 1, 10 ) ) {
 159                         # Let this process alone update the cache value
 160                         $unlocker = new ScopedCallback( function () use ( $wgMemc, $memcKey ) {
 161                                 $wgMemc->delete( $memcKey );
 162                         } );
 163                 } elseif ( is_array( $times ) ) {
 164                         # Could not acquire lock but an old cache exists, so use it
 165                         unset( $times['timestamp'] ); // hide from caller
 166                         wfProfileOut( __METHOD__ );
 167
 168                         return $times;
 169                 }
 170
 171                 $times = array();
 172                 foreach ( $serverIndexes as $i ) {
 173                         if ( $i == 0 ) { # Master
 174                                 $times[$i] = 0;
 175                         } elseif ( false !== ( $conn = $this->parent->getAnyOpenConnection( $i ) ) ) {
 176                                 $times[$i] = $conn->getLag();
 177                         } elseif ( false !== ( $conn = $this->parent->openConnection( $i, $wiki ) ) ) {
 178                                 $times[$i] = $conn->getLag();
 179                         }
 180                 }
 181
 182                 # Add a timestamp key so we know when it was cached
 183                 $times['timestamp'] = time();
 184                 $wgMemc->set( $memcKey, $times, $expiry + 10 );
 185                 unset( $times['timestamp'] ); // hide from caller
 186
 187                 wfProfileOut( __METHOD__ );
 188
 189                 return $times;
 190         }
 191
 192         /**
 193          * @param DatabaseBase|DatabaseMySQLBase $conn
 194          * @param int $threshold
 195          * @return int
 196          */
 197         function postConnectionBackoff( $conn, $threshold ) {
 198                 if ( !$threshold ) {
 199                         return 0;
 200                 }
 201                 $status = $conn->getMysqlStatus( "Thread%" );
 202                 if ( $status['Threads_running'] > $threshold ) {
 203                         $server = $conn->getProperty( 'mServer' );
 204                         wfLogDBError( "LB backoff from $server - Threads_running = {$status['Threads_running']}\n" );
 205
 206                         return $status['Threads_connected'];
 207                 } else {
 208                         return 0;
 209                 }
 210         }
 211 }