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   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
  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          $section = new ProfileSection( __METHOD__ );
  97  
  98          $expiry = 5;
  99          $requestRate = 10;
 100  
 101          $cache = $this->cache;
 102          $masterName = $this->parent->getServerName( 0 );
 103          $memcKey = wfMemcKey( 'lag_times', $masterName );
 104          $times = $cache->get( $memcKey );
 105          if ( is_array( $times ) ) {
 106              # Randomly recache with probability rising over $expiry
 107              $elapsed = time() - $times['timestamp'];
 108              $chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
 109              if ( mt_rand( 0, $chance ) != 0 ) {
 110                  unset( $times['timestamp'] ); // hide from caller
 111  
 112                  return $times;
 113              }
 114              wfIncrStats( 'lag_cache_miss_expired' );
 115          } else {
 116              wfIncrStats( 'lag_cache_miss_absent' );
 117          }
 118  
 119          # Cache key missing or expired
 120          if ( $cache->add( "$memcKey:lock", 1, 10 ) ) {
 121              # Let this process alone update the cache value
 122              $unlocker = new ScopedCallback( function () use ( $cache, $memcKey ) {
 123                  $cache->delete( $memcKey );
 124              } );
 125          } elseif ( is_array( $times ) ) {
 126              # Could not acquire lock but an old cache exists, so use it
 127              unset( $times['timestamp'] ); // hide from caller
 128  
 129              return $times;
 130          }
 131  
 132          $times = array();
 133          foreach ( $serverIndexes as $i ) {
 134              if ( $i == 0 ) { # Master
 135                  $times[$i] = 0;
 136              } elseif ( false !== ( $conn = $this->parent->getAnyOpenConnection( $i ) ) ) {
 137                  $times[$i] = $conn->getLag();
 138              } elseif ( false !== ( $conn = $this->parent->openConnection( $i, $wiki ) ) ) {
 139                  $times[$i] = $conn->getLag();
 140                  // Close the connection to avoid sleeper connections piling up.
 141                  // Note that the caller will pick one of these DBs and reconnect,
 142                  // which is slightly inefficient, but this only matters for the lag
 143                  // time cache miss cache, which is far less common that cache hits.
 144                  $this->parent->closeConnection( $conn );
 145              }
 146          }
 147  
 148          # Add a timestamp key so we know when it was cached
 149          $times['timestamp'] = time();
 150          $cache->set( $memcKey, $times, $expiry + 10 );
 151          unset( $times['timestamp'] ); // hide from caller
 152  
 153          return $times;
 154      }
 155  }