includes/db/LoadMonitor.php

   1 <?php
   2 /**
   3  * Database load monitoring
   4  *
   5  * @file
   6  * @ingroup Database
   7  */
   8
   9 /**
  10  * An interface for database load monitoring
  11  *
  12  * @ingroup Database
  13  */
  14 interface LoadMonitor {
  15         /**
  16          * Construct a new LoadMonitor with a given LoadBalancer parent
  17          *
  18          * @param LoadBalancer $parent
  19          */
  20         function __construct( $parent );
  21
  22         /**
  23          * Perform pre-connection load ratio adjustment.
  24          * @param $loads array
  25          * @param $group String: the selected query group
  26          * @param $wiki String
  27          */
  28         function scaleLoads( &$loads, $group = false, $wiki = false );
  29
  30         /**
  31          * Perform post-connection backoff.
  32          *
  33          * If the connection is in overload, this should return a backoff factor
  34          * which will be used to control polling time. The number of threads
  35          * connected is a good measure.
  36          *
  37          * If there is no overload, zero can be returned.
  38          *
  39          * A threshold thread count is given, the concrete class may compare this
  40          * to the running thread count. The threshold may be false, which indicates
  41          * that the sysadmin has not configured this feature.
  42          *
  43          * @param $conn DatabaseBase
  44          * @param $threshold Float
  45          */
  46         function postConnectionBackoff( $conn, $threshold );
  47
  48         /**
  49          * Return an estimate of replication lag for each server
  50          *
  51          * @param $serverIndexes
  52          * @param $wiki
  53          *
  54          * @return array
  55          */
  56         function getLagTimes( $serverIndexes, $wiki );
  57 }
  58
  59 class LoadMonitor_Null implements LoadMonitor {
  60         function __construct( $parent ) {
  61         }
  62
  63         function scaleLoads( &$loads, $group = false, $wiki = false ) {
  64         }
  65
  66         function postConnectionBackoff( $conn, $threshold ) {
  67         }
  68
  69         function getLagTimes( $serverIndexes, $wiki ) {
  70                 return array_fill_keys( $serverIndexes, 0 );
  71         }
  72 }
  73
  74
  75 /**
  76  * Basic MySQL load monitor with no external dependencies
  77  * Uses memcached to cache the replication lag for a short time
  78  *
  79  * @ingroup Database
  80  */
  81 class LoadMonitor_MySQL implements LoadMonitor {
  82
  83         /**
  84          * @var LoadBalancer
  85          */
  86         var $parent;
  87
  88         /**
  89          * @param LoadBalancer $parent
  90          */
  91         function __construct( $parent ) {
  92                 $this->parent = $parent;
  93         }
  94
  95         /**
  96          * @param $loads
  97          * @param $group bool
  98          * @param $wiki bool
  99          */
 100         function scaleLoads( &$loads, $group = false, $wiki = false ) {
 101         }
 102
 103         /**
 104          * @param $serverIndexes
 105          * @param $wiki
 106          * @return array
 107          */
 108         function getLagTimes( $serverIndexes, $wiki ) {
 109                 if ( count( $serverIndexes ) == 1 && reset( $serverIndexes ) == 0 ) {
 110                         // Single server only, just return zero without caching
 111                         return array( 0 => 0 );
 112                 }
 113
 114                 wfProfileIn( __METHOD__ );
 115                 $expiry = 5;
 116                 $requestRate = 10;
 117
 118                 global $wgMemc;
 119                 if ( empty( $wgMemc ) )
 120                         $wgMemc = wfGetMainCache();
 121
 122                 $masterName = $this->parent->getServerName( 0 );
 123                 $memcKey = wfMemcKey( 'lag_times', $masterName );
 124                 $times = $wgMemc->get( $memcKey );
 125                 if ( $times ) {
 126                         # Randomly recache with probability rising over $expiry
 127                         $elapsed = time() - $times['timestamp'];
 128                         $chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
 129                         if ( mt_rand( 0, $chance ) != 0 ) {
 130                                 unset( $times['timestamp'] );
 131                                 wfProfileOut( __METHOD__ );
 132                                 return $times;
 133                         }
 134                         wfIncrStats( 'lag_cache_miss_expired' );
 135                 } else {
 136                         wfIncrStats( 'lag_cache_miss_absent' );
 137                 }
 138
 139                 # Cache key missing or expired
 140
 141                 $times = array();
 142                 foreach ( $serverIndexes as $i ) {
 143                         if ($i == 0) { # Master
 144                                 $times[$i] = 0;
 145                         } elseif ( false !== ( $conn = $this->parent->getAnyOpenConnection( $i ) ) ) {
 146                                 $times[$i] = $conn->getLag();
 147                         } elseif ( false !== ( $conn = $this->parent->openConnection( $i, $wiki ) ) ) {
 148                                 $times[$i] = $conn->getLag();
 149                         }
 150                 }
 151
 152                 # Add a timestamp key so we know when it was cached
 153                 $times['timestamp'] = time();
 154                 $wgMemc->set( $memcKey, $times, $expiry );
 155
 156                 # But don't give the timestamp to the caller
 157                 unset($times['timestamp']);
 158                 $lagTimes = $times;
 159
 160                 wfProfileOut( __METHOD__ );
 161                 return $lagTimes;
 162         }
 163
 164         /**
 165          * @param $conn DatabaseBase
 166          * @param $threshold
 167          * @return int
 168          */
 169         function postConnectionBackoff( $conn, $threshold ) {
 170                 if ( !$threshold ) {
 171                         return 0;
 172                 }
 173                 $status = $conn->getMysqlStatus("Thread%");
 174                 if ( $status['Threads_running'] > $threshold ) {
 175                         $server = $conn->getProperty( 'mServer' );
 176                         wfLogDBError( "LB backoff from $server - Threads_running = {$status['Threads_running']}\n" );
 177                         return $status['Threads_connected'];
 178                 } else {
 179                         return 0;
 180                 }
 181         }
 182 }
 183