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         function getLagTimes( $serverIndexes, $wiki );
  52 }
  53
  54
  55 /**
  56  * Basic MySQL load monitor with no external dependencies
  57  * Uses memcached to cache the replication lag for a short time
  58  *
  59  * @ingroup Database
  60  */
  61 class LoadMonitor_MySQL implements LoadMonitor {
  62
  63         /**
  64          * @var LoadBalancer
  65          */
  66         var $parent;
  67
  68         /**
  69          * @param LoadBalancer $parent
  70          */
  71         function __construct( $parent ) {
  72                 $this->parent = $parent;
  73         }
  74
  75         /**
  76          * @param $loads
  77          * @param $group bool
  78          * @param $wiki bool
  79          */
  80         function scaleLoads( &$loads, $group = false, $wiki = false ) {
  81         }
  82
  83         /**
  84          * @param $serverIndexes
  85          * @param $wiki
  86          * @return array
  87          */
  88         function getLagTimes( $serverIndexes, $wiki ) {
  89                 wfProfileIn( __METHOD__ );
  90                 $expiry = 5;
  91                 $requestRate = 10;
  92
  93                 global $wgMemc;
  94                 if ( empty( $wgMemc ) )
  95                         $wgMemc = wfGetMainCache();
  96
  97                 $masterName = $this->parent->getServerName( 0 );
  98                 $memcKey = wfMemcKey( 'lag_times', $masterName );
  99                 $times = $wgMemc->get( $memcKey );
 100                 if ( $times ) {
 101                         # Randomly recache with probability rising over $expiry
 102                         $elapsed = time() - $times['timestamp'];
 103                         $chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
 104                         if ( mt_rand( 0, $chance ) != 0 ) {
 105                                 unset( $times['timestamp'] );
 106                                 wfProfileOut( __METHOD__ );
 107                                 return $times;
 108                         }
 109                         wfIncrStats( 'lag_cache_miss_expired' );
 110                 } else {
 111                         wfIncrStats( 'lag_cache_miss_absent' );
 112                 }
 113
 114                 # Cache key missing or expired
 115
 116                 $times = array();
 117                 foreach ( $serverIndexes as $i ) {
 118                         if ($i == 0) { # Master
 119                                 $times[$i] = 0;
 120                         } elseif ( false !== ( $conn = $this->parent->getAnyOpenConnection( $i ) ) ) {
 121                                 $times[$i] = $conn->getLag();
 122                         } elseif ( false !== ( $conn = $this->parent->openConnection( $i, $wiki ) ) ) {
 123                                 $times[$i] = $conn->getLag();
 124                         }
 125                 }
 126
 127                 # Add a timestamp key so we know when it was cached
 128                 $times['timestamp'] = time();
 129                 $wgMemc->set( $memcKey, $times, $expiry );
 130
 131                 # But don't give the timestamp to the caller
 132                 unset($times['timestamp']);
 133                 $lagTimes = $times;
 134
 135                 wfProfileOut( __METHOD__ );
 136                 return $lagTimes;
 137         }
 138
 139         /**
 140          * @param $conn DatabaseBase
 141          * @param $threshold
 142          * @return int
 143          */
 144         function postConnectionBackoff( $conn, $threshold ) {
 145                 if ( !$threshold ) {
 146                         return 0;
 147                 }
 148                 $status = $conn->getStatus("Thread%");
 149                 if ( $status['Threads_running'] > $threshold ) {
 150                         return $status['Threads_connected'];
 151                 } else {
 152                         return 0;
 153                 }
 154         }
 155 }
 156