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
  60 /**
  61  * Basic MySQL load monitor with no external dependencies
  62  * Uses memcached to cache the replication lag for a short time
  63  *
  64  * @ingroup Database
  65  */
  66 class LoadMonitor_MySQL implements LoadMonitor {
  67
  68         /**
  69          * @var LoadBalancer
  70          */
  71         var $parent;
  72
  73         /**
  74          * @param LoadBalancer $parent
  75          */
  76         function __construct( $parent ) {
  77                 $this->parent = $parent;
  78         }
  79
  80         /**
  81          * @param $loads
  82          * @param $group bool
  83          * @param $wiki bool
  84          */
  85         function scaleLoads( &$loads, $group = false, $wiki = false ) {
  86         }
  87
  88         /**
  89          * @param $serverIndexes
  90          * @param $wiki
  91          * @return array
  92          */
  93         function getLagTimes( $serverIndexes, $wiki ) {
  94                 wfProfileIn( __METHOD__ );
  95                 $expiry = 5;
  96                 $requestRate = 10;
  97
  98                 global $wgMemc;
  99                 if ( empty( $wgMemc ) )
 100                         $wgMemc = wfGetMainCache();
 101
 102                 $masterName = $this->parent->getServerName( 0 );
 103                 $memcKey = wfMemcKey( 'lag_times', $masterName );
 104                 $times = $wgMemc->get( $memcKey );
 105                 if ( $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'] );
 111                                 wfProfileOut( __METHOD__ );
 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
 121                 $times = array();
 122                 foreach ( $serverIndexes as $i ) {
 123                         if ($i == 0) { # Master
 124                                 $times[$i] = 0;
 125                         } elseif ( false !== ( $conn = $this->parent->getAnyOpenConnection( $i ) ) ) {
 126                                 $times[$i] = $conn->getLag();
 127                         } elseif ( false !== ( $conn = $this->parent->openConnection( $i, $wiki ) ) ) {
 128                                 $times[$i] = $conn->getLag();
 129                         }
 130                 }
 131
 132                 # Add a timestamp key so we know when it was cached
 133                 $times['timestamp'] = time();
 134                 $wgMemc->set( $memcKey, $times, $expiry );
 135
 136                 # But don't give the timestamp to the caller
 137                 unset($times['timestamp']);
 138                 $lagTimes = $times;
 139
 140                 wfProfileOut( __METHOD__ );
 141                 return $lagTimes;
 142         }
 143
 144         /**
 145          * @param $conn DatabaseBase
 146          * @param $threshold
 147          * @return int
 148          */
 149         function postConnectionBackoff( $conn, $threshold ) {
 150                 if ( !$threshold ) {
 151                         return 0;
 152                 }
 153                 $status = $conn->getStatus("Thread%");
 154                 if ( $status['Threads_running'] > $threshold ) {
 155                         return $status['Threads_connected'];
 156                 } else {
 157                         return 0;
 158                 }
 159         }
 160 }
 161