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 = $parent;
  73         }
  74
  75         function scaleLoads( &$loads, $group = false, $wiki = false ) {
  76         }
  77
  78         function getLagTimes( $serverIndexes, $wiki ) {
  79                 wfProfileIn( __METHOD__ );
  80                 $expiry = 5;
  81                 $requestRate = 10;
  82
  83                 global $wgMemc;
  84                 if ( empty( $wgMemc ) )
  85                         $wgMemc = wfGetMainCache();
  86
  87                 $masterName = $this->parent->getServerName( 0 );
  88                 $memcKey = wfMemcKey( 'lag_times', $masterName );
  89                 $times = $wgMemc->get( $memcKey );
  90                 if ( $times ) {
  91                         # Randomly recache with probability rising over $expiry
  92                         $elapsed = time() - $times['timestamp'];
  93                         $chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
  94                         if ( mt_rand( 0, $chance ) != 0 ) {
  95                                 unset( $times['timestamp'] );
  96                                 wfProfileOut( __METHOD__ );
  97                                 return $times;
  98                         }
  99                         wfIncrStats( 'lag_cache_miss_expired' );
 100                 } else {
 101                         wfIncrStats( 'lag_cache_miss_absent' );
 102                 }
 103
 104                 # Cache key missing or expired
 105
 106                 $times = array();
 107                 foreach ( $serverIndexes as $i ) {
 108                         if ($i == 0) { # Master
 109                                 $times[$i] = 0;
 110                         } elseif ( false !== ( $conn = $this->parent->getAnyOpenConnection( $i ) ) ) {
 111                                 $times[$i] = $conn->getLag();
 112                         } elseif ( false !== ( $conn = $this->parent->openConnection( $i, $wiki ) ) ) {
 113                                 $times[$i] = $conn->getLag();
 114                         }
 115                 }
 116
 117                 # Add a timestamp key so we know when it was cached
 118                 $times['timestamp'] = time();
 119                 $wgMemc->set( $memcKey, $times, $expiry );
 120
 121                 # But don't give the timestamp to the caller
 122                 unset($times['timestamp']);
 123                 $lagTimes = $times;
 124
 125                 wfProfileOut( __METHOD__ );
 126                 return $lagTimes;
 127         }
 128
 129         /**
 130          * @param $conn DatabaseBase
 131          * @param $threshold
 132          * @return int
 133          */
 134         function postConnectionBackoff( $conn, $threshold ) {
 135                 if ( !$threshold ) {
 136                         return 0;
 137                 }
 138                 $status = $conn->getStatus("Thread%");
 139                 if ( $status['Threads_running'] > $threshold ) {
 140                         return $status['Threads_connected'];
 141                 } else {
 142                         return 0;
 143                 }
 144         }
 145 }
 146