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         function __construct( $parent );
  19
  20         /**
  21          * Perform pre-connection load ratio adjustment.
  22          * @param $loads Array
  23          * @param $group String: the selected query group
  24          * @param $wiki String
  25          */
  26         function scaleLoads( &$loads, $group = false, $wiki = false );
  27
  28         /**
  29          * Perform post-connection backoff.
  30          *
  31          * If the connection is in overload, this should return a backoff factor
  32          * which will be used to control polling time. The number of threads
  33          * connected is a good measure.
  34          *
  35          * If there is no overload, zero can be returned.
  36          *
  37          * A threshold thread count is given, the concrete class may compare this
  38          * to the running thread count. The threshold may be false, which indicates
  39          * that the sysadmin has not configured this feature.
  40          *
  41          * @param $conn DatabaseBase
  42          * @param $threshold Float
  43          */
  44         function postConnectionBackoff( $conn, $threshold );
  45
  46         /**
  47          * Return an estimate of replication lag for each server
  48          */
  49         function getLagTimes( $serverIndexes, $wiki );
  50 }
  51
  52
  53 /**
  54  * Basic MySQL load monitor with no external dependencies
  55  * Uses memcached to cache the replication lag for a short time
  56  *
  57  * @ingroup Database
  58  */
  59 class LoadMonitor_MySQL implements LoadMonitor {
  60         var $parent; // LoadBalancer
  61
  62         function __construct( $parent ) {
  63                 $this->parent = $parent;
  64         }
  65
  66         function scaleLoads( &$loads, $group = false, $wiki = false ) {
  67         }
  68
  69         function getLagTimes( $serverIndexes, $wiki ) {
  70                 wfProfileIn( __METHOD__ );
  71                 $expiry = 5;
  72                 $requestRate = 10;
  73
  74                 global $wgMemc;
  75                 if ( empty( $wgMemc ) )
  76                         $wgMemc = wfGetMainCache();
  77
  78                 $masterName = $this->parent->getServerName( 0 );
  79                 $memcKey = wfMemcKey( 'lag_times', $masterName );
  80                 $times = $wgMemc->get( $memcKey );
  81                 if ( $times ) {
  82                         # Randomly recache with probability rising over $expiry
  83                         $elapsed = time() - $times['timestamp'];
  84                         $chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
  85                         if ( mt_rand( 0, $chance ) != 0 ) {
  86                                 unset( $times['timestamp'] );
  87                                 wfProfileOut( __METHOD__ );
  88                                 return $times;
  89                         }
  90                         wfIncrStats( 'lag_cache_miss_expired' );
  91                 } else {
  92                         wfIncrStats( 'lag_cache_miss_absent' );
  93                 }
  94
  95                 # Cache key missing or expired
  96
  97                 $times = array();
  98                 foreach ( $serverIndexes as $i ) {
  99                         if ($i == 0) { # Master
 100                                 $times[$i] = 0;
 101                         } elseif ( false !== ( $conn = $this->parent->getAnyOpenConnection( $i ) ) ) {
 102                                 $times[$i] = $conn->getLag();
 103                         } elseif ( false !== ( $conn = $this->parent->openConnection( $i, $wiki ) ) ) {
 104                                 $times[$i] = $conn->getLag();
 105                         }
 106                 }
 107
 108                 # Add a timestamp key so we know when it was cached
 109                 $times['timestamp'] = time();
 110                 $wgMemc->set( $memcKey, $times, $expiry );
 111
 112                 # But don't give the timestamp to the caller
 113                 unset($times['timestamp']);
 114                 $lagTimes = $times;
 115
 116                 wfProfileOut( __METHOD__ );
 117                 return $lagTimes;
 118         }
 119
 120         function postConnectionBackoff( $conn, $threshold ) {
 121                 if ( !$threshold ) {
 122                         return 0;
 123                 }
 124                 $status = $conn->getStatus("Thread%");
 125                 if ( $status['Threads_running'] > $threshold ) {
 126                         return $status['Threads_connected'];
 127                 } else {
 128                         return 0;
 129                 }
 130         }
 131 }
 132