includes/db/LoadMonitor.php

   1 <?php
   2
   3 /**
   4  * An interface for database load monitoring
   5  */
   6
   7 interface LoadMonitor {
   8         /**
   9          * Construct a new LoadMonitor with a given LoadBalancer parent
  10          */
  11         function __construct( $parent );
  12
  13         /**
  14          * Perform pre-connection load ratio adjustment.
  15          * @param array $loads
  16          * @param string $group The selected query group
  17          * @param string $wiki
  18          */
  19         function scaleLoads( &$loads, $group = false, $wiki = false );
  20
  21         /**
  22          * Perform post-connection backoff.
  23          *
  24          * If the connection is in overload, this should return a backoff factor
  25          * which will be used to control polling time. The number of threads
  26          * connected is a good measure.
  27          *
  28          * If there is no overload, zero can be returned.
  29          *
  30          * A threshold thread count is given, the concrete class may compare this
  31          * to the running thread count. The threshold may be false, which indicates
  32          * that the sysadmin has not configured this feature.
  33          *
  34          * @param Database $conn
  35          * @param float $threshold
  36          */
  37         function postConnectionBackoff( $conn, $threshold );
  38
  39         /**
  40          * Return an estimate of replication lag for each server
  41          */
  42         function getLagTimes( $serverIndexes, $wiki );
  43 }
  44
  45
  46 /**
  47  * Basic MySQL load monitor with no external dependencies
  48  * Uses memcached to cache the replication lag for a short time
  49  */
  50
  51 class LoadMonitor_MySQL implements LoadMonitor {
  52         var $parent; // LoadBalancer
  53
  54         function __construct( $parent ) {
  55                 $this->parent = $parent;
  56         }
  57
  58         function scaleLoads( &$loads, $group = false, $wiki = false ) {
  59         }
  60
  61         function getLagTimes( $serverIndexes, $wiki ) {
  62                 wfProfileIn( __METHOD__ );
  63                 $expiry = 5;
  64                 $requestRate = 10;
  65
  66                 global $wgMemc;
  67                 if ( empty( $wgMemc ) )
  68                         $wgMemc = wfGetMainCache();
  69
  70                 $masterName = $this->parent->getServerName( 0 );
  71                 $memcKey = wfMemcKey( 'lag_times', $masterName );
  72                 $times = $wgMemc->get( $memcKey );
  73                 if ( $times ) {
  74                         # Randomly recache with probability rising over $expiry
  75                         $elapsed = time() - $times['timestamp'];
  76                         $chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
  77                         if ( mt_rand( 0, $chance ) != 0 ) {
  78                                 unset( $times['timestamp'] );
  79                                 wfProfileOut( __METHOD__ );
  80                                 return $times;
  81                         }
  82                         wfIncrStats( 'lag_cache_miss_expired' );
  83                 } else {
  84                         wfIncrStats( 'lag_cache_miss_absent' );
  85                 }
  86
  87                 # Cache key missing or expired
  88
  89                 $times = array();
  90                 foreach ( $serverIndexes as $i ) {
  91                         if ($i == 0) { # Master
  92                                 $times[$i] = 0;
  93                         } elseif ( false !== ( $conn = $this->parent->getAnyOpenConnection( $i ) ) ) {
  94                                 $times[$i] = $conn->getLag();
  95                         } elseif ( false !== ( $conn = $this->parent->openConnection( $i, $wiki ) ) ) {
  96                                 $times[$i] = $conn->getLag();
  97                         }
  98                 }
  99
 100                 # Add a timestamp key so we know when it was cached
 101                 $times['timestamp'] = time();
 102                 $wgMemc->set( $memcKey, $times, $expiry );
 103
 104                 # But don't give the timestamp to the caller
 105                 unset($times['timestamp']);
 106                 $lagTimes = $times;
 107
 108                 wfProfileOut( __METHOD__ );
 109                 return $lagTimes;
 110         }
 111
 112         function postConnectionBackoff( $conn, $threshold ) {
 113                 if ( !$threshold ) {
 114                         return 0;
 115                 }
 116                 $status = $conn->getStatus("Thread%");
 117                 if ( $status['Threads_running'] > $threshold ) {
 118                         return $status['Threads_connected'];
 119                 } else {
 120                         return 0;
 121                 }
 122         }
 123 }
 124