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 class LoadMonitor_Null implements LoadMonitor {
  60         function __construct( $parent ) {
  61         }
  62
  63         function scaleLoads( &$loads, $group = false, $wiki = false ) {
  64         }
  65
  66         function postConnectionBackoff( $conn, $threshold ) {
  67         }
  68
  69         /**
  70          * @param $serverIndexes
  71          * @param $wiki
  72          * @return array
  73          */
  74         function getLagTimes( $serverIndexes, $wiki ) {
  75                 return array_fill_keys( $serverIndexes, 0 );
  76         }
  77 }
  78
  79 /**
  80  * Basic MySQL load monitor with no external dependencies
  81  * Uses memcached to cache the replication lag for a short time
  82  *
  83  * @ingroup Database
  84  */
  85 class LoadMonitor_MySQL implements LoadMonitor {
  86
  87         /**
  88          * @var LoadBalancer
  89          */
  90         var $parent;
  91
  92         /**
  93          * @param LoadBalancer $parent
  94          */
  95         function __construct( $parent ) {
  96                 $this->parent = $parent;
  97         }
  98
  99         /**
 100          * @param $loads
 101          * @param $group bool
 102          * @param $wiki bool
 103          */
 104         function scaleLoads( &$loads, $group = false, $wiki = false ) {
 105         }
 106
 107         /**
 108          * @param $serverIndexes
 109          * @param $wiki
 110          * @return array
 111          */
 112         function getLagTimes( $serverIndexes, $wiki ) {
 113                 if ( count( $serverIndexes ) == 1 && reset( $serverIndexes ) == 0 ) {
 114                         // Single server only, just return zero without caching
 115                         return array( 0 => 0 );
 116                 }
 117
 118                 wfProfileIn( __METHOD__ );
 119                 $expiry = 5;
 120                 $requestRate = 10;
 121
 122                 global $wgMemc;
 123                 if ( empty( $wgMemc ) )
 124                         $wgMemc = wfGetMainCache();
 125
 126                 $masterName = $this->parent->getServerName( 0 );
 127                 $memcKey = wfMemcKey( 'lag_times', $masterName );
 128                 $times = $wgMemc->get( $memcKey );
 129                 if ( $times ) {
 130                         # Randomly recache with probability rising over $expiry
 131                         $elapsed = time() - $times['timestamp'];
 132                         $chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
 133                         if ( mt_rand( 0, $chance ) != 0 ) {
 134                                 unset( $times['timestamp'] );
 135                                 wfProfileOut( __METHOD__ );
 136                                 return $times;
 137                         }
 138                         wfIncrStats( 'lag_cache_miss_expired' );
 139                 } else {
 140                         wfIncrStats( 'lag_cache_miss_absent' );
 141                 }
 142
 143                 # Cache key missing or expired
 144
 145                 $times = array();
 146                 foreach ( $serverIndexes as $i ) {
 147                         if ($i == 0) { # Master
 148                                 $times[$i] = 0;
 149                         } elseif ( false !== ( $conn = $this->parent->getAnyOpenConnection( $i ) ) ) {
 150                                 $times[$i] = $conn->getLag();
 151                         } elseif ( false !== ( $conn = $this->parent->openConnection( $i, $wiki ) ) ) {
 152                                 $times[$i] = $conn->getLag();
 153                         }
 154                 }
 155
 156                 # Add a timestamp key so we know when it was cached
 157                 $times['timestamp'] = time();
 158                 $wgMemc->set( $memcKey, $times, $expiry );
 159
 160                 # But don't give the timestamp to the caller
 161                 unset($times['timestamp']);
 162                 $lagTimes = $times;
 163
 164                 wfProfileOut( __METHOD__ );
 165                 return $lagTimes;
 166         }
 167
 168         /**
 169          * @param $conn DatabaseBase
 170          * @param $threshold
 171          * @return int
 172          */
 173         function postConnectionBackoff( $conn, $threshold ) {
 174                 if ( !$threshold ) {
 175                         return 0;
 176                 }
 177                 $status = $conn->getMysqlStatus("Thread%");
 178                 if ( $status['Threads_running'] > $threshold ) {
 179                         $server = $conn->getProperty( 'mServer' );
 180                         wfLogDBError( "LB backoff from $server - Threads_running = {$status['Threads_running']}\n" );
 181                         return $status['Threads_connected'];
 182                 } else {
 183                         return 0;
 184                 }
 185         }
 186 }
 187