includes/LoadBalancer.php

   1 <?php
   2 # Database load balancing object
   3
   4 require_once( "Database.php" );
   5
   6 # Valid database indexes
   7 # Operation-based indexes
   8 define( "DB_SLAVE", -1 );     # Read from the slave (or only server)
   9 define( "DB_MASTER", -2 );    # Write to master (or only server)
  10 define( "DB_LAST", -3 );     # Whatever database was used last
  11
  12 # Obsolete aliases
  13 define( "DB_READ", -1 );
  14 define( "DB_WRITE", -2 );
  15
  16 # Task-based indexes
  17 # ***NOT USED YET, EXPERIMENTAL***
  18 # These may be defined in $wgDBservers. If they aren't, the default reader or writer will be used
  19 # Even numbers are always readers, odd numbers are writers
  20 define( "DB_TASK_FIRST", 1000 );  # First in list
  21 define( "DB_SEARCH_R", 1000 );    # Search read
  22 define( "DB_SEARCH_W", 1001 );    # Search write
  23 define( "DB_ASKSQL_R", 1002 );    # Special:Asksql read
  24 define( "DB_WATCHLIST_R", 1004 ); # Watchlist read
  25 define( "DB_TASK_LAST", 1004) ;   # Last in list
  26
  27 define( "MASTER_WAIT_TIMEOUT", 15 ); # Time to wait for a slave to synchronise
  28
  29 class LoadBalancer {
  30         /* private */ var $mServers, $mConnections, $mLoads;
  31         /* private */ var $mFailFunction;
  32         /* private */ var $mForce, $mReadIndex, $mLastConn;
  33         /* private */ var $mWaitForFile, $mWaitForPos;
  34
  35         function LoadBalancer()
  36         {
  37                 $this->mServers = array();
  38                 $this->mConnections = array();
  39                 $this->mFailFunction = false;
  40                 $this->mReadIndex = -1;
  41                 $this->mForce = -1;
  42                 $this->mLastConn = false;
  43         }
  44
  45         function newFromParams( $servers, $failFunction = false )
  46         {
  47                 $lb = new LoadBalancer;
  48                 $lb->initialise( $servers, $failFunction = false );
  49                 return $lb;
  50         }
  51
  52         function initialise( $servers, $failFunction = false )
  53         {
  54                 $this->mServers = $servers;
  55                 $this->mFailFunction = $failFunction;
  56                 $this->mReadIndex = -1;
  57                 $this->mWriteIndex = -1;
  58                 $this->mForce = -1;
  59                 $this->mConnections = array();
  60                 $this->mLastConn = false;
  61                 $this->mLoads = array();
  62                 $this->mWaitForFile = false;
  63                 $this->mWaitForPos = false;
  64
  65                 foreach( $servers as $i => $server ) {
  66                         $this->mLoads[$i] = $server['load'];
  67                 }
  68         }
  69
  70         # Given an array of non-normalised probabilities, this function will select
  71         # an element and return the appropriate key
  72         function pickRandom( $weights )
  73         {
  74                 if ( !is_array( $weights ) || count( $weights ) == 0 ) {
  75                         return false;
  76                 }
  77
  78                 $sum = 0;
  79                 foreach ( $weights as $w ) {
  80                         $sum += $w;
  81                 }
  82                 $max = mt_getrandmax();
  83                 $rand = mt_rand(0, $max) / $max * $sum;
  84
  85                 $sum = 0;
  86                 foreach ( $weights as $i => $w ) {
  87                         $sum += $w;
  88                         if ( $sum >= $rand ) {
  89                                 break;
  90                         }
  91                 }
  92                 return $i;
  93         }
  94
  95         function &getReader()
  96         {
  97                 if ( $this->mForce >= 0 ) {
  98                         $conn =& $this->getConnection( $this->mForce );
  99                 } else {
 100                         if ( $this->mReadIndex >= 0 ) {
 101                                 $conn =& $this->getConnection( $this->mReadIndex );
 102                         } else {
 103                                 # $loads is $this->mLoads except with elements knocked out if they
 104                                 # don't work
 105                                 $loads = $this->mLoads;
 106                                 do {
 107                                         $i = $this->pickRandom( $loads );
 108                                         if ( $i !== false ) {
 109                                                 wfDebug( "Using reader #$i: {$this->mServers[$i]['host']}\n" );
 110
 111                                                 $conn =& $this->getConnection( $i );
 112                                                 if ( !$conn->isOpen() ) {
 113                                                         unset( $loads[$i] );
 114                                                 }
 115                                         }
 116                                 } while ( $i !== false && !$conn->isOpen() );
 117                                 if ( $conn->isOpen() ) {
 118                                         $this->mReadIndex = $i;
 119                                 }
 120                         }
 121                 }
 122                 if ( $conn === false || !$conn->isOpen() ) {
 123                         $this->reportConnectionError( $conn );
 124                         $conn = false;
 125                 }
 126                 return $conn;
 127         }
 128
 129         # Set the master wait position
 130         # If a DB_SLAVE connection has been opened already, waits
 131         # Otherwise sets a variable telling it to wait if such a connection is opened
 132         function waitFor( $file, $pos ) {
 133                 $this->mWaitForFile = false;
 134                 $this->mWaitForPos = false;
 135
 136                 if ( count( $this->mServers ) == 1 ) {
 137                         return;
 138                 }
 139
 140                 $this->mWaitForFile = $file;
 141                 $this->mWaitForPos = $pos;
 142
 143                 if ( $this->mReadIndex > 0 ) {
 144                         $this->doWait( $this->mReadIndex );
 145                 }
 146         }
 147
 148         # Wait for a given slave to catch up to the master pos stored in $this
 149         function doWait( $index ) {
 150                 global $wgMemc;
 151
 152                 $key = "masterpos:" . $index;
 153                 $memcPos = $wgMemc->get( $key );
 154                 if ( $memcPos ) {
 155                         list( $file, $pos ) = explode( ' ', $memcPos );
 156                         # If the saved position is later than the requested position, return now
 157                         if ( $file == $this->mWaitForFile && $this->mWaitForPos <= $pos ) {
 158                                 return;
 159                         }
 160                 }
 161
 162                 $conn =& $this->getConnection( $index );
 163                 $result = $conn->masterPosWait( $this->mWaitForFile, $this->mWaitForPos, MASTER_WAIT_TIMEOUT );
 164                 if ( $result == -1 ) {
 165                         # Timed out waiting for slave, use master instead
 166                         # This is not the ideal solution. If there are a large number of slaves, a slow
 167                         # replicated write query will cause the master to be swamped with reads. However
 168                         # that's a relatively graceful failure mode, so it will do for now.
 169                         $this->mReadIndex = 0;
 170                 }
 171         }
 172
 173         function &getConnection( $i, $fail = false )
 174         {
 175                 /*
 176                 # Task-based index
 177                 if ( $i >= DB_TASK_FIRST && $i < DB_TASK_LAST ) {
 178                         if ( $i % 2 ) {
 179                                 # Odd index use writer
 180                                 $i = DB_MASTER;
 181                         } else {
 182                                 # Even index use reader
 183                                 $i = DB_SLAVE;
 184                         }
 185                 }*/
 186
 187                 # Operation-based index
 188                 # Note, getReader() and getWriter() will re-enter this function
 189                 if ( $i == DB_SLAVE ) {
 190                         $this->mLastConn =& $this->getReader();
 191                 } elseif ( $i == DB_MASTER ) {
 192                         $this->mLastConn =& $this->getWriter();
 193                 } elseif ( $i == DB_LAST ) {
 194                         # Just use $this->mLastConn, which should already be set
 195                         if ( $this->mLastConn === false ) {
 196                                 # Oh dear, not set, best to use the writer for safety
 197                                 $this->mLastConn =& $this->getWriter();
 198                         }
 199                 } else {
 200                         # Explicit index
 201                         if ( !array_key_exists( $i, $this->mConnections ) || !$this->mConnections[$i]->isOpen() ) {
 202                                 $this->mConnections[$i] = $this->makeConnection( $this->mServers[$i] );
 203                                 if ( $i != 0 && $this->mWaitForFile ) {
 204                                         $this->doWait( $i );
 205                                 }
 206                         }
 207                         if ( !$this->mConnections[$i]->isOpen() ) {
 208                                 wfDebug( "Failed to connect to database $i at {$this->mServers[$i]['host']}\n" );
 209                                 if ( $fail ) {
 210                                         $this->reportConnectionError( $this->mConnections[$i] );
 211                                 }
 212                                 $this->mConnections[$i] = false;
 213                         }
 214                         $this->mLastConn =& $this->mConnections[$i];
 215                 }
 216                 return $this->mLastConn;
 217         }
 218
 219         /* private */ function makeConnection( &$server ) {
 220                         extract( $server );
 221                         # Get class for this database type
 222                         $class = 'Database' . ucfirst( $type );
 223                         if ( !class_exists( $class ) ) {
 224                                 require_once( "$class.php" );
 225                         }
 226
 227                         # Create object
 228                         return new $class( $host, $user, $password, $dbname, 1 );
 229         }
 230
 231         function reportConnectionError( &$conn )
 232         {
 233                 if ( !is_object( $conn ) ) {
 234                         $conn = new Database;
 235                 }
 236                 if ( $this->mFailFunction ) {
 237                         $conn->setFailFunction( $this->mFailFunction );
 238                 } else {
 239                         $conn->setFailFunction( "wfEmergencyAbort" );
 240                 }
 241                 $conn->reportConnectionError();
 242         }
 243
 244         function &getWriter()
 245         {
 246                 $c =& $this->getConnection( 0 );
 247                 if ( $c === false || !$c->isOpen() ) {
 248                         $this->reportConnectionError( $c );
 249                         $c = false;
 250                 }
 251                 return $c;
 252         }
 253
 254         function force( $i )
 255         {
 256                 $this->mForce = $i;
 257         }
 258
 259         function haveIndex( $i )
 260         {
 261                 return array_key_exists( $i, $this->mServers );
 262         }
 263
 264         # Get the number of defined servers (not the number of open connections)
 265         function getServerCount() {
 266                 return count( $this->mServers );
 267         }
 268
 269         # Save master pos to the session and to memcached, if the session exists
 270         function saveMasterPos() {
 271                 global $wgSessionStarted;
 272                 if ( $wgSessionStarted && count( $this->mServers ) > 1 ) {
 273                         # If this entire request was served from a slave without opening a connection to the
 274                         # master (however unlikely that may be), then we can fetch the position from the slave.
 275                         if ( empty( $this->mConnections[0] ) ) {
 276                                 $conn =& $this->getConnection( DB_SLAVE );
 277                                 list( $file, $pos ) = $conn->getSlavePos();
 278                         } else {
 279                                 $conn =& $this->getConnection( 0 );
 280                                 list( $file, $pos ) = $conn->getMasterPos();
 281                         }
 282                         if ( $file !== false ) {
 283                                 $_SESSION['master_log_file'] = $file;
 284                                 $_SESSION['master_pos'] = $pos;
 285                         }
 286                 }
 287         }
 288
 289         # Loads the master pos from the session, waits for it if necessary
 290         function loadMasterPos() {
 291                 if ( isset( $_SESSION['master_log_file'] ) && isset( $_SESSION['master_pos'] ) ) {
 292                         $this->waitFor( $_SESSION['master_log_file'], $_SESSION['master_pos'] );
 293                 }
 294         }
 295 }