includes/LoadBalancer.php

   1 <?php
   2 /**
   3  *
   4  * @package MediaWiki
   5  */
   6
   7 /**
   8  * Depends on the database object
   9  */
  10 require_once( 'Database.php' );
  11
  12 # Valid database indexes
  13 # Operation-based indexes
  14 define( 'DB_SLAVE', -1 );     # Read from the slave (or only server)
  15 define( 'DB_MASTER', -2 );    # Write to master (or only server)
  16 define( 'DB_LAST', -3 );     # Whatever database was used last
  17
  18 # Obsolete aliases
  19 define( 'DB_READ', -1 );
  20 define( 'DB_WRITE', -2 );
  21
  22 /**
  23  * Database load balancing object
  24  *
  25  * @todo document
  26  * @package MediaWiki
  27  */
  28 class LoadBalancer {
  29         /* private */ var $mServers, $mConnections, $mLoads, $mGroupLoads;
  30         /* private */ var $mFailFunction;
  31         /* private */ var $mForce, $mReadIndex, $mLastIndex;
  32         /* private */ var $mWaitForFile, $mWaitForPos, $mWaitTimeout;
  33         /* private */ var $mLaggedSlaveMode;
  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->mLastIndex = -1;
  43         }
  44
  45         function newFromParams( $servers, $failFunction = false, $waitTimeout = 10 )
  46         {
  47                 $lb = new LoadBalancer;
  48                 $lb->initialise( $servers, $failFunction, $waitTimeout );
  49                 return $lb;
  50         }
  51
  52         function initialise( $servers, $failFunction = false, $waitTimeout = 10 )
  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->mLastIndex = 1;
  61                 $this->mLoads = array();
  62                 $this->mWaitForFile = false;
  63                 $this->mWaitForPos = false;
  64                 $this->mWaitTimeout = $waitTimeout;
  65                 $this->mLaggedSlaveMode = false;
  66
  67                 foreach( $servers as $i => $server ) {
  68                         $this->mLoads[$i] = $server['load'];
  69                         if ( isset( $server['groupLoads'] ) ) {
  70                                 foreach ( $server['groupLoads'] as $group => $ratio ) {
  71                                         if ( !isset( $this->mGroupLoads[$group] ) ) {
  72                                                 $this->mGroupLoads[$group] = array();
  73                                         }
  74                                         $this->mGroupLoads[$group][$i] = $ratio;
  75                                 }
  76                         }
  77                 }
  78         }
  79
  80         /**
  81          * Given an array of non-normalised probabilities, this function will select
  82          * an element and return the appropriate key
  83          */
  84         function pickRandom( $weights )
  85         {
  86                 if ( !is_array( $weights ) || count( $weights ) == 0 ) {
  87                         return false;
  88                 }
  89
  90                 $sum = 0;
  91                 foreach ( $weights as $w ) {
  92                         $sum += $w;
  93                 }
  94
  95                 if ( $sum == 0 ) {
  96                         # No loads on any of them
  97                         # Just pick one at random
  98                         foreach ( $weights as $i => $w ) {
  99                                 $weights[$i] = 1;
 100                         }
 101                 }
 102                 $max = mt_getrandmax();
 103                 $rand = mt_rand(0, $max) / $max * $sum;
 104
 105                 $sum = 0;
 106                 foreach ( $weights as $i => $w ) {
 107                         $sum += $w;
 108                         if ( $sum >= $rand ) {
 109                                 break;
 110                         }
 111                 }
 112                 return $i;
 113         }
 114
 115         function getRandomNonLagged( $loads ) {
 116                 # Unset excessively lagged servers
 117                 $lags = $this->getLagTimes();
 118                 foreach ( $lags as $i => $lag ) {
 119                         if ( isset( $this->mServers[$i]['max lag'] ) && $lag > $this->mServers[$i]['max lag'] ) {
 120                                 unset( $loads[$i] );
 121                         }
 122                 }
 123
 124
 125                 # Find out if all the slaves with non-zero load are lagged
 126                 $sum = 0;
 127                 foreach ( $loads as $load ) {
 128                         $sum += $load;
 129                 }
 130                 if ( $sum == 0 ) {
 131                         # No appropriate DB servers except maybe the master and some slaves with zero load
 132                         # Do NOT use the master
 133                         # Instead, this function will return false, triggering read-only mode,
 134                         # and a lagged slave will be used instead.
 135                         unset ( $loads[0] );
 136                 }
 137
 138                 if ( count( $loads ) == 0 ) {
 139                         return false;
 140                 }
 141
 142                 #wfDebug( var_export( $loads, true ) );
 143
 144                 # Return a random representative of the remainder
 145                 return $this->pickRandom( $loads );
 146         }
 147
 148
 149         function getReaderIndex()
 150         {
 151                 global $wgMaxLag, $wgReadOnly, $wgDBClusterTimeout;
 152
 153                 $fname = 'LoadBalancer::getReaderIndex';
 154                 wfProfileIn( $fname );
 155
 156                 $i = false;
 157                 if ( $this->mForce >= 0 ) {
 158                         $i = $this->mForce;
 159                 } else {
 160                         if ( $this->mReadIndex >= 0 ) {
 161                                 $i = $this->mReadIndex;
 162                         } else {
 163                                 # $loads is $this->mLoads except with elements knocked out if they
 164                                 # don't work
 165                                 $loads = $this->mLoads;
 166                                 $done = false;
 167                                 $totalElapsed = 0;
 168                                 do {
 169                                         if ( $wgReadOnly ) {
 170                                                 $i = $this->pickRandom( $loads );
 171                                         } else {
 172                                                 $i = $this->getRandomNonLagged( $loads );
 173                                                 if ( $i === false && count( $loads ) != 0 )  {
 174                                                         # All slaves lagged. Switch to read-only mode
 175                                                         $wgReadOnly = wfMsgNoDB( 'readonly_lag' );
 176                                                         $i = $this->pickRandom( $loads );
 177                                                 }
 178                                         }
 179                                         if ( $i !== false ) {
 180                                                 wfDebug( "Using reader #$i: {$this->mServers[$i]['host']}...\n" );
 181                                                 $this->openConnection( $i );
 182
 183                                                 if ( !$this->isOpen( $i ) ) {
 184                                                         wfDebug( "Failed\n" );
 185                                                         unset( $loads[$i] );
 186                                                         $sleepTime = 0;
 187                                                 } else {
 188                                                         $status = $this->mConnections[$i]->getStatus();
 189                                                         if ( isset( $this->mServers[$i]['max threads'] ) &&
 190                                                           $status['Threads_running'] > $this->mServers[$i]['max threads'] )
 191                                                         {
 192                                                                 # Slave is lagged, wait for a while
 193                                                                 $sleepTime = 5000 * $status['Threads_connected'];
 194
 195                                                                 # If we reach the timeout and exit the loop, don't use it
 196                                                                 $i = false;
 197                                                         } else {
 198                                                                 $done = true;
 199                                                                 $sleepTime = 0;
 200                                                         }
 201                                                 }
 202                                         } else {
 203                                                 $sleepTime = 500000;
 204                                         }
 205                                         if ( $sleepTime ) {
 206                                                         $totalElapsed += $sleepTime;
 207                                                         usleep( $sleepTime );
 208                                         }
 209                                 } while ( count( $loads ) && !$done && $totalElapsed / 1e6 < $wgDBClusterTimeout );
 210
 211                                 if ( $i !== false && $this->isOpen( $i ) ) {
 212                                         # Wait for the session master pos for a short time
 213                                         if ( $this->mWaitForFile ) {
 214                                                 if ( !$this->doWait( $i ) ) {
 215                                                         $this->mServers[$i]['slave pos'] = $this->mConnections[$i]->getSlavePos();
 216                                                 }
 217                                         }
 218                                         if ( $i != false ) {
 219                                                 $this->mReadIndex = $i;
 220                                         }
 221                                 } else {
 222                                         $i = false;
 223                                 }
 224                         }
 225                 }
 226                 wfProfileOut( $fname );
 227                 return $i;
 228         }
 229
 230         /**
 231          * Get a random server to use in a query group
 232          */
 233         function getGroupIndex( $group ) {
 234                 if ( isset( $this->mGroupLoads[$group] ) ) {
 235                         $i = $this->pickRandom( $this->mGroupLoads[$group] );
 236                 } else {
 237                         $i = false;
 238                 }
 239                 wfDebug( "Query group $group => $i\n" );
 240                 return $i;
 241         }
 242
 243         /**
 244          * Set the master wait position
 245          * If a DB_SLAVE connection has been opened already, waits
 246          * Otherwise sets a variable telling it to wait if such a connection is opened
 247          */
 248         function waitFor( $file, $pos ) {
 249                 $fname = 'LoadBalancer::waitFor';
 250                 wfProfileIn( $fname );
 251
 252                 wfDebug( "User master pos: $file $pos\n" );
 253                 $this->mWaitForFile = false;
 254                 $this->mWaitForPos = false;
 255
 256                 if ( count( $this->mServers ) > 1 ) {
 257                         $this->mWaitForFile = $file;
 258                         $this->mWaitForPos = $pos;
 259                         $i = $this->mReadIndex;
 260
 261                         if ( $i > 0 ) {
 262                                 if ( !$this->doWait( $i ) ) {
 263                                         $this->mServers[$i]['slave pos'] = $this->mConnections[$i]->getSlavePos();
 264                                         $this->mLaggedSlaveMode = true;
 265                                 }
 266                         }
 267                 }
 268                 wfProfileOut( $fname );
 269         }
 270
 271         /**
 272          * Wait for a given slave to catch up to the master pos stored in $this
 273          */
 274         function doWait( $index ) {
 275                 global $wgMemc;
 276
 277                 $retVal = false;
 278
 279                 # Debugging hacks
 280                 if ( isset( $this->mServers[$index]['lagged slave'] ) ) {
 281                         return false;
 282                 } elseif ( isset( $this->mServers[$index]['fake slave'] ) ) {
 283                         return true;
 284                 }
 285
 286                 $key = 'masterpos:' . $index;
 287                 $memcPos = $wgMemc->get( $key );
 288                 if ( $memcPos ) {
 289                         list( $file, $pos ) = explode( ' ', $memcPos );
 290                         # If the saved position is later than the requested position, return now
 291                         if ( $file == $this->mWaitForFile && $this->mWaitForPos <= $pos ) {
 292                                 $retVal = true;
 293                         }
 294                 }
 295
 296                 if ( !$retVal && $this->isOpen( $index ) ) {
 297                         $conn =& $this->mConnections[$index];
 298                         wfDebug( "Waiting for slave #$index to catch up...\n" );
 299                         $result = $conn->masterPosWait( $this->mWaitForFile, $this->mWaitForPos, $this->mWaitTimeout );
 300
 301                         if ( $result == -1 || is_null( $result ) ) {
 302                                 # Timed out waiting for slave, use master instead
 303                                 wfDebug( "Timed out waiting for slave #$index pos {$this->mWaitForFile} {$this->mWaitForPos}\n" );
 304                                 $retVal = false;
 305                         } else {
 306                                 $retVal = true;
 307                                 wfDebug( "Done\n" );
 308                         }
 309                 }
 310                 return $retVal;
 311         }
 312
 313         /**
 314          * Get a connection by index
 315          */
 316         function &getConnection( $i, $fail = true, $groups = array() )
 317         {
 318                 $fname = 'LoadBalancer::getConnection';
 319                 wfProfileIn( $fname );
 320
 321                 # Query groups
 322                 $groupIndex = false;
 323                 foreach ( $groups as $group ) {
 324                         $groupIndex = $this->getGroupIndex( $group );
 325                         if ( $groupIndex !== false ) {
 326                                 $i = $groupIndex;
 327                                 break;
 328                         }
 329                 }
 330
 331                 # Operation-based index
 332                 if ( $i == DB_SLAVE ) {
 333                         $i = $this->getReaderIndex();
 334                 } elseif ( $i == DB_MASTER ) {
 335                         $i = $this->getWriterIndex();
 336                 } elseif ( $i == DB_LAST ) {
 337                         # Just use $this->mLastIndex, which should already be set
 338                         $i = $this->mLastIndex;
 339                         if ( $i === -1 ) {
 340                                 # Oh dear, not set, best to use the writer for safety
 341                                 wfDebug( "Warning: DB_LAST used when there was no previous index\n" );
 342                                 $i = $this->getWriterIndex();
 343                         }
 344                 }
 345                 # Now we have an explicit index into the servers array
 346                 $this->openConnection( $i, $fail );
 347
 348                 wfProfileOut( $fname );
 349                 return $this->mConnections[$i];
 350         }
 351
 352         /**
 353          * Open a connection to the server given by the specified index
 354          * Index must be an actual index into the array
 355          * Returns success
 356          * @private
 357          */
 358         function openConnection( $i, $fail = false ) {
 359                 $fname = 'LoadBalancer::openConnection';
 360                 wfProfileIn( $fname );
 361                 $success = true;
 362
 363                 if ( !$this->isOpen( $i ) ) {
 364                         $this->mConnections[$i] = $this->reallyOpenConnection( $this->mServers[$i] );
 365                 }
 366                 if ( !$this->isOpen( $i ) ) {
 367                         wfDebug( "Failed to connect to database $i at {$this->mServers[$i]['host']}\n" );
 368                         if ( $fail ) {
 369                                 $this->reportConnectionError( $this->mConnections[$i] );
 370                         }
 371                         $this->mConnections[$i] = false;
 372                         $success = false;
 373                 }
 374                 $this->mLastIndex = $i;
 375                 wfProfileOut( $fname );
 376                 return $success;
 377         }
 378
 379         /**
 380          * Test if the specified index represents an open connection
 381          * @private
 382          */
 383         function isOpen( $index ) {
 384                 if( !is_integer( $index ) ) {
 385                         return false;
 386                 }
 387                 if ( array_key_exists( $index, $this->mConnections ) && is_object( $this->mConnections[$index] ) &&
 388                   $this->mConnections[$index]->isOpen() )
 389                 {
 390                         return true;
 391                 } else {
 392                         return false;
 393                 }
 394         }
 395
 396         /**
 397          * Really opens a connection
 398          * @private
 399          */
 400         function reallyOpenConnection( &$server ) {
 401                 if( !is_array( $server ) ) {
 402                         wfDebugDieBacktrace( 'You must update your load-balancing configuration. See DefaultSettings.php entry for $wgDBservers.' );
 403                 }
 404
 405                 extract( $server );
 406                 # Get class for this database type
 407                 $class = 'Database' . ucfirst( $type );
 408                 if ( !class_exists( $class ) ) {
 409                         require_once( "$class.php" );
 410                 }
 411
 412                 # Create object
 413                 return new $class( $host, $user, $password, $dbname, 1, $flags );
 414         }
 415
 416         function reportConnectionError( &$conn )
 417         {
 418                 $fname = 'LoadBalancer::reportConnectionError';
 419                 wfProfileIn( $fname );
 420                 # Prevent infinite recursion
 421
 422                 static $reporting = false;
 423                 if ( !$reporting ) {
 424                         $reporting = true;
 425                         if ( !is_object( $conn ) ) {
 426                                 $conn = new Database;
 427                         }
 428                         if ( $this->mFailFunction ) {
 429                                 $conn->failFunction( $this->mFailFunction );
 430                         } else {
 431                                 $conn->failFunction( 'wfEmergencyAbort' );
 432                         }
 433                         $conn->reportConnectionError();
 434                         $reporting = false;
 435                 }
 436                 wfProfileOut( $fname );
 437         }
 438
 439         function getWriterIndex()
 440         {
 441                 return 0;
 442         }
 443
 444         function force( $i )
 445         {
 446                 $this->mForce = $i;
 447         }
 448
 449         function haveIndex( $i )
 450         {
 451                 return array_key_exists( $i, $this->mServers );
 452         }
 453
 454         /**
 455          * Get the number of defined servers (not the number of open connections)
 456          */
 457         function getServerCount() {
 458                 return count( $this->mServers );
 459         }
 460
 461         /**
 462          * Save master pos to the session and to memcached, if the session exists
 463          */
 464         function saveMasterPos() {
 465                 global $wgSessionStarted;
 466                 if ( $wgSessionStarted && count( $this->mServers ) > 1 ) {
 467                         # If this entire request was served from a slave without opening a connection to the
 468                         # master (however unlikely that may be), then we can fetch the position from the slave.
 469                         if ( empty( $this->mConnections[0] ) ) {
 470                                 $conn =& $this->getConnection( DB_SLAVE );
 471                                 list( $file, $pos ) = $conn->getSlavePos();
 472                                 wfDebug( "Saving master pos fetched from slave: $file $pos\n" );
 473                         } else {
 474                                 $conn =& $this->getConnection( 0 );
 475                                 list( $file, $pos ) = $conn->getMasterPos();
 476                                 wfDebug( "Saving master pos: $file $pos\n" );
 477                         }
 478                         if ( $file !== false ) {
 479                                 $_SESSION['master_log_file'] = $file;
 480                                 $_SESSION['master_pos'] = $pos;
 481                         }
 482                 }
 483         }
 484
 485         /**
 486          * Loads the master pos from the session, waits for it if necessary
 487          */
 488         function loadMasterPos() {
 489                 if ( isset( $_SESSION['master_log_file'] ) && isset( $_SESSION['master_pos'] ) ) {
 490                         $this->waitFor( $_SESSION['master_log_file'], $_SESSION['master_pos'] );
 491                 }
 492         }
 493
 494         /**
 495          * Close all open connections
 496          */
 497         function closeAll() {
 498                 foreach( $this->mConnections as $i => $conn ) {
 499                         if ( $this->isOpen( $i ) ) {
 500                                 // Need to use this syntax because $conn is a copy not a reference
 501                                 $this->mConnections[$i]->close();
 502                         }
 503                 }
 504         }
 505
 506         function commitAll() {
 507                 foreach( $this->mConnections as $i => $conn ) {
 508                         if ( $this->isOpen( $i ) ) {
 509                                 // Need to use this syntax because $conn is a copy not a reference
 510                                 $this->mConnections[$i]->immediateCommit();
 511                         }
 512                 }
 513         }
 514
 515         function waitTimeout( $value = NULL ) {
 516                 return wfSetVar( $this->mWaitTimeout, $value );
 517         }
 518
 519         function getLaggedSlaveMode() {
 520                 return $this->mLaggedSlaveMode;
 521         }
 522
 523         function pingAll() {
 524                 $success = true;
 525                 foreach ( $this->mConnections as $i => $conn ) {
 526                         if ( $this->isOpen( $i ) ) {
 527                                 if ( !$this->mConnections[$i]->ping() ) {
 528                                         $success = false;
 529                                 }
 530                         }
 531                 }
 532                 return $success;
 533         }
 534
 535         /**
 536          * Get the hostname and lag time of the most-lagged slave
 537          * This is useful for maintenance scripts that need to throttle their updates
 538          */
 539         function getMaxLag() {
 540                 $maxLag = -1;
 541                 $host = '';
 542                 foreach ( $this->mServers as $i => $conn ) {
 543                         if ( $this->openConnection( $i ) ) {
 544                                 $lag = $this->mConnections[$i]->getLag();
 545                                 if ( $lag > $maxLag ) {
 546                                         $maxLag = $lag;
 547                                         $host = $this->mServers[$i]['host'];
 548                                 }
 549                         }
 550                 }
 551                 return array( $host, $maxLag );
 552         }
 553
 554         /**
 555          * Get lag time for each DB
 556          * Results are cached for a short time in memcached
 557          */
 558         function getLagTimes() {
 559                 $expiry = 5;
 560                 $requestRate = 10;
 561
 562                 global $wgMemc;
 563                 $times = $wgMemc->get( 'lag_times' );
 564                 if ( $times ) {
 565                         # Randomly recache with probability rising over $expiry
 566                         $elapsed = time() - $times['timestamp'];
 567                         $chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
 568                         if ( mt_rand( 0, $chance ) != 0 ) {
 569                                 unset( $times['timestamp'] );
 570                                 return $times;
 571                         }
 572                 }
 573
 574                 # Cache key missing or expired
 575
 576                 $times = array();
 577                 foreach ( $this->mServers as $i => $conn ) {
 578                         if ( $this->openConnection( $i ) ) {
 579                                 $times[$i] = $this->mConnections[$i]->getLag();
 580                         }
 581                 }
 582
 583                 # Add a timestamp key so we know when it was cached
 584                 $times['timestamp'] = time();
 585                 $wgMemc->set( 'lag_times', $times, $expiry );
 586
 587                 # But don't give the timestamp to the caller
 588                 unset($times['timestamp']);
 589                 return $times;
 590         }
 591 }
 592
 593 ?>