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          * Get the index of the reader connection, which may be a slave
 150          * This takes into account load ratios and lag times. It should
 151          * always return a consistent index during a given invocation
 152          *
 153          * Side effect: opens connections to databases
 154          */
 155         function getReaderIndex()
 156         {
 157                 global $wgMaxLag, $wgReadOnly, $wgDBClusterTimeout;
 158
 159                 $fname = 'LoadBalancer::getReaderIndex';
 160                 wfProfileIn( $fname );
 161
 162                 $i = false;
 163                 if ( $this->mForce >= 0 ) {
 164                         $i = $this->mForce;
 165                 } else {
 166                         if ( $this->mReadIndex >= 0 ) {
 167                                 $i = $this->mReadIndex;
 168                         } else {
 169                                 # $loads is $this->mLoads except with elements knocked out if they
 170                                 # don't work
 171                                 $loads = $this->mLoads;
 172                                 $done = false;
 173                                 $totalElapsed = 0;
 174                                 do {
 175                                         if ( $wgReadOnly ) {
 176                                                 $i = $this->pickRandom( $loads );
 177                                         } else {
 178                                                 $i = $this->getRandomNonLagged( $loads );
 179                                                 if ( $i === false && count( $loads ) != 0 )  {
 180                                                         # All slaves lagged. Switch to read-only mode
 181                                                         $wgReadOnly = wfMsgNoDB( 'readonly_lag' );
 182                                                         $i = $this->pickRandom( $loads );
 183                                                 }
 184                                         }
 185                                         if ( $i !== false ) {
 186                                                 wfDebug( "Using reader #$i: {$this->mServers[$i]['host']}...\n" );
 187                                                 $this->openConnection( $i );
 188
 189                                                 if ( !$this->isOpen( $i ) ) {
 190                                                         wfDebug( "Failed\n" );
 191                                                         unset( $loads[$i] );
 192                                                         $sleepTime = 0;
 193                                                 } else {
 194                                                         $status = $this->mConnections[$i]->getStatus();
 195                                                         if ( isset( $this->mServers[$i]['max threads'] ) &&
 196                                                           $status['Threads_running'] > $this->mServers[$i]['max threads'] )
 197                                                         {
 198                                                                 # Slave is lagged, wait for a while
 199                                                                 $sleepTime = 5000 * $status['Threads_connected'];
 200
 201                                                                 # If we reach the timeout and exit the loop, don't use it
 202                                                                 $i = false;
 203                                                         } else {
 204                                                                 $done = true;
 205                                                                 $sleepTime = 0;
 206                                                         }
 207                                                 }
 208                                         } else {
 209                                                 $sleepTime = 500000;
 210                                         }
 211                                         if ( $sleepTime ) {
 212                                                         $totalElapsed += $sleepTime;
 213                                                         usleep( $sleepTime );
 214                                         }
 215                                 } while ( count( $loads ) && !$done && $totalElapsed / 1e6 < $wgDBClusterTimeout );
 216
 217                                 if ( $i !== false && $this->isOpen( $i ) ) {
 218                                         # Wait for the session master pos for a short time
 219                                         if ( $this->mWaitForFile ) {
 220                                                 if ( !$this->doWait( $i ) ) {
 221                                                         $this->mServers[$i]['slave pos'] = $this->mConnections[$i]->getSlavePos();
 222                                                 }
 223                                         }
 224                                         if ( $i !== false ) {
 225                                                 $this->mReadIndex = $i;
 226                                         }
 227                                 } else {
 228                                         $i = false;
 229                                 }
 230                         }
 231                 }
 232                 wfProfileOut( $fname );
 233                 return $i;
 234         }
 235
 236         /**
 237          * Get a random server to use in a query group
 238          */
 239         function getGroupIndex( $group ) {
 240                 if ( isset( $this->mGroupLoads[$group] ) ) {
 241                         $i = $this->pickRandom( $this->mGroupLoads[$group] );
 242                 } else {
 243                         $i = false;
 244                 }
 245                 wfDebug( "Query group $group => $i\n" );
 246                 return $i;
 247         }
 248
 249         /**
 250          * Set the master wait position
 251          * If a DB_SLAVE connection has been opened already, waits
 252          * Otherwise sets a variable telling it to wait if such a connection is opened
 253          */
 254         function waitFor( $file, $pos ) {
 255                 $fname = 'LoadBalancer::waitFor';
 256                 wfProfileIn( $fname );
 257
 258                 wfDebug( "User master pos: $file $pos\n" );
 259                 $this->mWaitForFile = false;
 260                 $this->mWaitForPos = false;
 261
 262                 if ( count( $this->mServers ) > 1 ) {
 263                         $this->mWaitForFile = $file;
 264                         $this->mWaitForPos = $pos;
 265                         $i = $this->mReadIndex;
 266
 267                         if ( $i > 0 ) {
 268                                 if ( !$this->doWait( $i ) ) {
 269                                         $this->mServers[$i]['slave pos'] = $this->mConnections[$i]->getSlavePos();
 270                                         $this->mLaggedSlaveMode = true;
 271                                 }
 272                         }
 273                 }
 274                 wfProfileOut( $fname );
 275         }
 276
 277         /**
 278          * Wait for a given slave to catch up to the master pos stored in $this
 279          */
 280         function doWait( $index ) {
 281                 global $wgMemc;
 282
 283                 $retVal = false;
 284
 285                 # Debugging hacks
 286                 if ( isset( $this->mServers[$index]['lagged slave'] ) ) {
 287                         return false;
 288                 } elseif ( isset( $this->mServers[$index]['fake slave'] ) ) {
 289                         return true;
 290                 }
 291
 292                 $key = 'masterpos:' . $index;
 293                 $memcPos = $wgMemc->get( $key );
 294                 if ( $memcPos ) {
 295                         list( $file, $pos ) = explode( ' ', $memcPos );
 296                         # If the saved position is later than the requested position, return now
 297                         if ( $file == $this->mWaitForFile && $this->mWaitForPos <= $pos ) {
 298                                 $retVal = true;
 299                         }
 300                 }
 301
 302                 if ( !$retVal && $this->isOpen( $index ) ) {
 303                         $conn =& $this->mConnections[$index];
 304                         wfDebug( "Waiting for slave #$index to catch up...\n" );
 305                         $result = $conn->masterPosWait( $this->mWaitForFile, $this->mWaitForPos, $this->mWaitTimeout );
 306
 307                         if ( $result == -1 || is_null( $result ) ) {
 308                                 # Timed out waiting for slave, use master instead
 309                                 wfDebug( "Timed out waiting for slave #$index pos {$this->mWaitForFile} {$this->mWaitForPos}\n" );
 310                                 $retVal = false;
 311                         } else {
 312                                 $retVal = true;
 313                                 wfDebug( "Done\n" );
 314                         }
 315                 }
 316                 return $retVal;
 317         }
 318
 319         /**
 320          * Get a connection by index
 321          */
 322         function &getConnection( $i, $fail = true, $groups = array() )
 323         {
 324                 $fname = 'LoadBalancer::getConnection';
 325                 wfProfileIn( $fname );
 326
 327                 # Query groups
 328                 $groupIndex = false;
 329                 foreach ( $groups as $group ) {
 330                         $groupIndex = $this->getGroupIndex( $group );
 331                         if ( $groupIndex !== false ) {
 332                                 $i = $groupIndex;
 333                                 break;
 334                         }
 335                 }
 336
 337                 # Operation-based index
 338                 if ( $i == DB_SLAVE ) {
 339                         $i = $this->getReaderIndex();
 340                 } elseif ( $i == DB_MASTER ) {
 341                         $i = $this->getWriterIndex();
 342                 } elseif ( $i == DB_LAST ) {
 343                         # Just use $this->mLastIndex, which should already be set
 344                         $i = $this->mLastIndex;
 345                         if ( $i === -1 ) {
 346                                 # Oh dear, not set, best to use the writer for safety
 347                                 wfDebug( "Warning: DB_LAST used when there was no previous index\n" );
 348                                 $i = $this->getWriterIndex();
 349                         }
 350                 }
 351                 # Now we have an explicit index into the servers array
 352                 $this->openConnection( $i, $fail );
 353
 354                 wfProfileOut( $fname );
 355                 return $this->mConnections[$i];
 356         }
 357
 358         /**
 359          * Open a connection to the server given by the specified index
 360          * Index must be an actual index into the array
 361          * Returns success
 362          * @private
 363          */
 364         function openConnection( $i, $fail = false ) {
 365                 $fname = 'LoadBalancer::openConnection';
 366                 wfProfileIn( $fname );
 367                 $success = true;
 368
 369                 if ( !$this->isOpen( $i ) ) {
 370                         $this->mConnections[$i] = $this->reallyOpenConnection( $this->mServers[$i] );
 371                 }
 372                 if ( !$this->isOpen( $i ) ) {
 373                         wfDebug( "Failed to connect to database $i at {$this->mServers[$i]['host']}\n" );
 374                         if ( $fail ) {
 375                                 $this->reportConnectionError( $this->mConnections[$i] );
 376                         }
 377                         $this->mConnections[$i] = false;
 378                         $success = false;
 379                 }
 380                 $this->mLastIndex = $i;
 381                 wfProfileOut( $fname );
 382                 return $success;
 383         }
 384
 385         /**
 386          * Test if the specified index represents an open connection
 387          * @private
 388          */
 389         function isOpen( $index ) {
 390                 if( !is_integer( $index ) ) {
 391                         return false;
 392                 }
 393                 if ( array_key_exists( $index, $this->mConnections ) && is_object( $this->mConnections[$index] ) &&
 394                   $this->mConnections[$index]->isOpen() )
 395                 {
 396                         return true;
 397                 } else {
 398                         return false;
 399                 }
 400         }
 401
 402         /**
 403          * Really opens a connection
 404          * @private
 405          */
 406         function reallyOpenConnection( &$server ) {
 407                 if( !is_array( $server ) ) {
 408                         wfDebugDieBacktrace( 'You must update your load-balancing configuration. See DefaultSettings.php entry for $wgDBservers.' );
 409                 }
 410
 411                 extract( $server );
 412                 # Get class for this database type
 413                 $class = 'Database' . ucfirst( $type );
 414                 if ( !class_exists( $class ) ) {
 415                         require_once( "$class.php" );
 416                 }
 417
 418                 # Create object
 419                 return new $class( $host, $user, $password, $dbname, 1, $flags );
 420         }
 421
 422         function reportConnectionError( &$conn )
 423         {
 424                 $fname = 'LoadBalancer::reportConnectionError';
 425                 wfProfileIn( $fname );
 426                 # Prevent infinite recursion
 427
 428                 static $reporting = false;
 429                 if ( !$reporting ) {
 430                         $reporting = true;
 431                         if ( !is_object( $conn ) ) {
 432                                 $conn = new Database;
 433                         }
 434                         if ( $this->mFailFunction ) {
 435                                 $conn->failFunction( $this->mFailFunction );
 436                         } else {
 437                                 $conn->failFunction( 'wfEmergencyAbort' );
 438                         }
 439                         $conn->reportConnectionError();
 440                         $reporting = false;
 441                 }
 442                 wfProfileOut( $fname );
 443         }
 444
 445         function getWriterIndex()
 446         {
 447                 return 0;
 448         }
 449
 450         function force( $i )
 451         {
 452                 $this->mForce = $i;
 453         }
 454
 455         function haveIndex( $i )
 456         {
 457                 return array_key_exists( $i, $this->mServers );
 458         }
 459
 460         /**
 461          * Get the number of defined servers (not the number of open connections)
 462          */
 463         function getServerCount() {
 464                 return count( $this->mServers );
 465         }
 466
 467         /**
 468          * Save master pos to the session and to memcached, if the session exists
 469          */
 470         function saveMasterPos() {
 471                 global $wgSessionStarted;
 472                 if ( $wgSessionStarted && count( $this->mServers ) > 1 ) {
 473                         # If this entire request was served from a slave without opening a connection to the
 474                         # master (however unlikely that may be), then we can fetch the position from the slave.
 475                         if ( empty( $this->mConnections[0] ) ) {
 476                                 $conn =& $this->getConnection( DB_SLAVE );
 477                                 list( $file, $pos ) = $conn->getSlavePos();
 478                                 wfDebug( "Saving master pos fetched from slave: $file $pos\n" );
 479                         } else {
 480                                 $conn =& $this->getConnection( 0 );
 481                                 list( $file, $pos ) = $conn->getMasterPos();
 482                                 wfDebug( "Saving master pos: $file $pos\n" );
 483                         }
 484                         if ( $file !== false ) {
 485                                 $_SESSION['master_log_file'] = $file;
 486                                 $_SESSION['master_pos'] = $pos;
 487                         }
 488                 }
 489         }
 490
 491         /**
 492          * Loads the master pos from the session, waits for it if necessary
 493          */
 494         function loadMasterPos() {
 495                 if ( isset( $_SESSION['master_log_file'] ) && isset( $_SESSION['master_pos'] ) ) {
 496                         $this->waitFor( $_SESSION['master_log_file'], $_SESSION['master_pos'] );
 497                 }
 498         }
 499
 500         /**
 501          * Close all open connections
 502          */
 503         function closeAll() {
 504                 foreach( $this->mConnections as $i => $conn ) {
 505                         if ( $this->isOpen( $i ) ) {
 506                                 // Need to use this syntax because $conn is a copy not a reference
 507                                 $this->mConnections[$i]->close();
 508                         }
 509                 }
 510         }
 511
 512         function commitAll() {
 513                 foreach( $this->mConnections as $i => $conn ) {
 514                         if ( $this->isOpen( $i ) ) {
 515                                 // Need to use this syntax because $conn is a copy not a reference
 516                                 $this->mConnections[$i]->immediateCommit();
 517                         }
 518                 }
 519         }
 520
 521         function waitTimeout( $value = NULL ) {
 522                 return wfSetVar( $this->mWaitTimeout, $value );
 523         }
 524
 525         function getLaggedSlaveMode() {
 526                 return $this->mLaggedSlaveMode;
 527         }
 528
 529         function pingAll() {
 530                 $success = true;
 531                 foreach ( $this->mConnections as $i => $conn ) {
 532                         if ( $this->isOpen( $i ) ) {
 533                                 if ( !$this->mConnections[$i]->ping() ) {
 534                                         $success = false;
 535                                 }
 536                         }
 537                 }
 538                 return $success;
 539         }
 540
 541         /**
 542          * Get the hostname and lag time of the most-lagged slave
 543          * This is useful for maintenance scripts that need to throttle their updates
 544          */
 545         function getMaxLag() {
 546                 $maxLag = -1;
 547                 $host = '';
 548                 foreach ( $this->mServers as $i => $conn ) {
 549                         if ( $this->openConnection( $i ) ) {
 550                                 $lag = $this->mConnections[$i]->getLag();
 551                                 if ( $lag > $maxLag ) {
 552                                         $maxLag = $lag;
 553                                         $host = $this->mServers[$i]['host'];
 554                                 }
 555                         }
 556                 }
 557                 return array( $host, $maxLag );
 558         }
 559
 560         /**
 561          * Get lag time for each DB
 562          * Results are cached for a short time in memcached
 563          */
 564         function getLagTimes() {
 565                 $expiry = 5;
 566                 $requestRate = 10;
 567
 568                 global $wgMemc;
 569                 $times = $wgMemc->get( 'lag_times' );
 570                 if ( $times ) {
 571                         # Randomly recache with probability rising over $expiry
 572                         $elapsed = time() - $times['timestamp'];
 573                         $chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
 574                         if ( mt_rand( 0, $chance ) != 0 ) {
 575                                 unset( $times['timestamp'] );
 576                                 return $times;
 577                         }
 578                 }
 579
 580                 # Cache key missing or expired
 581
 582                 $times = array();
 583                 foreach ( $this->mServers as $i => $conn ) {
 584                         if ( $this->openConnection( $i ) ) {
 585                                 $times[$i] = $this->mConnections[$i]->getLag();
 586                         }
 587                 }
 588
 589                 # Add a timestamp key so we know when it was cached
 590                 $times['timestamp'] = time();
 591                 $wgMemc->set( 'lag_times', $times, $expiry );
 592
 593                 # But don't give the timestamp to the caller
 594                 unset($times['timestamp']);
 595                 return $times;
 596         }
 597 }
 598
 599 ?>