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, $mErrorConnection;
  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                 $this->mErrorConnection = false;
  44         }
  45
  46         function newFromParams( $servers, $failFunction = false, $waitTimeout = 10 )
  47         {
  48                 $lb = new LoadBalancer;
  49                 $lb->initialise( $servers, $failFunction, $waitTimeout );
  50                 return $lb;
  51         }
  52
  53         function initialise( $servers, $failFunction = false, $waitTimeout = 10 )
  54         {
  55                 $this->mServers = $servers;
  56                 $this->mFailFunction = $failFunction;
  57                 $this->mReadIndex = -1;
  58                 $this->mWriteIndex = -1;
  59                 $this->mForce = -1;
  60                 $this->mConnections = array();
  61                 $this->mLastIndex = 1;
  62                 $this->mLoads = array();
  63                 $this->mWaitForFile = false;
  64                 $this->mWaitForPos = false;
  65                 $this->mWaitTimeout = $waitTimeout;
  66                 $this->mLaggedSlaveMode = false;
  67
  68                 foreach( $servers as $i => $server ) {
  69                         $this->mLoads[$i] = $server['load'];
  70                         if ( isset( $server['groupLoads'] ) ) {
  71                                 foreach ( $server['groupLoads'] as $group => $ratio ) {
  72                                         if ( !isset( $this->mGroupLoads[$group] ) ) {
  73                                                 $this->mGroupLoads[$group] = array();
  74                                         }
  75                                         $this->mGroupLoads[$group][$i] = $ratio;
  76                                 }
  77                         }
  78                 }
  79         }
  80
  81         /**
  82          * Given an array of non-normalised probabilities, this function will select
  83          * an element and return the appropriate key
  84          */
  85         function pickRandom( $weights )
  86         {
  87                 if ( !is_array( $weights ) || count( $weights ) == 0 ) {
  88                         return false;
  89                 }
  90
  91                 $sum = 0;
  92                 foreach ( $weights as $w ) {
  93                         $sum += $w;
  94                 }
  95
  96                 if ( $sum == 0 ) {
  97                         # No loads on any of them
  98                         # Just pick one at random
  99                         foreach ( $weights as $i => $w ) {
 100                                 $weights[$i] = 1;
 101                         }
 102                 }
 103                 $max = mt_getrandmax();
 104                 $rand = mt_rand(0, $max) / $max * $sum;
 105
 106                 $sum = 0;
 107                 foreach ( $weights as $i => $w ) {
 108                         $sum += $w;
 109                         if ( $sum >= $rand ) {
 110                                 break;
 111                         }
 112                 }
 113                 return $i;
 114         }
 115
 116         function getRandomNonLagged( $loads ) {
 117                 # Unset excessively lagged servers
 118                 $lags = $this->getLagTimes();
 119                 foreach ( $lags as $i => $lag ) {
 120                         if ( isset( $this->mServers[$i]['max lag'] ) && $lag > $this->mServers[$i]['max lag'] ) {
 121                                 unset( $loads[$i] );
 122                         }
 123                 }
 124
 125
 126                 # Find out if all the slaves with non-zero load are lagged
 127                 $sum = 0;
 128                 foreach ( $loads as $load ) {
 129                         $sum += $load;
 130                 }
 131                 if ( $sum == 0 ) {
 132                         # No appropriate DB servers except maybe the master and some slaves with zero load
 133                         # Do NOT use the master
 134                         # Instead, this function will return false, triggering read-only mode,
 135                         # and a lagged slave will be used instead.
 136                         unset ( $loads[0] );
 137                 }
 138
 139                 if ( count( $loads ) == 0 ) {
 140                         return false;
 141                 }
 142
 143                 #wfDebug( var_export( $loads, true ) );
 144
 145                 # Return a random representative of the remainder
 146                 return $this->pickRandom( $loads );
 147         }
 148
 149         /**
 150          * Get the index of the reader connection, which may be a slave
 151          * This takes into account load ratios and lag times. It should
 152          * always return a consistent index during a given invocation
 153          *
 154          * Side effect: opens connections to databases
 155          */
 156         function getReaderIndex()
 157         {
 158                 global $wgMaxLag, $wgReadOnly, $wgDBClusterTimeout;
 159
 160                 $fname = 'LoadBalancer::getReaderIndex';
 161                 wfProfileIn( $fname );
 162
 163                 $i = false;
 164                 if ( $this->mForce >= 0 ) {
 165                         $i = $this->mForce;
 166                 } else {
 167                         if ( $this->mReadIndex >= 0 ) {
 168                                 $i = $this->mReadIndex;
 169                         } else {
 170                                 # $loads is $this->mLoads except with elements knocked out if they
 171                                 # don't work
 172                                 $loads = $this->mLoads;
 173                                 $done = false;
 174                                 $totalElapsed = 0;
 175                                 do {
 176                                         if ( $wgReadOnly ) {
 177                                                 $i = $this->pickRandom( $loads );
 178                                         } else {
 179                                                 $i = $this->getRandomNonLagged( $loads );
 180                                                 if ( $i === false && count( $loads ) != 0 )  {
 181                                                         # All slaves lagged. Switch to read-only mode
 182                                                         $wgReadOnly = wfMsgNoDB( 'readonly_lag' );
 183                                                         $i = $this->pickRandom( $loads );
 184                                                 }
 185                                         }
 186                                         if ( $i !== false ) {
 187                                                 wfDebug( "Using reader #$i: {$this->mServers[$i]['host']}...\n" );
 188                                                 $this->openConnection( $i );
 189
 190                                                 if ( !$this->isOpen( $i ) ) {
 191                                                         wfDebug( "Failed\n" );
 192                                                         unset( $loads[$i] );
 193                                                         $sleepTime = 0;
 194                                                 } else {
 195                                                         $status = $this->mConnections[$i]->getStatus();
 196                                                         if ( isset( $this->mServers[$i]['max threads'] ) &&
 197                                                           $status['Threads_running'] > $this->mServers[$i]['max threads'] )
 198                                                         {
 199                                                                 # Slave is lagged, wait for a while
 200                                                                 $sleepTime = 5000 * $status['Threads_connected'];
 201
 202                                                                 # If we reach the timeout and exit the loop, don't use it
 203                                                                 $i = false;
 204                                                         } else {
 205                                                                 $done = true;
 206                                                                 $sleepTime = 0;
 207                                                         }
 208                                                 }
 209                                         } else {
 210                                                 $sleepTime = 500000;
 211                                         }
 212                                         if ( $sleepTime ) {
 213                                                         $totalElapsed += $sleepTime;
 214                                                         usleep( $sleepTime );
 215                                         }
 216                                 } while ( count( $loads ) && !$done && $totalElapsed / 1e6 < $wgDBClusterTimeout );
 217
 218                                 if ( $i !== false && $this->isOpen( $i ) ) {
 219                                         # Wait for the session master pos for a short time
 220                                         if ( $this->mWaitForFile ) {
 221                                                 if ( !$this->doWait( $i ) ) {
 222                                                         $this->mServers[$i]['slave pos'] = $this->mConnections[$i]->getSlavePos();
 223                                                 }
 224                                         }
 225                                         if ( $i !== false ) {
 226                                                 $this->mReadIndex = $i;
 227                                         }
 228                                 } else {
 229                                         $i = false;
 230                                 }
 231                         }
 232                 }
 233                 wfProfileOut( $fname );
 234                 return $i;
 235         }
 236
 237         /**
 238          * Get a random server to use in a query group
 239          */
 240         function getGroupIndex( $group ) {
 241                 if ( isset( $this->mGroupLoads[$group] ) ) {
 242                         $i = $this->pickRandom( $this->mGroupLoads[$group] );
 243                 } else {
 244                         $i = false;
 245                 }
 246                 wfDebug( "Query group $group => $i\n" );
 247                 return $i;
 248         }
 249
 250         /**
 251          * Set the master wait position
 252          * If a DB_SLAVE connection has been opened already, waits
 253          * Otherwise sets a variable telling it to wait if such a connection is opened
 254          */
 255         function waitFor( $file, $pos ) {
 256                 $fname = 'LoadBalancer::waitFor';
 257                 wfProfileIn( $fname );
 258
 259                 wfDebug( "User master pos: $file $pos\n" );
 260                 $this->mWaitForFile = false;
 261                 $this->mWaitForPos = false;
 262
 263                 if ( count( $this->mServers ) > 1 ) {
 264                         $this->mWaitForFile = $file;
 265                         $this->mWaitForPos = $pos;
 266                         $i = $this->mReadIndex;
 267
 268                         if ( $i > 0 ) {
 269                                 if ( !$this->doWait( $i ) ) {
 270                                         $this->mServers[$i]['slave pos'] = $this->mConnections[$i]->getSlavePos();
 271                                         $this->mLaggedSlaveMode = true;
 272                                 }
 273                         }
 274                 }
 275                 wfProfileOut( $fname );
 276         }
 277
 278         /**
 279          * Wait for a given slave to catch up to the master pos stored in $this
 280          */
 281         function doWait( $index ) {
 282                 global $wgMemc;
 283
 284                 $retVal = false;
 285
 286                 # Debugging hacks
 287                 if ( isset( $this->mServers[$index]['lagged slave'] ) ) {
 288                         return false;
 289                 } elseif ( isset( $this->mServers[$index]['fake slave'] ) ) {
 290                         return true;
 291                 }
 292
 293                 $key = 'masterpos:' . $index;
 294                 $memcPos = $wgMemc->get( $key );
 295                 if ( $memcPos ) {
 296                         list( $file, $pos ) = explode( ' ', $memcPos );
 297                         # If the saved position is later than the requested position, return now
 298                         if ( $file == $this->mWaitForFile && $this->mWaitForPos <= $pos ) {
 299                                 $retVal = true;
 300                         }
 301                 }
 302
 303                 if ( !$retVal && $this->isOpen( $index ) ) {
 304                         $conn =& $this->mConnections[$index];
 305                         wfDebug( "Waiting for slave #$index to catch up...\n" );
 306                         $result = $conn->masterPosWait( $this->mWaitForFile, $this->mWaitForPos, $this->mWaitTimeout );
 307
 308                         if ( $result == -1 || is_null( $result ) ) {
 309                                 # Timed out waiting for slave, use master instead
 310                                 wfDebug( "Timed out waiting for slave #$index pos {$this->mWaitForFile} {$this->mWaitForPos}\n" );
 311                                 $retVal = false;
 312                         } else {
 313                                 $retVal = true;
 314                                 wfDebug( "Done\n" );
 315                         }
 316                 }
 317                 return $retVal;
 318         }
 319
 320         /**
 321          * Get a connection by index
 322          */
 323         function &getConnection( $i, $fail = true, $groups = array() )
 324         {
 325                 $fname = 'LoadBalancer::getConnection';
 326                 wfProfileIn( $fname );
 327
 328                 # Query groups
 329                 $groupIndex = false;
 330                 foreach ( $groups as $group ) {
 331                         $groupIndex = $this->getGroupIndex( $group );
 332                         if ( $groupIndex !== false ) {
 333                                 $i = $groupIndex;
 334                                 break;
 335                         }
 336                 }
 337
 338                 # Operation-based index
 339                 if ( $i == DB_SLAVE ) {
 340                         $i = $this->getReaderIndex();
 341                 } elseif ( $i == DB_MASTER ) {
 342                         $i = $this->getWriterIndex();
 343                 } elseif ( $i == DB_LAST ) {
 344                         # Just use $this->mLastIndex, which should already be set
 345                         $i = $this->mLastIndex;
 346                         if ( $i === -1 ) {
 347                                 # Oh dear, not set, best to use the writer for safety
 348                                 wfDebug( "Warning: DB_LAST used when there was no previous index\n" );
 349                                 $i = $this->getWriterIndex();
 350                         }
 351                 }
 352                 # Couldn't find a working server in getReaderIndex()?
 353                 if ( $i === false ) {
 354                         $this->reportConnectionError( $this->mErrorConnection );
 355                 }
 356                 # Now we have an explicit index into the servers array
 357                 $this->openConnection( $i, $fail );
 358
 359                 wfProfileOut( $fname );
 360                 return $this->mConnections[$i];
 361         }
 362
 363         /**
 364          * Open a connection to the server given by the specified index
 365          * Index must be an actual index into the array
 366          * Returns success
 367          * @private
 368          */
 369         function openConnection( $i, $fail = false ) {
 370                 $fname = 'LoadBalancer::openConnection';
 371                 wfProfileIn( $fname );
 372                 $success = true;
 373
 374                 if ( !$this->isOpen( $i ) ) {
 375                         $this->mConnections[$i] = $this->reallyOpenConnection( $this->mServers[$i] );
 376                 }
 377
 378                 if ( !$this->isOpen( $i ) ) {
 379                         wfDebug( "Failed to connect to database $i at {$this->mServers[$i]['host']}\n" );
 380                         if ( $fail ) {
 381                                 $this->reportConnectionError( $this->mConnections[$i] );
 382                         }
 383                         $this->mErrorConnection = $this->mConnections[$i];
 384                         $this->mConnections[$i] = false;
 385                         $success = false;
 386                 }
 387                 $this->mLastIndex = $i;
 388                 wfProfileOut( $fname );
 389                 return $success;
 390         }
 391
 392         /**
 393          * Test if the specified index represents an open connection
 394          * @private
 395          */
 396         function isOpen( $index ) {
 397                 if( !is_integer( $index ) ) {
 398                         return false;
 399                 }
 400                 if ( array_key_exists( $index, $this->mConnections ) && is_object( $this->mConnections[$index] ) &&
 401                   $this->mConnections[$index]->isOpen() )
 402                 {
 403                         return true;
 404                 } else {
 405                         return false;
 406                 }
 407         }
 408
 409         /**
 410          * Really opens a connection
 411          * @private
 412          */
 413         function reallyOpenConnection( &$server ) {
 414                 if( !is_array( $server ) ) {
 415                         wfDebugDieBacktrace( 'You must update your load-balancing configuration. See DefaultSettings.php entry for $wgDBservers.' );
 416                 }
 417
 418                 extract( $server );
 419                 # Get class for this database type
 420                 $class = 'Database' . ucfirst( $type );
 421                 if ( !class_exists( $class ) ) {
 422                         require_once( "$class.php" );
 423                 }
 424
 425                 # Create object
 426                 return new $class( $host, $user, $password, $dbname, 1, $flags );
 427         }
 428
 429         function reportConnectionError( &$conn )
 430         {
 431                 $fname = 'LoadBalancer::reportConnectionError';
 432                 wfProfileIn( $fname );
 433                 # Prevent infinite recursion
 434
 435                 static $reporting = false;
 436                 if ( !$reporting ) {
 437                         $reporting = true;
 438                         if ( !is_object( $conn ) ) {
 439                                 $conn = new Database;
 440                         }
 441                         if ( $this->mFailFunction ) {
 442                                 $conn->failFunction( $this->mFailFunction );
 443                         } else {
 444                                 $conn->failFunction( false );
 445                         }
 446                         $conn->reportConnectionError();
 447                         $reporting = false;
 448                 }
 449                 wfProfileOut( $fname );
 450         }
 451
 452         function getWriterIndex()
 453         {
 454                 return 0;
 455         }
 456
 457         function force( $i )
 458         {
 459                 $this->mForce = $i;
 460         }
 461
 462         function haveIndex( $i )
 463         {
 464                 return array_key_exists( $i, $this->mServers );
 465         }
 466
 467         /**
 468          * Get the number of defined servers (not the number of open connections)
 469          */
 470         function getServerCount() {
 471                 return count( $this->mServers );
 472         }
 473
 474         /**
 475          * Save master pos to the session and to memcached, if the session exists
 476          */
 477         function saveMasterPos() {
 478                 global $wgSessionStarted;
 479                 if ( $wgSessionStarted && count( $this->mServers ) > 1 ) {
 480                         # If this entire request was served from a slave without opening a connection to the
 481                         # master (however unlikely that may be), then we can fetch the position from the slave.
 482                         if ( empty( $this->mConnections[0] ) ) {
 483                                 $conn =& $this->getConnection( DB_SLAVE );
 484                                 list( $file, $pos ) = $conn->getSlavePos();
 485                                 wfDebug( "Saving master pos fetched from slave: $file $pos\n" );
 486                         } else {
 487                                 $conn =& $this->getConnection( 0 );
 488                                 list( $file, $pos ) = $conn->getMasterPos();
 489                                 wfDebug( "Saving master pos: $file $pos\n" );
 490                         }
 491                         if ( $file !== false ) {
 492                                 $_SESSION['master_log_file'] = $file;
 493                                 $_SESSION['master_pos'] = $pos;
 494                         }
 495                 }
 496         }
 497
 498         /**
 499          * Loads the master pos from the session, waits for it if necessary
 500          */
 501         function loadMasterPos() {
 502                 if ( isset( $_SESSION['master_log_file'] ) && isset( $_SESSION['master_pos'] ) ) {
 503                         $this->waitFor( $_SESSION['master_log_file'], $_SESSION['master_pos'] );
 504                 }
 505         }
 506
 507         /**
 508          * Close all open connections
 509          */
 510         function closeAll() {
 511                 foreach( $this->mConnections as $i => $conn ) {
 512                         if ( $this->isOpen( $i ) ) {
 513                                 // Need to use this syntax because $conn is a copy not a reference
 514                                 $this->mConnections[$i]->close();
 515                         }
 516                 }
 517         }
 518
 519         function commitAll() {
 520                 foreach( $this->mConnections as $i => $conn ) {
 521                         if ( $this->isOpen( $i ) ) {
 522                                 // Need to use this syntax because $conn is a copy not a reference
 523                                 $this->mConnections[$i]->immediateCommit();
 524                         }
 525                 }
 526         }
 527
 528         function waitTimeout( $value = NULL ) {
 529                 return wfSetVar( $this->mWaitTimeout, $value );
 530         }
 531
 532         function getLaggedSlaveMode() {
 533                 return $this->mLaggedSlaveMode;
 534         }
 535
 536         function pingAll() {
 537                 $success = true;
 538                 foreach ( $this->mConnections as $i => $conn ) {
 539                         if ( $this->isOpen( $i ) ) {
 540                                 if ( !$this->mConnections[$i]->ping() ) {
 541                                         $success = false;
 542                                 }
 543                         }
 544                 }
 545                 return $success;
 546         }
 547
 548         /**
 549          * Get the hostname and lag time of the most-lagged slave
 550          * This is useful for maintenance scripts that need to throttle their updates
 551          */
 552         function getMaxLag() {
 553                 $maxLag = -1;
 554                 $host = '';
 555                 foreach ( $this->mServers as $i => $conn ) {
 556                         if ( $this->openConnection( $i ) ) {
 557                                 $lag = $this->mConnections[$i]->getLag();
 558                                 if ( $lag > $maxLag ) {
 559                                         $maxLag = $lag;
 560                                         $host = $this->mServers[$i]['host'];
 561                                 }
 562                         }
 563                 }
 564                 return array( $host, $maxLag );
 565         }
 566
 567         /**
 568          * Get lag time for each DB
 569          * Results are cached for a short time in memcached
 570          */
 571         function getLagTimes() {
 572                 $expiry = 5;
 573                 $requestRate = 10;
 574
 575                 global $wgMemc;
 576                 $times = $wgMemc->get( 'lag_times' );
 577                 if ( $times ) {
 578                         # Randomly recache with probability rising over $expiry
 579                         $elapsed = time() - $times['timestamp'];
 580                         $chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
 581                         if ( mt_rand( 0, $chance ) != 0 ) {
 582                                 unset( $times['timestamp'] );
 583                                 return $times;
 584                         }
 585                 }
 586
 587                 # Cache key missing or expired
 588
 589                 $times = array();
 590                 foreach ( $this->mServers as $i => $conn ) {
 591                         if ( $this->openConnection( $i ) ) {
 592                                 $times[$i] = $this->mConnections[$i]->getLag();
 593                         }
 594                 }
 595
 596                 # Add a timestamp key so we know when it was cached
 597                 $times['timestamp'] = time();
 598                 $wgMemc->set( 'lag_times', $times, $expiry );
 599
 600                 # But don't give the timestamp to the caller
 601                 unset($times['timestamp']);
 602                 return $times;
 603         }
 604 }
 605
 606 ?>