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 # Scale polling time so that under overload conditions, the database server
  24 # receives a SHOW STATUS query at an average interval of this many microseconds
  25 define( 'AVG_STATUS_POLL', 2000 );
  26
  27
  28 /**
  29  * Database load balancing object
  30  *
  31  * @todo document
  32  * @package MediaWiki
  33  */
  34 class LoadBalancer {
  35         /* private */ var $mServers, $mConnections, $mLoads, $mGroupLoads;
  36         /* private */ var $mFailFunction, $mErrorConnection;
  37         /* private */ var $mForce, $mReadIndex, $mLastIndex;
  38         /* private */ var $mWaitForFile, $mWaitForPos, $mWaitTimeout;
  39         /* private */ var $mLaggedSlaveMode;
  40
  41         function LoadBalancer()
  42         {
  43                 $this->mServers = array();
  44                 $this->mConnections = array();
  45                 $this->mFailFunction = false;
  46                 $this->mReadIndex = -1;
  47                 $this->mForce = -1;
  48                 $this->mLastIndex = -1;
  49                 $this->mErrorConnection = false;
  50         }
  51
  52         function newFromParams( $servers, $failFunction = false, $waitTimeout = 10 )
  53         {
  54                 $lb = new LoadBalancer;
  55                 $lb->initialise( $servers, $failFunction, $waitTimeout );
  56                 return $lb;
  57         }
  58
  59         function initialise( $servers, $failFunction = false, $waitTimeout = 10 )
  60         {
  61                 $this->mServers = $servers;
  62                 $this->mFailFunction = $failFunction;
  63                 $this->mReadIndex = -1;
  64                 $this->mWriteIndex = -1;
  65                 $this->mForce = -1;
  66                 $this->mConnections = array();
  67                 $this->mLastIndex = 1;
  68                 $this->mLoads = array();
  69                 $this->mWaitForFile = false;
  70                 $this->mWaitForPos = false;
  71                 $this->mWaitTimeout = $waitTimeout;
  72                 $this->mLaggedSlaveMode = false;
  73
  74                 foreach( $servers as $i => $server ) {
  75                         $this->mLoads[$i] = $server['load'];
  76                         if ( isset( $server['groupLoads'] ) ) {
  77                                 foreach ( $server['groupLoads'] as $group => $ratio ) {
  78                                         if ( !isset( $this->mGroupLoads[$group] ) ) {
  79                                                 $this->mGroupLoads[$group] = array();
  80                                         }
  81                                         $this->mGroupLoads[$group][$i] = $ratio;
  82                                 }
  83                         }
  84                 }
  85         }
  86
  87         /**
  88          * Given an array of non-normalised probabilities, this function will select
  89          * an element and return the appropriate key
  90          */
  91         function pickRandom( $weights )
  92         {
  93                 if ( !is_array( $weights ) || count( $weights ) == 0 ) {
  94                         return false;
  95                 }
  96
  97                 $sum = 0;
  98                 foreach ( $weights as $w ) {
  99                         $sum += $w;
 100                 }
 101
 102                 if ( $sum == 0 ) {
 103                         # No loads on any of them
 104                         # Just pick one at random
 105                         foreach ( $weights as $i => $w ) {
 106                                 $weights[$i] = 1;
 107                         }
 108                 }
 109                 $max = mt_getrandmax();
 110                 $rand = mt_rand(0, $max) / $max * $sum;
 111
 112                 $sum = 0;
 113                 foreach ( $weights as $i => $w ) {
 114                         $sum += $w;
 115                         if ( $sum >= $rand ) {
 116                                 break;
 117                         }
 118                 }
 119                 return $i;
 120         }
 121
 122         function getRandomNonLagged( $loads ) {
 123                 # Unset excessively lagged servers
 124                 $lags = $this->getLagTimes();
 125                 foreach ( $lags as $i => $lag ) {
 126                         if ( isset( $this->mServers[$i]['max lag'] ) && $lag > $this->mServers[$i]['max lag'] ) {
 127                                 unset( $loads[$i] );
 128                         }
 129                 }
 130
 131
 132                 # Find out if all the slaves with non-zero load are lagged
 133                 $sum = 0;
 134                 foreach ( $loads as $load ) {
 135                         $sum += $load;
 136                 }
 137                 if ( $sum == 0 ) {
 138                         # No appropriate DB servers except maybe the master and some slaves with zero load
 139                         # Do NOT use the master
 140                         # Instead, this function will return false, triggering read-only mode,
 141                         # and a lagged slave will be used instead.
 142                         unset ( $loads[0] );
 143                 }
 144
 145                 if ( count( $loads ) == 0 ) {
 146                         return false;
 147                 }
 148
 149                 #wfDebugLog( 'connect', var_export( $loads, true ) );
 150
 151                 # Return a random representative of the remainder
 152                 return $this->pickRandom( $loads );
 153         }
 154
 155         /**
 156          * Get the index of the reader connection, which may be a slave
 157          * This takes into account load ratios and lag times. It should
 158          * always return a consistent index during a given invocation
 159          *
 160          * Side effect: opens connections to databases
 161          */
 162         function getReaderIndex()
 163         {
 164                 global $wgMaxLag, $wgReadOnly, $wgDBClusterTimeout;
 165
 166                 $fname = 'LoadBalancer::getReaderIndex';
 167                 wfProfileIn( $fname );
 168
 169                 $i = false;
 170                 if ( $this->mForce >= 0 ) {
 171                         $i = $this->mForce;
 172                 } else {
 173                         if ( $this->mReadIndex >= 0 ) {
 174                                 $i = $this->mReadIndex;
 175                         } else {
 176                                 # $loads is $this->mLoads except with elements knocked out if they
 177                                 # don't work
 178                                 $loads = $this->mLoads;
 179                                 $done = false;
 180                                 $totalElapsed = 0;
 181                                 do {
 182                                         if ( $wgReadOnly ) {
 183                                                 $i = $this->pickRandom( $loads );
 184                                         } else {
 185                                                 $i = $this->getRandomNonLagged( $loads );
 186                                                 if ( $i === false && count( $loads ) != 0 )  {
 187                                                         # All slaves lagged. Switch to read-only mode
 188                                                         $wgReadOnly = wfMsgNoDB( 'readonly_lag' );
 189                                                         $i = $this->pickRandom( $loads );
 190                                                 }
 191                                         }
 192                                         $serverIndex = $i;
 193                                         if ( $i !== false ) {
 194                                                 wfDebugLog( 'connect', "Using reader #$i: {$this->mServers[$i]['host']}...\n" );
 195                                                 $this->openConnection( $i );
 196
 197                                                 if ( !$this->isOpen( $i ) ) {
 198                                                         wfDebug( "Failed\n" );
 199                                                         unset( $loads[$i] );
 200                                                         $sleepTime = 0;
 201                                                 } else {
 202                                                         $status = $this->mConnections[$i]->getStatus();
 203                                                         if ( isset( $this->mServers[$i]['max threads'] ) &&
 204                                                           $status['Threads_running'] > $this->mServers[$i]['max threads'] )
 205                                                         {
 206                                                                 # Slave is lagged, wait for a while
 207                                                                 $sleepTime = AVG_STATUS_POLL * $status['Threads_connected'];
 208
 209                                                                 # If we reach the timeout and exit the loop, don't use it
 210                                                                 $i = false;
 211                                                         } else {
 212                                                                 $done = true;
 213                                                                 $sleepTime = 0;
 214                                                         }
 215                                                 }
 216                                         } else {
 217                                                 $sleepTime = 500000;
 218                                         }
 219                                         if ( $sleepTime ) {
 220                                                         $totalElapsed += $sleepTime;
 221                                                         $x = "{$this->mServers[$serverIndex]['host']} [$serverIndex]";
 222                                                         wfProfileIn( "$fname-sleep $x" );
 223                                                         usleep( $sleepTime );
 224                                                         wfProfileOut( "$fname-sleep $x" );
 225                                         }
 226                                 } while ( count( $loads ) && !$done && $totalElapsed / 1e6 < $wgDBClusterTimeout );
 227
 228                                 if ( $i !== false && $this->isOpen( $i ) ) {
 229                                         # Wait for the session master pos for a short time
 230                                         if ( $this->mWaitForFile ) {
 231                                                 if ( !$this->doWait( $i ) ) {
 232                                                         $this->mServers[$i]['slave pos'] = $this->mConnections[$i]->getSlavePos();
 233                                                 }
 234                                         }
 235                                         if ( $i !== false ) {
 236                                                 $this->mReadIndex = $i;
 237                                         }
 238                                 } else {
 239                                         $i = false;
 240                                 }
 241                         }
 242                 }
 243                 wfProfileOut( $fname );
 244                 return $i;
 245         }
 246
 247         /**
 248          * Get a random server to use in a query group
 249          */
 250         function getGroupIndex( $group ) {
 251                 if ( isset( $this->mGroupLoads[$group] ) ) {
 252                         $i = $this->pickRandom( $this->mGroupLoads[$group] );
 253                 } else {
 254                         $i = false;
 255                 }
 256                 wfDebug( "Query group $group => $i\n" );
 257                 return $i;
 258         }
 259
 260         /**
 261          * Set the master wait position
 262          * If a DB_SLAVE connection has been opened already, waits
 263          * Otherwise sets a variable telling it to wait if such a connection is opened
 264          */
 265         function waitFor( $file, $pos ) {
 266                 $fname = 'LoadBalancer::waitFor';
 267                 wfProfileIn( $fname );
 268
 269                 wfDebug( "User master pos: $file $pos\n" );
 270                 $this->mWaitForFile = false;
 271                 $this->mWaitForPos = false;
 272
 273                 if ( count( $this->mServers ) > 1 ) {
 274                         $this->mWaitForFile = $file;
 275                         $this->mWaitForPos = $pos;
 276                         $i = $this->mReadIndex;
 277
 278                         if ( $i > 0 ) {
 279                                 if ( !$this->doWait( $i ) ) {
 280                                         $this->mServers[$i]['slave pos'] = $this->mConnections[$i]->getSlavePos();
 281                                         $this->mLaggedSlaveMode = true;
 282                                 }
 283                         }
 284                 }
 285                 wfProfileOut( $fname );
 286         }
 287
 288         /**
 289          * Wait for a given slave to catch up to the master pos stored in $this
 290          */
 291         function doWait( $index ) {
 292                 global $wgMemc;
 293
 294                 $retVal = false;
 295
 296                 # Debugging hacks
 297                 if ( isset( $this->mServers[$index]['lagged slave'] ) ) {
 298                         return false;
 299                 } elseif ( isset( $this->mServers[$index]['fake slave'] ) ) {
 300                         return true;
 301                 }
 302
 303                 $key = 'masterpos:' . $index;
 304                 $memcPos = $wgMemc->get( $key );
 305                 if ( $memcPos ) {
 306                         list( $file, $pos ) = explode( ' ', $memcPos );
 307                         # If the saved position is later than the requested position, return now
 308                         if ( $file == $this->mWaitForFile && $this->mWaitForPos <= $pos ) {
 309                                 $retVal = true;
 310                         }
 311                 }
 312
 313                 if ( !$retVal && $this->isOpen( $index ) ) {
 314                         $conn =& $this->mConnections[$index];
 315                         wfDebug( "Waiting for slave #$index to catch up...\n" );
 316                         $result = $conn->masterPosWait( $this->mWaitForFile, $this->mWaitForPos, $this->mWaitTimeout );
 317
 318                         if ( $result == -1 || is_null( $result ) ) {
 319                                 # Timed out waiting for slave, use master instead
 320                                 wfDebug( "Timed out waiting for slave #$index pos {$this->mWaitForFile} {$this->mWaitForPos}\n" );
 321                                 $retVal = false;
 322                         } else {
 323                                 $retVal = true;
 324                                 wfDebug( "Done\n" );
 325                         }
 326                 }
 327                 return $retVal;
 328         }
 329
 330         /**
 331          * Get a connection by index
 332          */
 333         function &getConnection( $i, $fail = true, $groups = array() )
 334         {
 335                 $fname = 'LoadBalancer::getConnection';
 336                 wfProfileIn( $fname );
 337
 338                 # Query groups
 339                 $groupIndex = false;
 340                 foreach ( $groups as $group ) {
 341                         $groupIndex = $this->getGroupIndex( $group );
 342                         if ( $groupIndex !== false ) {
 343                                 $i = $groupIndex;
 344                                 break;
 345                         }
 346                 }
 347
 348                 # Operation-based index
 349                 if ( $i == DB_SLAVE ) {
 350                         $i = $this->getReaderIndex();
 351                 } elseif ( $i == DB_MASTER ) {
 352                         $i = $this->getWriterIndex();
 353                 } elseif ( $i == DB_LAST ) {
 354                         # Just use $this->mLastIndex, which should already be set
 355                         $i = $this->mLastIndex;
 356                         if ( $i === -1 ) {
 357                                 # Oh dear, not set, best to use the writer for safety
 358                                 wfDebug( "Warning: DB_LAST used when there was no previous index\n" );
 359                                 $i = $this->getWriterIndex();
 360                         }
 361                 }
 362                 # Couldn't find a working server in getReaderIndex()?
 363                 if ( $i === false ) {
 364                         $this->reportConnectionError( $this->mErrorConnection );
 365                 }
 366                 # Now we have an explicit index into the servers array
 367                 $this->openConnection( $i, $fail );
 368
 369                 wfProfileOut( $fname );
 370                 return $this->mConnections[$i];
 371         }
 372
 373         /**
 374          * Open a connection to the server given by the specified index
 375          * Index must be an actual index into the array
 376          * Returns success
 377          * @private
 378          */
 379         function openConnection( $i, $fail = false ) {
 380                 $fname = 'LoadBalancer::openConnection';
 381                 wfProfileIn( $fname );
 382                 $success = true;
 383
 384                 if ( !$this->isOpen( $i ) ) {
 385                         $this->mConnections[$i] = $this->reallyOpenConnection( $this->mServers[$i] );
 386                 }
 387
 388                 if ( !$this->isOpen( $i ) ) {
 389                         wfDebug( "Failed to connect to database $i at {$this->mServers[$i]['host']}\n" );
 390                         if ( $fail ) {
 391                                 $this->reportConnectionError( $this->mConnections[$i] );
 392                         }
 393                         $this->mErrorConnection = $this->mConnections[$i];
 394                         $this->mConnections[$i] = false;
 395                         $success = false;
 396                 }
 397                 $this->mLastIndex = $i;
 398                 wfProfileOut( $fname );
 399                 return $success;
 400         }
 401
 402         /**
 403          * Test if the specified index represents an open connection
 404          * @private
 405          */
 406         function isOpen( $index ) {
 407                 if( !is_integer( $index ) ) {
 408                         return false;
 409                 }
 410                 if ( array_key_exists( $index, $this->mConnections ) && is_object( $this->mConnections[$index] ) &&
 411                   $this->mConnections[$index]->isOpen() )
 412                 {
 413                         return true;
 414                 } else {
 415                         return false;
 416                 }
 417         }
 418
 419         /**
 420          * Really opens a connection
 421          * @private
 422          */
 423         function reallyOpenConnection( &$server ) {
 424                 if( !is_array( $server ) ) {
 425                         wfDebugDieBacktrace( 'You must update your load-balancing configuration. See DefaultSettings.php entry for $wgDBservers.' );
 426                 }
 427
 428                 extract( $server );
 429                 # Get class for this database type
 430                 $class = 'Database' . ucfirst( $type );
 431                 if ( !class_exists( $class ) ) {
 432                         require_once( "$class.php" );
 433                 }
 434
 435                 # Create object
 436                 return new $class( $host, $user, $password, $dbname, 1, $flags );
 437         }
 438
 439         function reportConnectionError( &$conn )
 440         {
 441                 $fname = 'LoadBalancer::reportConnectionError';
 442                 wfProfileIn( $fname );
 443                 # Prevent infinite recursion
 444
 445                 static $reporting = false;
 446                 if ( !$reporting ) {
 447                         $reporting = true;
 448                         if ( !is_object( $conn ) ) {
 449                                 $conn = new Database;
 450                         }
 451                         if ( $this->mFailFunction ) {
 452                                 $conn->failFunction( $this->mFailFunction );
 453                         } else {
 454                                 $conn->failFunction( false );
 455                         }
 456                         $conn->reportConnectionError();
 457                         $reporting = false;
 458                 }
 459                 wfProfileOut( $fname );
 460         }
 461
 462         function getWriterIndex()
 463         {
 464                 return 0;
 465         }
 466
 467         function force( $i )
 468         {
 469                 $this->mForce = $i;
 470         }
 471
 472         function haveIndex( $i )
 473         {
 474                 return array_key_exists( $i, $this->mServers );
 475         }
 476
 477         /**
 478          * Get the number of defined servers (not the number of open connections)
 479          */
 480         function getServerCount() {
 481                 return count( $this->mServers );
 482         }
 483
 484         /**
 485          * Save master pos to the session and to memcached, if the session exists
 486          */
 487         function saveMasterPos() {
 488                 global $wgSessionStarted;
 489                 if ( $wgSessionStarted && count( $this->mServers ) > 1 ) {
 490                         # If this entire request was served from a slave without opening a connection to the
 491                         # master (however unlikely that may be), then we can fetch the position from the slave.
 492                         if ( empty( $this->mConnections[0] ) ) {
 493                                 $conn =& $this->getConnection( DB_SLAVE );
 494                                 list( $file, $pos ) = $conn->getSlavePos();
 495                                 wfDebug( "Saving master pos fetched from slave: $file $pos\n" );
 496                         } else {
 497                                 $conn =& $this->getConnection( 0 );
 498                                 list( $file, $pos ) = $conn->getMasterPos();
 499                                 wfDebug( "Saving master pos: $file $pos\n" );
 500                         }
 501                         if ( $file !== false ) {
 502                                 $_SESSION['master_log_file'] = $file;
 503                                 $_SESSION['master_pos'] = $pos;
 504                         }
 505                 }
 506         }
 507
 508         /**
 509          * Loads the master pos from the session, waits for it if necessary
 510          */
 511         function loadMasterPos() {
 512                 if ( isset( $_SESSION['master_log_file'] ) && isset( $_SESSION['master_pos'] ) ) {
 513                         $this->waitFor( $_SESSION['master_log_file'], $_SESSION['master_pos'] );
 514                 }
 515         }
 516
 517         /**
 518          * Close all open connections
 519          */
 520         function closeAll() {
 521                 foreach( $this->mConnections as $i => $conn ) {
 522                         if ( $this->isOpen( $i ) ) {
 523                                 // Need to use this syntax because $conn is a copy not a reference
 524                                 $this->mConnections[$i]->close();
 525                         }
 526                 }
 527         }
 528
 529         function commitAll() {
 530                 foreach( $this->mConnections as $i => $conn ) {
 531                         if ( $this->isOpen( $i ) ) {
 532                                 // Need to use this syntax because $conn is a copy not a reference
 533                                 $this->mConnections[$i]->immediateCommit();
 534                         }
 535                 }
 536         }
 537
 538         function waitTimeout( $value = NULL ) {
 539                 return wfSetVar( $this->mWaitTimeout, $value );
 540         }
 541
 542         function getLaggedSlaveMode() {
 543                 return $this->mLaggedSlaveMode;
 544         }
 545
 546         function pingAll() {
 547                 $success = true;
 548                 foreach ( $this->mConnections as $i => $conn ) {
 549                         if ( $this->isOpen( $i ) ) {
 550                                 if ( !$this->mConnections[$i]->ping() ) {
 551                                         $success = false;
 552                                 }
 553                         }
 554                 }
 555                 return $success;
 556         }
 557
 558         /**
 559          * Get the hostname and lag time of the most-lagged slave
 560          * This is useful for maintenance scripts that need to throttle their updates
 561          */
 562         function getMaxLag() {
 563                 $maxLag = -1;
 564                 $host = '';
 565                 foreach ( $this->mServers as $i => $conn ) {
 566                         if ( $this->openConnection( $i ) ) {
 567                                 $lag = $this->mConnections[$i]->getLag();
 568                                 if ( $lag > $maxLag ) {
 569                                         $maxLag = $lag;
 570                                         $host = $this->mServers[$i]['host'];
 571                                 }
 572                         }
 573                 }
 574                 return array( $host, $maxLag );
 575         }
 576
 577         /**
 578          * Get lag time for each DB
 579          * Results are cached for a short time in memcached
 580          */
 581         function getLagTimes() {
 582                 $expiry = 5;
 583                 $requestRate = 10;
 584
 585                 global $wgMemc;
 586                 $times = $wgMemc->get( 'lag_times' );
 587                 if ( $times ) {
 588                         # Randomly recache with probability rising over $expiry
 589                         $elapsed = time() - $times['timestamp'];
 590                         $chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
 591                         if ( mt_rand( 0, $chance ) != 0 ) {
 592                                 unset( $times['timestamp'] );
 593                                 return $times;
 594                         }
 595                 }
 596
 597                 # Cache key missing or expired
 598
 599                 $times = array();
 600                 foreach ( $this->mServers as $i => $conn ) {
 601                         if ( $this->openConnection( $i ) ) {
 602                                 $times[$i] = $this->mConnections[$i]->getLag();
 603                         }
 604                 }
 605
 606                 # Add a timestamp key so we know when it was cached
 607                 $times['timestamp'] = time();
 608                 $wgMemc->set( 'lag_times', $times, $expiry );
 609
 610                 # But don't give the timestamp to the caller
 611                 unset($times['timestamp']);
 612                 return $times;
 613         }
 614 }
 615
 616 ?>