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, $mLastError = 'Unknown error';
  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 ( $totalElapsed / 1e6 >= $wgDBClusterTimeout ) {
 229                                         $this->mErrorConnection = false;
 230                                         $this->mLastError = 'All servers busy';
 231                                 }
 232
 233                                 if ( $i !== false && $this->isOpen( $i ) ) {
 234                                         # Wait for the session master pos for a short time
 235                                         if ( $this->mWaitForFile ) {
 236                                                 if ( !$this->doWait( $i ) ) {
 237                                                         $this->mServers[$i]['slave pos'] = $this->mConnections[$i]->getSlavePos();
 238                                                 }
 239                                         }
 240                                         if ( $i !== false ) {
 241                                                 $this->mReadIndex = $i;
 242                                         }
 243                                 } else {
 244                                         $i = false;
 245                                 }
 246                         }
 247                 }
 248                 wfProfileOut( $fname );
 249                 return $i;
 250         }
 251
 252         /**
 253          * Get a random server to use in a query group
 254          */
 255         function getGroupIndex( $group ) {
 256                 if ( isset( $this->mGroupLoads[$group] ) ) {
 257                         $i = $this->pickRandom( $this->mGroupLoads[$group] );
 258                 } else {
 259                         $i = false;
 260                 }
 261                 wfDebug( "Query group $group => $i\n" );
 262                 return $i;
 263         }
 264
 265         /**
 266          * Set the master wait position
 267          * If a DB_SLAVE connection has been opened already, waits
 268          * Otherwise sets a variable telling it to wait if such a connection is opened
 269          */
 270         function waitFor( $file, $pos ) {
 271                 $fname = 'LoadBalancer::waitFor';
 272                 wfProfileIn( $fname );
 273
 274                 wfDebug( "User master pos: $file $pos\n" );
 275                 $this->mWaitForFile = false;
 276                 $this->mWaitForPos = false;
 277
 278                 if ( count( $this->mServers ) > 1 ) {
 279                         $this->mWaitForFile = $file;
 280                         $this->mWaitForPos = $pos;
 281                         $i = $this->mReadIndex;
 282
 283                         if ( $i > 0 ) {
 284                                 if ( !$this->doWait( $i ) ) {
 285                                         $this->mServers[$i]['slave pos'] = $this->mConnections[$i]->getSlavePos();
 286                                         $this->mLaggedSlaveMode = true;
 287                                 }
 288                         }
 289                 }
 290                 wfProfileOut( $fname );
 291         }
 292
 293         /**
 294          * Wait for a given slave to catch up to the master pos stored in $this
 295          */
 296         function doWait( $index ) {
 297                 global $wgMemc;
 298
 299                 $retVal = false;
 300
 301                 # Debugging hacks
 302                 if ( isset( $this->mServers[$index]['lagged slave'] ) ) {
 303                         return false;
 304                 } elseif ( isset( $this->mServers[$index]['fake slave'] ) ) {
 305                         return true;
 306                 }
 307
 308                 $key = 'masterpos:' . $index;
 309                 $memcPos = $wgMemc->get( $key );
 310                 if ( $memcPos ) {
 311                         list( $file, $pos ) = explode( ' ', $memcPos );
 312                         # If the saved position is later than the requested position, return now
 313                         if ( $file == $this->mWaitForFile && $this->mWaitForPos <= $pos ) {
 314                                 $retVal = true;
 315                         }
 316                 }
 317
 318                 if ( !$retVal && $this->isOpen( $index ) ) {
 319                         $conn =& $this->mConnections[$index];
 320                         wfDebug( "Waiting for slave #$index to catch up...\n" );
 321                         $result = $conn->masterPosWait( $this->mWaitForFile, $this->mWaitForPos, $this->mWaitTimeout );
 322
 323                         if ( $result == -1 || is_null( $result ) ) {
 324                                 # Timed out waiting for slave, use master instead
 325                                 wfDebug( "Timed out waiting for slave #$index pos {$this->mWaitForFile} {$this->mWaitForPos}\n" );
 326                                 $retVal = false;
 327                         } else {
 328                                 $retVal = true;
 329                                 wfDebug( "Done\n" );
 330                         }
 331                 }
 332                 return $retVal;
 333         }
 334
 335         /**
 336          * Get a connection by index
 337          */
 338         function &getConnection( $i, $fail = true, $groups = array() )
 339         {
 340                 $fname = 'LoadBalancer::getConnection';
 341                 wfProfileIn( $fname );
 342
 343                 # Query groups
 344                 $groupIndex = false;
 345                 foreach ( $groups as $group ) {
 346                         $groupIndex = $this->getGroupIndex( $group );
 347                         if ( $groupIndex !== false ) {
 348                                 $i = $groupIndex;
 349                                 break;
 350                         }
 351                 }
 352
 353                 # Operation-based index
 354                 if ( $i == DB_SLAVE ) {
 355                         $i = $this->getReaderIndex();
 356                 } elseif ( $i == DB_MASTER ) {
 357                         $i = $this->getWriterIndex();
 358                 } elseif ( $i == DB_LAST ) {
 359                         # Just use $this->mLastIndex, which should already be set
 360                         $i = $this->mLastIndex;
 361                         if ( $i === -1 ) {
 362                                 # Oh dear, not set, best to use the writer for safety
 363                                 wfDebug( "Warning: DB_LAST used when there was no previous index\n" );
 364                                 $i = $this->getWriterIndex();
 365                         }
 366                 }
 367                 # Couldn't find a working server in getReaderIndex()?
 368                 if ( $i === false ) {
 369                         $this->reportConnectionError( $this->mErrorConnection );
 370                 }
 371                 # Now we have an explicit index into the servers array
 372                 $this->openConnection( $i, $fail );
 373
 374                 wfProfileOut( $fname );
 375                 return $this->mConnections[$i];
 376         }
 377
 378         /**
 379          * Open a connection to the server given by the specified index
 380          * Index must be an actual index into the array
 381          * Returns success
 382          * @private
 383          */
 384         function openConnection( $i, $fail = false ) {
 385                 $fname = 'LoadBalancer::openConnection';
 386                 wfProfileIn( $fname );
 387                 $success = true;
 388
 389                 if ( !$this->isOpen( $i ) ) {
 390                         $this->mConnections[$i] = $this->reallyOpenConnection( $this->mServers[$i] );
 391                 }
 392
 393                 if ( !$this->isOpen( $i ) ) {
 394                         wfDebug( "Failed to connect to database $i at {$this->mServers[$i]['host']}\n" );
 395                         if ( $fail ) {
 396                                 $this->reportConnectionError( $this->mConnections[$i] );
 397                         }
 398                         $this->mErrorConnection = $this->mConnections[$i];
 399                         $this->mConnections[$i] = false;
 400                         $success = false;
 401                 }
 402                 $this->mLastIndex = $i;
 403                 wfProfileOut( $fname );
 404                 return $success;
 405         }
 406
 407         /**
 408          * Test if the specified index represents an open connection
 409          * @private
 410          */
 411         function isOpen( $index ) {
 412                 if( !is_integer( $index ) ) {
 413                         return false;
 414                 }
 415                 if ( array_key_exists( $index, $this->mConnections ) && is_object( $this->mConnections[$index] ) &&
 416                   $this->mConnections[$index]->isOpen() )
 417                 {
 418                         return true;
 419                 } else {
 420                         return false;
 421                 }
 422         }
 423
 424         /**
 425          * Really opens a connection
 426          * @private
 427          */
 428         function reallyOpenConnection( &$server ) {
 429                 if( !is_array( $server ) ) {
 430                         wfDebugDieBacktrace( 'You must update your load-balancing configuration. See DefaultSettings.php entry for $wgDBservers.' );
 431                 }
 432
 433                 extract( $server );
 434                 # Get class for this database type
 435                 $class = 'Database' . ucfirst( $type );
 436                 if ( !class_exists( $class ) ) {
 437                         require_once( "$class.php" );
 438                 }
 439
 440                 # Create object
 441                 return new $class( $host, $user, $password, $dbname, 1, $flags );
 442         }
 443
 444         function reportConnectionError( &$conn )
 445         {
 446                 $fname = 'LoadBalancer::reportConnectionError';
 447                 wfProfileIn( $fname );
 448                 # Prevent infinite recursion
 449
 450                 static $reporting = false;
 451                 if ( !$reporting ) {
 452                         $reporting = true;
 453                         if ( !is_object( $conn ) ) {
 454                                 // No last connection, probably due to all servers being too busy
 455                                 $conn = new Database;
 456                                 if ( $this->mFailFunction ) {
 457                                         $conn->failFunction( $this->mFailFunction );
 458                                         $conn->reportConnectionError( $this->mLastError );
 459                                 } else {
 460                                         // If all servers were busy, mLastError will contain something sensible
 461                                         wfEmergencyAbort( $conn, $this->mLastError );
 462                                 }
 463                         } else {
 464                                 if ( $this->mFailFunction ) {
 465                                         $conn->failFunction( $this->mFailFunction );
 466                                 } else {
 467                                         $conn->failFunction( false );
 468                                 }
 469                                 $conn->reportConnectionError( "{$this->mLastError} ({$conn->mServer})" );
 470                         }
 471                         $reporting = false;
 472                 }
 473                 wfProfileOut( $fname );
 474         }
 475
 476         function getWriterIndex()
 477         {
 478                 return 0;
 479         }
 480
 481         function force( $i )
 482         {
 483                 $this->mForce = $i;
 484         }
 485
 486         function haveIndex( $i )
 487         {
 488                 return array_key_exists( $i, $this->mServers );
 489         }
 490
 491         /**
 492          * Get the number of defined servers (not the number of open connections)
 493          */
 494         function getServerCount() {
 495                 return count( $this->mServers );
 496         }
 497
 498         /**
 499          * Save master pos to the session and to memcached, if the session exists
 500          */
 501         function saveMasterPos() {
 502                 global $wgSessionStarted;
 503                 if ( $wgSessionStarted && count( $this->mServers ) > 1 ) {
 504                         # If this entire request was served from a slave without opening a connection to the
 505                         # master (however unlikely that may be), then we can fetch the position from the slave.
 506                         if ( empty( $this->mConnections[0] ) ) {
 507                                 $conn =& $this->getConnection( DB_SLAVE );
 508                                 list( $file, $pos ) = $conn->getSlavePos();
 509                                 wfDebug( "Saving master pos fetched from slave: $file $pos\n" );
 510                         } else {
 511                                 $conn =& $this->getConnection( 0 );
 512                                 list( $file, $pos ) = $conn->getMasterPos();
 513                                 wfDebug( "Saving master pos: $file $pos\n" );
 514                         }
 515                         if ( $file !== false ) {
 516                                 $_SESSION['master_log_file'] = $file;
 517                                 $_SESSION['master_pos'] = $pos;
 518                         }
 519                 }
 520         }
 521
 522         /**
 523          * Loads the master pos from the session, waits for it if necessary
 524          */
 525         function loadMasterPos() {
 526                 if ( isset( $_SESSION['master_log_file'] ) && isset( $_SESSION['master_pos'] ) ) {
 527                         $this->waitFor( $_SESSION['master_log_file'], $_SESSION['master_pos'] );
 528                 }
 529         }
 530
 531         /**
 532          * Close all open connections
 533          */
 534         function closeAll() {
 535                 foreach( $this->mConnections as $i => $conn ) {
 536                         if ( $this->isOpen( $i ) ) {
 537                                 // Need to use this syntax because $conn is a copy not a reference
 538                                 $this->mConnections[$i]->close();
 539                         }
 540                 }
 541         }
 542
 543         function commitAll() {
 544                 foreach( $this->mConnections as $i => $conn ) {
 545                         if ( $this->isOpen( $i ) ) {
 546                                 // Need to use this syntax because $conn is a copy not a reference
 547                                 $this->mConnections[$i]->immediateCommit();
 548                         }
 549                 }
 550         }
 551
 552         function waitTimeout( $value = NULL ) {
 553                 return wfSetVar( $this->mWaitTimeout, $value );
 554         }
 555
 556         function getLaggedSlaveMode() {
 557                 return $this->mLaggedSlaveMode;
 558         }
 559
 560         function pingAll() {
 561                 $success = true;
 562                 foreach ( $this->mConnections as $i => $conn ) {
 563                         if ( $this->isOpen( $i ) ) {
 564                                 if ( !$this->mConnections[$i]->ping() ) {
 565                                         $success = false;
 566                                 }
 567                         }
 568                 }
 569                 return $success;
 570         }
 571
 572         /**
 573          * Get the hostname and lag time of the most-lagged slave
 574          * This is useful for maintenance scripts that need to throttle their updates
 575          */
 576         function getMaxLag() {
 577                 $maxLag = -1;
 578                 $host = '';
 579                 foreach ( $this->mServers as $i => $conn ) {
 580                         if ( $this->openConnection( $i ) ) {
 581                                 $lag = $this->mConnections[$i]->getLag();
 582                                 if ( $lag > $maxLag ) {
 583                                         $maxLag = $lag;
 584                                         $host = $this->mServers[$i]['host'];
 585                                 }
 586                         }
 587                 }
 588                 return array( $host, $maxLag );
 589         }
 590
 591         /**
 592          * Get lag time for each DB
 593          * Results are cached for a short time in memcached
 594          */
 595         function getLagTimes() {
 596                 global $wgDBname;
 597
 598                 $expiry = 5;
 599                 $requestRate = 10;
 600
 601                 global $wgMemc;
 602                 $times = $wgMemc->get( "$wgDBname:lag_times" );
 603                 if ( $times ) {
 604                         # Randomly recache with probability rising over $expiry
 605                         $elapsed = time() - $times['timestamp'];
 606                         $chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
 607                         if ( mt_rand( 0, $chance ) != 0 ) {
 608                                 unset( $times['timestamp'] );
 609                                 return $times;
 610                         }
 611                 }
 612
 613                 # Cache key missing or expired
 614
 615                 $times = array();
 616                 foreach ( $this->mServers as $i => $conn ) {
 617                         if ( $this->openConnection( $i ) ) {
 618                                 $times[$i] = $this->mConnections[$i]->getLag();
 619                         }
 620                 }
 621
 622                 # Add a timestamp key so we know when it was cached
 623                 $times['timestamp'] = time();
 624                 $wgMemc->set( "$wgDBname:lag_times", $times, $expiry );
 625
 626                 # But don't give the timestamp to the caller
 627                 unset($times['timestamp']);
 628                 return $times;
 629         }
 630 }
 631
 632 ?>