includes/db/LoadBalancer.php

   1 <?php
   2 /**
   3  * Database load balancing.
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU General Public License as published by
   7  * the Free Software Foundation; either version 2 of the License, or
   8  * (at your option) any later version.
   9  *
  10  * This program is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  13  * GNU General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU General Public License along
  16  * with this program; if not, write to the Free Software Foundation, Inc.,
  17  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  18  * http://www.gnu.org/copyleft/gpl.html
  19  *
  20  * @file
  21  * @ingroup Database
  22  */
  23
  24 /**
  25  * Database load balancing object
  26  *
  27  * @todo document
  28  * @ingroup Database
  29  */
  30 class LoadBalancer {
  31         /** @var array[] Map of (server index => server config array) */
  32         private $mServers;
  33         /** @var array[] Map of (local/foreignUsed/foreignFree => server index => DatabaseBase array) */
  34         private $mConns;
  35         /** @var array Map of (server index => weight) */
  36         private $mLoads;
  37         /** @var array[] Map of (group => server index => weight) */
  38         private $mGroupLoads;
  39         /** @var bool Whether to disregard slave lag as a factor in slave selection */
  40         private $mAllowLagged;
  41         /** @var integer Seconds to spend waiting on slave lag to resolve */
  42         private $mWaitTimeout;
  43
  44         /** @var array LBFactory information */
  45         private $mParentInfo;
  46         /** @var string The LoadMonitor subclass name */
  47         private $mLoadMonitorClass;
  48         /** @var LoadMonitor */
  49         private $mLoadMonitor;
  50
  51         /** @var bool|DatabaseBase Database connection that caused a problem */
  52         private $mErrorConnection;
  53         /** @var integer The generic (not query grouped) slave index (of $mServers) */
  54         private $mReadIndex;
  55         /** @var bool|DBMasterPos False if not set */
  56         private $mWaitForPos;
  57         /** @var bool Whether the generic reader fell back to a lagged slave */
  58         private $mLaggedSlaveMode;
  59         /** @var string The last DB selection or connection error */
  60         private $mLastError = 'Unknown error';
  61         /** @var integer Total connections opened */
  62         private $connsOpened = 0;
  63
  64         /** @var integer Warn when this many connection are held */
  65         const CONN_HELD_WARN_THRESHOLD = 10;
  66
  67         /**
  68          * @param array $params Array with keys:
  69          *   servers           Required. Array of server info structures.
  70          *   loadMonitor       Name of a class used to fetch server lag and load.
  71          * @throws MWException
  72          */
  73         public function __construct( array $params ) {
  74                 if ( !isset( $params['servers'] ) ) {
  75                         throw new MWException( __CLASS__ . ': missing servers parameter' );
  76                 }
  77                 $this->mServers = $params['servers'];
  78                 $this->mWaitTimeout = 10;
  79
  80                 $this->mReadIndex = -1;
  81                 $this->mWriteIndex = -1;
  82                 $this->mConns = array(
  83                         'local' => array(),
  84                         'foreignUsed' => array(),
  85                         'foreignFree' => array() );
  86                 $this->mLoads = array();
  87                 $this->mWaitForPos = false;
  88                 $this->mLaggedSlaveMode = false;
  89                 $this->mErrorConnection = false;
  90                 $this->mAllowLagged = false;
  91
  92                 if ( isset( $params['loadMonitor'] ) ) {
  93                         $this->mLoadMonitorClass = $params['loadMonitor'];
  94                 } else {
  95                         $master = reset( $params['servers'] );
  96                         if ( isset( $master['type'] ) && $master['type'] === 'mysql' ) {
  97                                 $this->mLoadMonitorClass = 'LoadMonitorMySQL';
  98                         } else {
  99                                 $this->mLoadMonitorClass = 'LoadMonitorNull';
 100                         }
 101                 }
 102
 103                 foreach ( $params['servers'] as $i => $server ) {
 104                         $this->mLoads[$i] = $server['load'];
 105                         if ( isset( $server['groupLoads'] ) ) {
 106                                 foreach ( $server['groupLoads'] as $group => $ratio ) {
 107                                         if ( !isset( $this->mGroupLoads[$group] ) ) {
 108                                                 $this->mGroupLoads[$group] = array();
 109                                         }
 110                                         $this->mGroupLoads[$group][$i] = $ratio;
 111                                 }
 112                         }
 113                 }
 114         }
 115
 116         /**
 117          * Get a LoadMonitor instance
 118          *
 119          * @return LoadMonitor
 120          */
 121         private function getLoadMonitor() {
 122                 if ( !isset( $this->mLoadMonitor ) ) {
 123                         $class = $this->mLoadMonitorClass;
 124                         $this->mLoadMonitor = new $class( $this );
 125                 }
 126
 127                 return $this->mLoadMonitor;
 128         }
 129
 130         /**
 131          * Get or set arbitrary data used by the parent object, usually an LBFactory
 132          * @param mixed $x
 133          * @return mixed
 134          */
 135         public function parentInfo( $x = null ) {
 136                 return wfSetVar( $this->mParentInfo, $x );
 137         }
 138
 139         /**
 140          * Given an array of non-normalised probabilities, this function will select
 141          * an element and return the appropriate key
 142          *
 143          * @deprecated since 1.21, use ArrayUtils::pickRandom()
 144          *
 145          * @param array $weights
 146          * @return bool|int|string
 147          */
 148         public function pickRandom( array $weights ) {
 149                 return ArrayUtils::pickRandom( $weights );
 150         }
 151
 152         /**
 153          * @param array $loads
 154          * @param bool|string $wiki Wiki to get non-lagged for
 155          * @param float $maxLag Restrict the maximum allowed lag to this many seconds
 156          * @return bool|int|string
 157          */
 158         private function getRandomNonLagged( array $loads, $wiki = false, $maxLag = INF ) {
 159                 $lags = $this->getLagTimes( $wiki );
 160
 161                 # Unset excessively lagged servers
 162                 foreach ( $lags as $i => $lag ) {
 163                         if ( $i != 0 ) {
 164                                 $maxServerLag = $maxLag;
 165                                 if ( isset( $this->mServers[$i]['max lag'] ) ) {
 166                                         $maxServerLag = min( $maxServerLag, $this->mServers[$i]['max lag'] );
 167                                 }
 168                                 if ( $lag === false ) {
 169                                         wfDebugLog( 'replication', "Server #$i is not replicating" );
 170                                         unset( $loads[$i] );
 171                                 } elseif ( $lag > $maxServerLag ) {
 172                                         wfDebugLog( 'replication', "Server #$i is excessively lagged ($lag seconds)" );
 173                                         unset( $loads[$i] );
 174                                 }
 175                         }
 176                 }
 177
 178                 # Find out if all the slaves with non-zero load are lagged
 179                 $sum = 0;
 180                 foreach ( $loads as $load ) {
 181                         $sum += $load;
 182                 }
 183                 if ( $sum == 0 ) {
 184                         # No appropriate DB servers except maybe the master and some slaves with zero load
 185                         # Do NOT use the master
 186                         # Instead, this function will return false, triggering read-only mode,
 187                         # and a lagged slave will be used instead.
 188                         return false;
 189                 }
 190
 191                 if ( count( $loads ) == 0 ) {
 192                         return false;
 193                 }
 194
 195                 #wfDebugLog( 'connect', var_export( $loads, true ) );
 196
 197                 # Return a random representative of the remainder
 198                 return ArrayUtils::pickRandom( $loads );
 199         }
 200
 201         /**
 202          * Get the index of the reader connection, which may be a slave
 203          * This takes into account load ratios and lag times. It should
 204          * always return a consistent index during a given invocation
 205          *
 206          * Side effect: opens connections to databases
 207          * @param string|bool $group Query group, or false for the generic reader
 208          * @param string|bool $wiki Wiki ID, or false for the current wiki
 209          * @throws MWException
 210          * @return bool|int|string
 211          */
 212         public function getReaderIndex( $group = false, $wiki = false ) {
 213                 global $wgDBtype;
 214
 215                 # @todo FIXME: For now, only go through all this for mysql databases
 216                 if ( $wgDBtype != 'mysql' ) {
 217                         return $this->getWriterIndex();
 218                 }
 219
 220                 if ( count( $this->mServers ) == 1 ) {
 221                         # Skip the load balancing if there's only one server
 222                         return 0;
 223                 } elseif ( $group === false && $this->mReadIndex >= 0 ) {
 224                         # Shortcut if generic reader exists already
 225                         return $this->mReadIndex;
 226                 }
 227
 228                 # Find the relevant load array
 229                 if ( $group !== false ) {
 230                         if ( isset( $this->mGroupLoads[$group] ) ) {
 231                                 $nonErrorLoads = $this->mGroupLoads[$group];
 232                         } else {
 233                                 # No loads for this group, return false and the caller can use some other group
 234                                 wfDebug( __METHOD__ . ": no loads for group $group\n" );
 235
 236                                 return false;
 237                         }
 238                 } else {
 239                         $nonErrorLoads = $this->mLoads;
 240                 }
 241
 242                 if ( !count( $nonErrorLoads ) ) {
 243                         throw new MWException( "Empty server array given to LoadBalancer" );
 244                 }
 245
 246                 # Scale the configured load ratios according to the dynamic load (if the load monitor supports it)
 247                 $this->getLoadMonitor()->scaleLoads( $nonErrorLoads, $group, $wiki );
 248
 249                 $laggedSlaveMode = false;
 250
 251                 # No server found yet
 252                 $i = false;
 253                 # First try quickly looking through the available servers for a server that
 254                 # meets our criteria
 255                 $currentLoads = $nonErrorLoads;
 256                 while ( count( $currentLoads ) ) {
 257                         if ( $this->mAllowLagged || $laggedSlaveMode ) {
 258                                 $i = ArrayUtils::pickRandom( $currentLoads );
 259                         } else {
 260                                 $i = false;
 261                                 if ( $this->mWaitForPos && $this->mWaitForPos->asOfTime() ) {
 262                                         # ChronologyProtecter causes mWaitForPos to be set via sessions.
 263                                         # This triggers doWait() after connect, so it's especially good to
 264                                         # avoid lagged servers so as to avoid just blocking in that method.
 265                                         $ago = microtime( true ) - $this->mWaitForPos->asOfTime();
 266                                         # Aim for <= 1 second of waiting (being too picky can backfire)
 267                                         $i = $this->getRandomNonLagged( $currentLoads, $wiki, $ago + 1 );
 268                                 }
 269                                 if ( $i === false ) {
 270                                         # Any server with less lag than it's 'max lag' param is preferable
 271                                         $i = $this->getRandomNonLagged( $currentLoads, $wiki );
 272                                 }
 273                                 if ( $i === false && count( $currentLoads ) != 0 ) {
 274                                         # All slaves lagged. Switch to read-only mode
 275                                         wfDebugLog( 'replication', "All slaves lagged. Switch to read-only mode" );
 276                                         $i = ArrayUtils::pickRandom( $currentLoads );
 277                                         $laggedSlaveMode = true;
 278                                 }
 279                         }
 280
 281                         if ( $i === false ) {
 282                                 # pickRandom() returned false
 283                                 # This is permanent and means the configuration or the load monitor
 284                                 # wants us to return false.
 285                                 wfDebugLog( 'connect', __METHOD__ . ": pickRandom() returned false" );
 286
 287                                 return false;
 288                         }
 289
 290                         $serverName = $this->getServerName( $i );
 291                         wfDebugLog( 'connect', __METHOD__ . ": Using reader #$i: $serverName..." );
 292
 293                         $conn = $this->openConnection( $i, $wiki );
 294                         if ( !$conn ) {
 295                                 wfDebugLog( 'connect', __METHOD__ . ": Failed connecting to $i/$wiki" );
 296                                 unset( $nonErrorLoads[$i] );
 297                                 unset( $currentLoads[$i] );
 298                                 $i = false;
 299                                 continue;
 300                         }
 301
 302                         // Decrement reference counter, we are finished with this connection.
 303                         // It will be incremented for the caller later.
 304                         if ( $wiki !== false ) {
 305                                 $this->reuseConnection( $conn );
 306                         }
 307
 308                         # Return this server
 309                         break;
 310                 }
 311
 312                 # If all servers were down, quit now
 313                 if ( !count( $nonErrorLoads ) ) {
 314                         wfDebugLog( 'connect', "All servers down" );
 315                 }
 316
 317                 if ( $i !== false ) {
 318                         # Slave connection successful
 319                         # Wait for the session master pos for a short time
 320                         if ( $this->mWaitForPos && $i > 0 ) {
 321                                 if ( !$this->doWait( $i ) ) {
 322                                         $this->mServers[$i]['slave pos'] = $conn->getSlavePos();
 323                                 }
 324                         }
 325                         if ( $this->mReadIndex <= 0 && $this->mLoads[$i] > 0 && $group === false ) {
 326                                 $this->mReadIndex = $i;
 327                                 # Record if the generic reader index is in "lagged slave" mode
 328                                 if ( $laggedSlaveMode ) {
 329                                         $this->mLaggedSlaveMode = true;
 330                                 }
 331                         }
 332                         $serverName = $this->getServerName( $i );
 333                         wfDebug( __METHOD__ . ": using server $serverName for group '$group'\n" );
 334                 }
 335
 336                 return $i;
 337         }
 338
 339         /**
 340          * Set the master wait position
 341          * If a DB_SLAVE connection has been opened already, waits
 342          * Otherwise sets a variable telling it to wait if such a connection is opened
 343          * @param DBMasterPos $pos
 344          */
 345         public function waitFor( $pos ) {
 346                 $this->mWaitForPos = $pos;
 347                 $i = $this->mReadIndex;
 348
 349                 if ( $i > 0 ) {
 350                         if ( !$this->doWait( $i ) ) {
 351                                 $this->mServers[$i]['slave pos'] = $this->getAnyOpenConnection( $i )->getSlavePos();
 352                                 $this->mLaggedSlaveMode = true;
 353                         }
 354                 }
 355         }
 356
 357         /**
 358          * Set the master wait position and wait for a "generic" slave to catch up to it
 359          *
 360          * This can be used a faster proxy for waitForAll()
 361          *
 362          * @param DBMasterPos $pos
 363          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 364          * @return bool Success (able to connect and no timeouts reached)
 365          * @since 1.26
 366          */
 367         public function waitForOne( $pos, $timeout = null ) {
 368                 $this->mWaitForPos = $pos;
 369
 370                 $i = $this->mReadIndex;
 371                 if ( $i <= 0 ) {
 372                         // Pick a generic slave if there isn't one yet
 373                         $readLoads = $this->mLoads;
 374                         unset( $readLoads[$this->getWriterIndex()] ); // slaves only
 375                         $readLoads = array_filter( $readLoads ); // with non-zero load
 376                         $i = ArrayUtils::pickRandom( $readLoads );
 377                 }
 378
 379                 if ( $i > 0 ) {
 380                         $ok = $this->doWait( $i, true, $timeout );
 381                 } else {
 382                         $ok = true; // no applicable loads
 383                 }
 384
 385                 return $ok;
 386         }
 387
 388         /**
 389          * Set the master wait position and wait for ALL slaves to catch up to it
 390          * @param DBMasterPos $pos
 391          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 392          * @return bool Success (able to connect and no timeouts reached)
 393          */
 394         public function waitForAll( $pos, $timeout = null ) {
 395                 $this->mWaitForPos = $pos;
 396                 $serverCount = count( $this->mServers );
 397
 398                 $ok = true;
 399                 for ( $i = 1; $i < $serverCount; $i++ ) {
 400                         if ( $this->mLoads[$i] > 0 ) {
 401                                 $ok = $this->doWait( $i, true, $timeout ) && $ok;
 402                         }
 403                 }
 404
 405                 return $ok;
 406         }
 407
 408         /**
 409          * Get any open connection to a given server index, local or foreign
 410          * Returns false if there is no connection open
 411          *
 412          * @param int $i
 413          * @return DatabaseBase|bool False on failure
 414          */
 415         public function getAnyOpenConnection( $i ) {
 416                 foreach ( $this->mConns as $conns ) {
 417                         if ( !empty( $conns[$i] ) ) {
 418                                 return reset( $conns[$i] );
 419                         }
 420                 }
 421
 422                 return false;
 423         }
 424
 425         /**
 426          * Wait for a given slave to catch up to the master pos stored in $this
 427          * @param int $index Server index
 428          * @param bool $open Check the server even if a new connection has to be made
 429          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 430          * @return bool
 431          */
 432         protected function doWait( $index, $open = false, $timeout = null ) {
 433                 $close = false; // close the connection afterwards
 434
 435                 # Find a connection to wait on, creating one if needed and allowed
 436                 $conn = $this->getAnyOpenConnection( $index );
 437                 if ( !$conn ) {
 438                         if ( !$open ) {
 439                                 wfDebug( __METHOD__ . ": no connection open\n" );
 440
 441                                 return false;
 442                         } else {
 443                                 $conn = $this->openConnection( $index, '' );
 444                                 if ( !$conn ) {
 445                                         wfDebug( __METHOD__ . ": failed to open connection\n" );
 446
 447                                         return false;
 448                                 }
 449                                 // Avoid connection spam in waitForAll() when connections
 450                                 // are made just for the sake of doing this lag check.
 451                                 $close = true;
 452                         }
 453                 }
 454
 455                 wfDebug( __METHOD__ . ": Waiting for slave #$index to catch up...\n" );
 456                 $timeout = $timeout ?: $this->mWaitTimeout;
 457                 $result = $conn->masterPosWait( $this->mWaitForPos, $timeout );
 458
 459                 if ( $result == -1 || is_null( $result ) ) {
 460                         # Timed out waiting for slave, use master instead
 461                         $server = $server = $this->getServerName( $index );
 462                         $msg = __METHOD__ . ": Timed out waiting on $server pos {$this->mWaitForPos}";
 463                         wfDebug( "$msg\n" );
 464                         wfDebugLog( 'DBPerformance', "$msg:\n" . wfBacktrace( true ) );
 465                         $ok = false;
 466                 } else {
 467                         wfDebug( __METHOD__ . ": Done\n" );
 468                         $ok = true;
 469                 }
 470
 471                 if ( $close ) {
 472                         $this->closeConnection( $conn );
 473                 }
 474
 475                 return $ok;
 476         }
 477
 478         /**
 479          * Get a connection by index
 480          * This is the main entry point for this class.
 481          *
 482          * @param int $i Server index
 483          * @param array|string|bool $groups Query group(s), or false for the generic reader
 484          * @param string|bool $wiki Wiki ID, or false for the current wiki
 485          *
 486          * @throws MWException
 487          * @return DatabaseBase
 488          */
 489         public function getConnection( $i, $groups = array(), $wiki = false ) {
 490                 if ( $i === null || $i === false ) {
 491                         throw new MWException( 'Attempt to call ' . __METHOD__ .
 492                                 ' with invalid server index' );
 493                 }
 494
 495                 if ( $wiki === wfWikiID() ) {
 496                         $wiki = false;
 497                 }
 498
 499                 $groups = ( $groups === false || $groups === array() )
 500                         ? array( false ) // check one "group": the generic pool
 501                         : (array)$groups;
 502
 503                 $masterOnly = ( $i == DB_MASTER || $i == $this->getWriterIndex() );
 504                 $oldConnsOpened = $this->connsOpened; // connections open now
 505
 506                 if ( $i == DB_MASTER ) {
 507                         $i = $this->getWriterIndex();
 508                 } else {
 509                         # Try to find an available server in any the query groups (in order)
 510                         foreach ( $groups as $group ) {
 511                                 $groupIndex = $this->getReaderIndex( $group, $wiki );
 512                                 if ( $groupIndex !== false ) {
 513                                         $i = $groupIndex;
 514                                         break;
 515                                 }
 516                         }
 517                 }
 518
 519                 # Operation-based index
 520                 if ( $i == DB_SLAVE ) {
 521                         $this->mLastError = 'Unknown error'; // reset error string
 522                         # Try the general server pool if $groups are unavailable.
 523                         $i = in_array( false, $groups, true )
 524                                 ? false // don't bother with this if that is what was tried above
 525                                 : $this->getReaderIndex( false, $wiki );
 526                         # Couldn't find a working server in getReaderIndex()?
 527                         if ( $i === false ) {
 528                                 $this->mLastError = 'No working slave server: ' . $this->mLastError;
 529
 530                                 return $this->reportConnectionError();
 531                         }
 532                 }
 533
 534                 # Now we have an explicit index into the servers array
 535                 $conn = $this->openConnection( $i, $wiki );
 536                 if ( !$conn ) {
 537                         return $this->reportConnectionError();
 538                 }
 539
 540                 # Profile any new connections that happen
 541                 if ( $this->connsOpened > $oldConnsOpened ) {
 542                         $host = $conn->getServer();
 543                         $dbname = $conn->getDBname();
 544                         $trxProf = Profiler::instance()->getTransactionProfiler();
 545                         $trxProf->recordConnection( $host, $dbname, $masterOnly );
 546                 }
 547
 548                 return $conn;
 549         }
 550
 551         /**
 552          * Mark a foreign connection as being available for reuse under a different
 553          * DB name or prefix. This mechanism is reference-counted, and must be called
 554          * the same number of times as getConnection() to work.
 555          *
 556          * @param DatabaseBase $conn
 557          * @throws MWException
 558          */
 559         public function reuseConnection( $conn ) {
 560                 $serverIndex = $conn->getLBInfo( 'serverIndex' );
 561                 $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
 562                 if ( $serverIndex === null || $refCount === null ) {
 563                         wfDebug( __METHOD__ . ": this connection was not opened as a foreign connection\n" );
 564
 565                         /**
 566                          * This can happen in code like:
 567                          *   foreach ( $dbs as $db ) {
 568                          *     $conn = $lb->getConnection( DB_SLAVE, array(), $db );
 569                          *     ...
 570                          *     $lb->reuseConnection( $conn );
 571                          *   }
 572                          * When a connection to the local DB is opened in this way, reuseConnection()
 573                          * should be ignored
 574                          */
 575
 576                         return;
 577                 }
 578
 579                 $dbName = $conn->getDBname();
 580                 $prefix = $conn->tablePrefix();
 581                 if ( strval( $prefix ) !== '' ) {
 582                         $wiki = "$dbName-$prefix";
 583                 } else {
 584                         $wiki = $dbName;
 585                 }
 586                 if ( $this->mConns['foreignUsed'][$serverIndex][$wiki] !== $conn ) {
 587                         throw new MWException( __METHOD__ . ": connection not found, has " .
 588                                 "the connection been freed already?" );
 589                 }
 590                 $conn->setLBInfo( 'foreignPoolRefCount', --$refCount );
 591                 if ( $refCount <= 0 ) {
 592                         $this->mConns['foreignFree'][$serverIndex][$wiki] = $conn;
 593                         unset( $this->mConns['foreignUsed'][$serverIndex][$wiki] );
 594                         wfDebug( __METHOD__ . ": freed connection $serverIndex/$wiki\n" );
 595                 } else {
 596                         wfDebug( __METHOD__ . ": reference count for $serverIndex/$wiki reduced to $refCount\n" );
 597                 }
 598         }
 599
 600         /**
 601          * Get a database connection handle reference
 602          *
 603          * The handle's methods wrap simply wrap those of a DatabaseBase handle
 604          *
 605          * @see LoadBalancer::getConnection() for parameter information
 606          *
 607          * @param int $db
 608          * @param array|string|bool $groups Query group(s), or false for the generic reader
 609          * @param string|bool $wiki Wiki ID, or false for the current wiki
 610          * @return DBConnRef
 611          */
 612         public function getConnectionRef( $db, $groups = array(), $wiki = false ) {
 613                 return new DBConnRef( $this, $this->getConnection( $db, $groups, $wiki ) );
 614         }
 615
 616         /**
 617          * Get a database connection handle reference without connecting yet
 618          *
 619          * The handle's methods wrap simply wrap those of a DatabaseBase handle
 620          *
 621          * @see LoadBalancer::getConnection() for parameter information
 622          *
 623          * @param int $db
 624          * @param array|string|bool $groups Query group(s), or false for the generic reader
 625          * @param string|bool $wiki Wiki ID, or false for the current wiki
 626          * @return DBConnRef
 627          */
 628         public function getLazyConnectionRef( $db, $groups = array(), $wiki = false ) {
 629                 return new DBConnRef( $this, array( $db, $groups, $wiki ) );
 630         }
 631
 632         /**
 633          * Open a connection to the server given by the specified index
 634          * Index must be an actual index into the array.
 635          * If the server is already open, returns it.
 636          *
 637          * On error, returns false, and the connection which caused the
 638          * error will be available via $this->mErrorConnection.
 639          *
 640          * @param int $i Server index
 641          * @param string|bool $wiki Wiki ID, or false for the current wiki
 642          * @return DatabaseBase
 643          *
 644          * @access private
 645          */
 646         public function openConnection( $i, $wiki = false ) {
 647                 if ( $wiki !== false ) {
 648                         $conn = $this->openForeignConnection( $i, $wiki );
 649                 } elseif ( isset( $this->mConns['local'][$i][0] ) ) {
 650                         $conn = $this->mConns['local'][$i][0];
 651                 } else {
 652                         $server = $this->mServers[$i];
 653                         $server['serverIndex'] = $i;
 654                         $conn = $this->reallyOpenConnection( $server, false );
 655                         $serverName = $this->getServerName( $i );
 656                         if ( $conn->isOpen() ) {
 657                                 wfDebug( "Connected to database $i at $serverName\n" );
 658                                 $this->mConns['local'][$i][0] = $conn;
 659                         } else {
 660                                 wfDebug( "Failed to connect to database $i at $serverName\n" );
 661                                 $this->mErrorConnection = $conn;
 662                                 $conn = false;
 663                         }
 664                 }
 665
 666                 if ( $conn && !$conn->isOpen() ) {
 667                         // Connection was made but later unrecoverably lost for some reason.
 668                         // Do not return a handle that will just throw exceptions on use,
 669                         // but let the calling code (e.g. getReaderIndex) try another server.
 670                         // See DatabaseMyslBase::ping() for how this can happen.
 671                         $this->mErrorConnection = $conn;
 672                         $conn = false;
 673                 }
 674
 675                 return $conn;
 676         }
 677
 678         /**
 679          * Open a connection to a foreign DB, or return one if it is already open.
 680          *
 681          * Increments a reference count on the returned connection which locks the
 682          * connection to the requested wiki. This reference count can be
 683          * decremented by calling reuseConnection().
 684          *
 685          * If a connection is open to the appropriate server already, but with the wrong
 686          * database, it will be switched to the right database and returned, as long as
 687          * it has been freed first with reuseConnection().
 688          *
 689          * On error, returns false, and the connection which caused the
 690          * error will be available via $this->mErrorConnection.
 691          *
 692          * @param int $i Server index
 693          * @param string $wiki Wiki ID to open
 694          * @return DatabaseBase
 695          */
 696         private function openForeignConnection( $i, $wiki ) {
 697                 list( $dbName, $prefix ) = wfSplitWikiID( $wiki );
 698                 if ( isset( $this->mConns['foreignUsed'][$i][$wiki] ) ) {
 699                         // Reuse an already-used connection
 700                         $conn = $this->mConns['foreignUsed'][$i][$wiki];
 701                         wfDebug( __METHOD__ . ": reusing connection $i/$wiki\n" );
 702                 } elseif ( isset( $this->mConns['foreignFree'][$i][$wiki] ) ) {
 703                         // Reuse a free connection for the same wiki
 704                         $conn = $this->mConns['foreignFree'][$i][$wiki];
 705                         unset( $this->mConns['foreignFree'][$i][$wiki] );
 706                         $this->mConns['foreignUsed'][$i][$wiki] = $conn;
 707                         wfDebug( __METHOD__ . ": reusing free connection $i/$wiki\n" );
 708                 } elseif ( !empty( $this->mConns['foreignFree'][$i] ) ) {
 709                         // Reuse a connection from another wiki
 710                         $conn = reset( $this->mConns['foreignFree'][$i] );
 711                         $oldWiki = key( $this->mConns['foreignFree'][$i] );
 712
 713                         // The empty string as a DB name means "don't care".
 714                         // DatabaseMysqlBase::open() already handle this on connection.
 715                         if ( $dbName !== '' && !$conn->selectDB( $dbName ) ) {
 716                                 $this->mLastError = "Error selecting database $dbName on server " .
 717                                         $conn->getServer() . " from client host " . wfHostname() . "\n";
 718                                 $this->mErrorConnection = $conn;
 719                                 $conn = false;
 720                         } else {
 721                                 $conn->tablePrefix( $prefix );
 722                                 unset( $this->mConns['foreignFree'][$i][$oldWiki] );
 723                                 $this->mConns['foreignUsed'][$i][$wiki] = $conn;
 724                                 wfDebug( __METHOD__ . ": reusing free connection from $oldWiki for $wiki\n" );
 725                         }
 726                 } else {
 727                         // Open a new connection
 728                         $server = $this->mServers[$i];
 729                         $server['serverIndex'] = $i;
 730                         $server['foreignPoolRefCount'] = 0;
 731                         $server['foreign'] = true;
 732                         $conn = $this->reallyOpenConnection( $server, $dbName );
 733                         if ( !$conn->isOpen() ) {
 734                                 wfDebug( __METHOD__ . ": error opening connection for $i/$wiki\n" );
 735                                 $this->mErrorConnection = $conn;
 736                                 $conn = false;
 737                         } else {
 738                                 $conn->tablePrefix( $prefix );
 739                                 $this->mConns['foreignUsed'][$i][$wiki] = $conn;
 740                                 wfDebug( __METHOD__ . ": opened new connection for $i/$wiki\n" );
 741                         }
 742                 }
 743
 744                 // Increment reference count
 745                 if ( $conn ) {
 746                         $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
 747                         $conn->setLBInfo( 'foreignPoolRefCount', $refCount + 1 );
 748                 }
 749
 750                 return $conn;
 751         }
 752
 753         /**
 754          * Test if the specified index represents an open connection
 755          *
 756          * @param int $index Server index
 757          * @access private
 758          * @return bool
 759          */
 760         private function isOpen( $index ) {
 761                 if ( !is_integer( $index ) ) {
 762                         return false;
 763                 }
 764
 765                 return (bool)$this->getAnyOpenConnection( $index );
 766         }
 767
 768         /**
 769          * Really opens a connection. Uncached.
 770          * Returns a Database object whether or not the connection was successful.
 771          * @access private
 772          *
 773          * @param array $server
 774          * @param bool $dbNameOverride
 775          * @throws MWException
 776          * @return DatabaseBase
 777          */
 778         protected function reallyOpenConnection( $server, $dbNameOverride = false ) {
 779                 if ( !is_array( $server ) ) {
 780                         throw new MWException( 'You must update your load-balancing configuration. ' .
 781                                 'See DefaultSettings.php entry for $wgDBservers.' );
 782                 }
 783
 784                 if ( $dbNameOverride !== false ) {
 785                         $server['dbname'] = $dbNameOverride;
 786                 }
 787
 788                 // Log when many connection are made on requests
 789                 if ( ++$this->connsOpened >= self::CONN_HELD_WARN_THRESHOLD ) {
 790                         $masterAddr = $this->getServerName( 0 );
 791                         wfDebugLog( 'DBPerformance', __METHOD__ . ": " .
 792                                 "{$this->connsOpened}+ connections made (master=$masterAddr)\n" .
 793                                 wfBacktrace( true ) );
 794                 }
 795
 796                 # Create object
 797                 try {
 798                         $db = DatabaseBase::factory( $server['type'], $server );
 799                 } catch ( DBConnectionError $e ) {
 800                         // FIXME: This is probably the ugliest thing I have ever done to
 801                         // PHP. I'm half-expecting it to segfault, just out of disgust. -- TS
 802                         $db = $e->db;
 803                 }
 804
 805                 $db->setLBInfo( $server );
 806                 if ( isset( $server['fakeSlaveLag'] ) ) {
 807                         $db->setFakeSlaveLag( $server['fakeSlaveLag'] );
 808                 }
 809                 if ( isset( $server['fakeMaster'] ) ) {
 810                         $db->setFakeMaster( true );
 811                 }
 812
 813                 return $db;
 814         }
 815
 816         /**
 817          * @throws DBConnectionError
 818          * @return bool
 819          */
 820         private function reportConnectionError() {
 821                 $conn = $this->mErrorConnection; // The connection which caused the error
 822                 $context = array(
 823                         'method' => __METHOD__,
 824                         'last_error' => $this->mLastError,
 825                 );
 826
 827                 if ( !is_object( $conn ) ) {
 828                         // No last connection, probably due to all servers being too busy
 829                         wfLogDBError(
 830                                 "LB failure with no last connection. Connection error: {last_error}",
 831                                 $context
 832                         );
 833
 834                         // If all servers were busy, mLastError will contain something sensible
 835                         throw new DBConnectionError( null, $this->mLastError );
 836                 } else {
 837                         $context['db_server'] = $conn->getProperty( 'mServer' );
 838                         wfLogDBError(
 839                                 "Connection error: {last_error} ({db_server})",
 840                                 $context
 841                         );
 842                         $conn->reportConnectionError( "{$this->mLastError} ({$context['db_server']})" ); // throws DBConnectionError
 843                 }
 844
 845                 return false; /* not reached */
 846         }
 847
 848         /**
 849          * @return int
 850          * @since 1.26
 851          */
 852         public function getWriterIndex() {
 853                 return 0;
 854         }
 855
 856         /**
 857          * Returns true if the specified index is a valid server index
 858          *
 859          * @param string $i
 860          * @return bool
 861          */
 862         public function haveIndex( $i ) {
 863                 return array_key_exists( $i, $this->mServers );
 864         }
 865
 866         /**
 867          * Returns true if the specified index is valid and has non-zero load
 868          *
 869          * @param string $i
 870          * @return bool
 871          */
 872         public function isNonZeroLoad( $i ) {
 873                 return array_key_exists( $i, $this->mServers ) && $this->mLoads[$i] != 0;
 874         }
 875
 876         /**
 877          * Get the number of defined servers (not the number of open connections)
 878          *
 879          * @return int
 880          */
 881         public function getServerCount() {
 882                 return count( $this->mServers );
 883         }
 884
 885         /**
 886          * Get the host name or IP address of the server with the specified index
 887          * Prefer a readable name if available.
 888          * @param string $i
 889          * @return string
 890          */
 891         public function getServerName( $i ) {
 892                 if ( isset( $this->mServers[$i]['hostName'] ) ) {
 893                         $name = $this->mServers[$i]['hostName'];
 894                 } elseif ( isset( $this->mServers[$i]['host'] ) ) {
 895                         $name = $this->mServers[$i]['host'];
 896                 } else {
 897                         $name = '';
 898                 }
 899
 900                 return ( $name != '' ) ? $name : 'localhost';
 901         }
 902
 903         /**
 904          * Return the server info structure for a given index, or false if the index is invalid.
 905          * @param int $i
 906          * @return array|bool
 907          */
 908         public function getServerInfo( $i ) {
 909                 if ( isset( $this->mServers[$i] ) ) {
 910                         return $this->mServers[$i];
 911                 } else {
 912                         return false;
 913                 }
 914         }
 915
 916         /**
 917          * Sets the server info structure for the given index. Entry at index $i
 918          * is created if it doesn't exist
 919          * @param int $i
 920          * @param array $serverInfo
 921          */
 922         public function setServerInfo( $i, array $serverInfo ) {
 923                 $this->mServers[$i] = $serverInfo;
 924         }
 925
 926         /**
 927          * Get the current master position for chronology control purposes
 928          * @return mixed
 929          */
 930         public function getMasterPos() {
 931                 # If this entire request was served from a slave without opening a connection to the
 932                 # master (however unlikely that may be), then we can fetch the position from the slave.
 933                 $masterConn = $this->getAnyOpenConnection( 0 );
 934                 if ( !$masterConn ) {
 935                         $serverCount = count( $this->mServers );
 936                         for ( $i = 1; $i < $serverCount; $i++ ) {
 937                                 $conn = $this->getAnyOpenConnection( $i );
 938                                 if ( $conn ) {
 939                                         wfDebug( "Master pos fetched from slave\n" );
 940
 941                                         return $conn->getSlavePos();
 942                                 }
 943                         }
 944                 } else {
 945                         wfDebug( "Master pos fetched from master\n" );
 946
 947                         return $masterConn->getMasterPos();
 948                 }
 949
 950                 return false;
 951         }
 952
 953         /**
 954          * Close all open connections
 955          */
 956         public function closeAll() {
 957                 foreach ( $this->mConns as $conns2 ) {
 958                         foreach ( $conns2 as $conns3 ) {
 959                                 /** @var DatabaseBase $conn */
 960                                 foreach ( $conns3 as $conn ) {
 961                                         $conn->close();
 962                                 }
 963                         }
 964                 }
 965                 $this->mConns = array(
 966                         'local' => array(),
 967                         'foreignFree' => array(),
 968                         'foreignUsed' => array(),
 969                 );
 970                 $this->connsOpened = 0;
 971         }
 972
 973         /**
 974          * Close a connection
 975          * Using this function makes sure the LoadBalancer knows the connection is closed.
 976          * If you use $conn->close() directly, the load balancer won't update its state.
 977          * @param DatabaseBase $conn
 978          */
 979         public function closeConnection( $conn ) {
 980                 $done = false;
 981                 foreach ( $this->mConns as $i1 => $conns2 ) {
 982                         foreach ( $conns2 as $i2 => $conns3 ) {
 983                                 foreach ( $conns3 as $i3 => $candidateConn ) {
 984                                         if ( $conn === $candidateConn ) {
 985                                                 $conn->close();
 986                                                 unset( $this->mConns[$i1][$i2][$i3] );
 987                                                 --$this->connsOpened;
 988                                                 $done = true;
 989                                                 break;
 990                                         }
 991                                 }
 992                         }
 993                 }
 994                 if ( !$done ) {
 995                         $conn->close();
 996                 }
 997         }
 998
 999         /**
1000          * Commit transactions on all open connections
1001          */
1002         public function commitAll() {
1003                 foreach ( $this->mConns as $conns2 ) {
1004                         foreach ( $conns2 as $conns3 ) {
1005                                 /** @var DatabaseBase[] $conns3 */
1006                                 foreach ( $conns3 as $conn ) {
1007                                         if ( $conn->trxLevel() ) {
1008                                                 $conn->commit( __METHOD__, 'flush' );
1009                                         }
1010                                 }
1011                         }
1012                 }
1013         }
1014
1015         /**
1016          *  Issue COMMIT only on master, only if queries were done on connection
1017          */
1018         public function commitMasterChanges() {
1019                 $masterIndex = $this->getWriterIndex();
1020                 foreach ( $this->mConns as $conns2 ) {
1021                         if ( empty( $conns2[$masterIndex] ) ) {
1022                                 continue;
1023                         }
1024                         /** @var DatabaseBase $conn */
1025                         foreach ( $conns2[$masterIndex] as $conn ) {
1026                                 if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
1027                                         $conn->commit( __METHOD__, 'flush' );
1028                                 }
1029                         }
1030                 }
1031         }
1032
1033         /**
1034          * Issue ROLLBACK only on master, only if queries were done on connection
1035          * @since 1.23
1036          */
1037         public function rollbackMasterChanges() {
1038                 $failedServers = array();
1039
1040                 $masterIndex = $this->getWriterIndex();
1041                 foreach ( $this->mConns as $conns2 ) {
1042                         if ( empty( $conns2[$masterIndex] ) ) {
1043                                 continue;
1044                         }
1045                         /** @var DatabaseBase $conn */
1046                         foreach ( $conns2[$masterIndex] as $conn ) {
1047                                 if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
1048                                         try {
1049                                                 $conn->rollback( __METHOD__, 'flush' );
1050                                         } catch ( DBError $e ) {
1051                                                 MWExceptionHandler::logException( $e );
1052                                                 $failedServers[] = $conn->getServer();
1053                                         }
1054                                 }
1055                         }
1056                 }
1057
1058                 if ( $failedServers ) {
1059                         throw new DBExpectedError( null, "Rollback failed on server(s) " .
1060                                 implode( ', ', array_unique( $failedServers ) ) );
1061                 }
1062         }
1063
1064         /**
1065          * @return bool Whether a master connection is already open
1066          * @since 1.24
1067          */
1068         public function hasMasterConnection() {
1069                 return $this->isOpen( $this->getWriterIndex() );
1070         }
1071
1072         /**
1073          * Determine if there are pending changes in a transaction by this thread
1074          * @since 1.23
1075          * @return bool
1076          */
1077         public function hasMasterChanges() {
1078                 $masterIndex = $this->getWriterIndex();
1079                 foreach ( $this->mConns as $conns2 ) {
1080                         if ( empty( $conns2[$masterIndex] ) ) {
1081                                 continue;
1082                         }
1083                         /** @var DatabaseBase $conn */
1084                         foreach ( $conns2[$masterIndex] as $conn ) {
1085                                 if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
1086                                         return true;
1087                                 }
1088                         }
1089                 }
1090                 return false;
1091         }
1092
1093         /**
1094          * Get the timestamp of the latest write query done by this thread
1095          * @since 1.25
1096          * @return float|bool UNIX timestamp or false
1097          */
1098         public function lastMasterChangeTimestamp() {
1099                 $lastTime = false;
1100                 $masterIndex = $this->getWriterIndex();
1101                 foreach ( $this->mConns as $conns2 ) {
1102                         if ( empty( $conns2[$masterIndex] ) ) {
1103                                 continue;
1104                         }
1105                         /** @var DatabaseBase $conn */
1106                         foreach ( $conns2[$masterIndex] as $conn ) {
1107                                 $lastTime = max( $lastTime, $conn->lastDoneWrites() );
1108                         }
1109                 }
1110                 return $lastTime;
1111         }
1112
1113         /**
1114          * Check if this load balancer object had any recent or still
1115          * pending writes issued against it by this PHP thread
1116          *
1117          * @param float $age How many seconds ago is "recent" [defaults to mWaitTimeout]
1118          * @return bool
1119          * @since 1.25
1120          */
1121         public function hasOrMadeRecentMasterChanges( $age = null ) {
1122                 $age = ( $age === null ) ? $this->mWaitTimeout : $age;
1123
1124                 return ( $this->hasMasterChanges()
1125                         || $this->lastMasterChangeTimestamp() > microtime( true ) - $age );
1126         }
1127
1128         /**
1129          * @param mixed $value
1130          * @return mixed
1131          */
1132         public function waitTimeout( $value = null ) {
1133                 return wfSetVar( $this->mWaitTimeout, $value );
1134         }
1135
1136         /**
1137          * @return bool Whether the generic connection for reads is highly "lagged"
1138          */
1139         public function getLaggedSlaveMode() {
1140                 # Get a generic reader connection
1141                 $this->getConnection( DB_SLAVE );
1142
1143                 return $this->mLaggedSlaveMode;
1144         }
1145
1146         /**
1147          * Disables/enables lag checks
1148          * @param null|bool $mode
1149          * @return bool
1150          */
1151         public function allowLagged( $mode = null ) {
1152                 if ( $mode === null ) {
1153                         return $this->mAllowLagged;
1154                 }
1155                 $this->mAllowLagged = $mode;
1156
1157                 return $this->mAllowLagged;
1158         }
1159
1160         /**
1161          * @return bool
1162          */
1163         public function pingAll() {
1164                 $success = true;
1165                 foreach ( $this->mConns as $conns2 ) {
1166                         foreach ( $conns2 as $conns3 ) {
1167                                 /** @var DatabaseBase[] $conns3 */
1168                                 foreach ( $conns3 as $conn ) {
1169                                         if ( !$conn->ping() ) {
1170                                                 $success = false;
1171                                         }
1172                                 }
1173                         }
1174                 }
1175
1176                 return $success;
1177         }
1178
1179         /**
1180          * Call a function with each open connection object
1181          * @param callable $callback
1182          * @param array $params
1183          */
1184         public function forEachOpenConnection( $callback, array $params = array() ) {
1185                 foreach ( $this->mConns as $conns2 ) {
1186                         foreach ( $conns2 as $conns3 ) {
1187                                 foreach ( $conns3 as $conn ) {
1188                                         $mergedParams = array_merge( array( $conn ), $params );
1189                                         call_user_func_array( $callback, $mergedParams );
1190                                 }
1191                         }
1192                 }
1193         }
1194
1195         /**
1196          * Get the hostname and lag time of the most-lagged slave
1197          *
1198          * This is useful for maintenance scripts that need to throttle their updates.
1199          * May attempt to open connections to slaves on the default DB. If there is
1200          * no lag, the maximum lag will be reported as -1.
1201          *
1202          * @param bool|string $wiki Wiki ID, or false for the default database
1203          * @return array ( host, max lag, index of max lagged host )
1204          */
1205         public function getMaxLag( $wiki = false ) {
1206                 $maxLag = -1;
1207                 $host = '';
1208                 $maxIndex = 0;
1209
1210                 if ( $this->getServerCount() <= 1 ) {
1211                         return array( $host, $maxLag, $maxIndex ); // no replication = no lag
1212                 }
1213
1214                 $lagTimes = $this->getLagTimes( $wiki );
1215                 foreach ( $lagTimes as $i => $lag ) {
1216                         if ( $lag > $maxLag ) {
1217                                 $maxLag = $lag;
1218                                 $host = $this->mServers[$i]['host'];
1219                                 $maxIndex = $i;
1220                         }
1221                 }
1222
1223                 return array( $host, $maxLag, $maxIndex );
1224         }
1225
1226         /**
1227          * Get lag time for each server
1228          *
1229          * Results are cached for a short time in memcached/process cache
1230          *
1231          * @param string|bool $wiki
1232          * @return int[] Map of (server index => seconds)
1233          */
1234         public function getLagTimes( $wiki = false ) {
1235                 if ( $this->getServerCount() <= 1 ) {
1236                         return array( 0 => 0 ); // no replication = no lag
1237                 }
1238
1239                 # Send the request to the load monitor
1240                 return $this->getLoadMonitor()->getLagTimes( array_keys( $this->mServers ), $wiki );
1241         }
1242
1243         /**
1244          * Get the lag in seconds for a given connection, or zero if this load
1245          * balancer does not have replication enabled.
1246          *
1247          * This should be used in preference to Database::getLag() in cases where
1248          * replication may not be in use, since there is no way to determine if
1249          * replication is in use at the connection level without running
1250          * potentially restricted queries such as SHOW SLAVE STATUS. Using this
1251          * function instead of Database::getLag() avoids a fatal error in this
1252          * case on many installations.
1253          *
1254          * @param DatabaseBase $conn
1255          * @return int
1256          */
1257         public function safeGetLag( $conn ) {
1258                 if ( $this->getServerCount() == 1 ) {
1259                         return 0;
1260                 } else {
1261                         return $conn->getLag();
1262                 }
1263         }
1264
1265         /**
1266          * Clear the cache for slag lag delay times
1267          *
1268          * This is only used for testing
1269          */
1270         public function clearLagTimeCache() {
1271                 $this->getLoadMonitor()->clearCaches();
1272         }
1273 }