includes/db/loadbalancer/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         /** @var BagOStuff */
  51         private $srvCache;
  52
  53         /** @var bool|DatabaseBase Database connection that caused a problem */
  54         private $mErrorConnection;
  55         /** @var integer The generic (not query grouped) slave index (of $mServers) */
  56         private $mReadIndex;
  57         /** @var bool|DBMasterPos False if not set */
  58         private $mWaitForPos;
  59         /** @var bool Whether the generic reader fell back to a lagged slave */
  60         private $laggedSlaveMode = false;
  61         /** @var bool Whether the generic reader fell back to a lagged slave */
  62         private $slavesDownMode = false;
  63         /** @var string The last DB selection or connection error */
  64         private $mLastError = 'Unknown error';
  65         /** @var string|bool Reason the LB is read-only or false if not */
  66         private $readOnlyReason = false;
  67         /** @var integer Total connections opened */
  68         private $connsOpened = 0;
  69
  70         /** @var integer Warn when this many connection are held */
  71         const CONN_HELD_WARN_THRESHOLD = 10;
  72         /** @var integer Default 'max lag' when unspecified */
  73         const MAX_LAG = 10;
  74         /** @var integer Max time to wait for a slave to catch up (e.g. ChronologyProtector) */
  75         const POS_WAIT_TIMEOUT = 10;
  76
  77         /**
  78          * @param array $params Array with keys:
  79          *  - servers : Required. Array of server info structures.
  80          *  - loadMonitor : Name of a class used to fetch server lag and load.
  81          *  - readOnlyReason : Reason the master DB is read-only if so [optional]
  82          * @throws MWException
  83          */
  84         public function __construct( array $params ) {
  85                 if ( !isset( $params['servers'] ) ) {
  86                         throw new MWException( __CLASS__ . ': missing servers parameter' );
  87                 }
  88                 $this->mServers = $params['servers'];
  89                 $this->mWaitTimeout = self::POS_WAIT_TIMEOUT;
  90
  91                 $this->mReadIndex = -1;
  92                 $this->mWriteIndex = -1;
  93                 $this->mConns = array(
  94                         'local' => array(),
  95                         'foreignUsed' => array(),
  96                         'foreignFree' => array() );
  97                 $this->mLoads = array();
  98                 $this->mWaitForPos = false;
  99                 $this->mErrorConnection = false;
 100                 $this->mAllowLagged = false;
 101
 102                 if ( isset( $params['readOnlyReason'] ) && is_string( $params['readOnlyReason'] ) ) {
 103                         $this->readOnlyReason = $params['readOnlyReason'];
 104                 }
 105
 106                 if ( isset( $params['loadMonitor'] ) ) {
 107                         $this->mLoadMonitorClass = $params['loadMonitor'];
 108                 } else {
 109                         $master = reset( $params['servers'] );
 110                         if ( isset( $master['type'] ) && $master['type'] === 'mysql' ) {
 111                                 $this->mLoadMonitorClass = 'LoadMonitorMySQL';
 112                         } else {
 113                                 $this->mLoadMonitorClass = 'LoadMonitorNull';
 114                         }
 115                 }
 116
 117                 foreach ( $params['servers'] as $i => $server ) {
 118                         $this->mLoads[$i] = $server['load'];
 119                         if ( isset( $server['groupLoads'] ) ) {
 120                                 foreach ( $server['groupLoads'] as $group => $ratio ) {
 121                                         if ( !isset( $this->mGroupLoads[$group] ) ) {
 122                                                 $this->mGroupLoads[$group] = array();
 123                                         }
 124                                         $this->mGroupLoads[$group][$i] = $ratio;
 125                                 }
 126                         }
 127                 }
 128
 129                 $this->srvCache = ObjectCache::getLocalServerInstance();
 130         }
 131
 132         /**
 133          * Get a LoadMonitor instance
 134          *
 135          * @return LoadMonitor
 136          */
 137         private function getLoadMonitor() {
 138                 if ( !isset( $this->mLoadMonitor ) ) {
 139                         $class = $this->mLoadMonitorClass;
 140                         $this->mLoadMonitor = new $class( $this );
 141                 }
 142
 143                 return $this->mLoadMonitor;
 144         }
 145
 146         /**
 147          * Get or set arbitrary data used by the parent object, usually an LBFactory
 148          * @param mixed $x
 149          * @return mixed
 150          */
 151         public function parentInfo( $x = null ) {
 152                 return wfSetVar( $this->mParentInfo, $x );
 153         }
 154
 155         /**
 156          * Given an array of non-normalised probabilities, this function will select
 157          * an element and return the appropriate key
 158          *
 159          * @deprecated since 1.21, use ArrayUtils::pickRandom()
 160          *
 161          * @param array $weights
 162          * @return bool|int|string
 163          */
 164         public function pickRandom( array $weights ) {
 165                 return ArrayUtils::pickRandom( $weights );
 166         }
 167
 168         /**
 169          * @param array $loads
 170          * @param bool|string $wiki Wiki to get non-lagged for
 171          * @param int $maxLag Restrict the maximum allowed lag to this many seconds
 172          * @return bool|int|string
 173          */
 174         private function getRandomNonLagged( array $loads, $wiki = false, $maxLag = self::MAX_LAG ) {
 175                 $lags = $this->getLagTimes( $wiki );
 176
 177                 # Unset excessively lagged servers
 178                 foreach ( $lags as $i => $lag ) {
 179                         if ( $i != 0 ) {
 180                                 $maxServerLag = $maxLag;
 181                                 if ( isset( $this->mServers[$i]['max lag'] ) ) {
 182                                         $maxServerLag = min( $maxServerLag, $this->mServers[$i]['max lag'] );
 183                                 }
 184                                 if ( $lag === false ) {
 185                                         wfDebugLog( 'replication', "Server #$i is not replicating" );
 186                                         unset( $loads[$i] );
 187                                 } elseif ( $lag > $maxServerLag ) {
 188                                         wfDebugLog( 'replication', "Server #$i is excessively lagged ($lag seconds)" );
 189                                         unset( $loads[$i] );
 190                                 }
 191                         }
 192                 }
 193
 194                 # Find out if all the slaves with non-zero load are lagged
 195                 $sum = 0;
 196                 foreach ( $loads as $load ) {
 197                         $sum += $load;
 198                 }
 199                 if ( $sum == 0 ) {
 200                         # No appropriate DB servers except maybe the master and some slaves with zero load
 201                         # Do NOT use the master
 202                         # Instead, this function will return false, triggering read-only mode,
 203                         # and a lagged slave will be used instead.
 204                         return false;
 205                 }
 206
 207                 if ( count( $loads ) == 0 ) {
 208                         return false;
 209                 }
 210
 211                 # wfDebugLog( 'connect', var_export( $loads, true ) );
 212
 213                 # Return a random representative of the remainder
 214                 return ArrayUtils::pickRandom( $loads );
 215         }
 216
 217         /**
 218          * Get the index of the reader connection, which may be a slave
 219          * This takes into account load ratios and lag times. It should
 220          * always return a consistent index during a given invocation
 221          *
 222          * Side effect: opens connections to databases
 223          * @param string|bool $group Query group, or false for the generic reader
 224          * @param string|bool $wiki Wiki ID, or false for the current wiki
 225          * @throws MWException
 226          * @return bool|int|string
 227          */
 228         public function getReaderIndex( $group = false, $wiki = false ) {
 229                 global $wgDBtype;
 230
 231                 # @todo FIXME: For now, only go through all this for mysql databases
 232                 if ( $wgDBtype != 'mysql' ) {
 233                         return $this->getWriterIndex();
 234                 }
 235
 236                 if ( count( $this->mServers ) == 1 ) {
 237                         # Skip the load balancing if there's only one server
 238                         return 0;
 239                 } elseif ( $group === false && $this->mReadIndex >= 0 ) {
 240                         # Shortcut if generic reader exists already
 241                         return $this->mReadIndex;
 242                 }
 243
 244                 # Find the relevant load array
 245                 if ( $group !== false ) {
 246                         if ( isset( $this->mGroupLoads[$group] ) ) {
 247                                 $nonErrorLoads = $this->mGroupLoads[$group];
 248                         } else {
 249                                 # No loads for this group, return false and the caller can use some other group
 250                                 wfDebug( __METHOD__ . ": no loads for group $group\n" );
 251
 252                                 return false;
 253                         }
 254                 } else {
 255                         $nonErrorLoads = $this->mLoads;
 256                 }
 257
 258                 if ( !count( $nonErrorLoads ) ) {
 259                         throw new MWException( "Empty server array given to LoadBalancer" );
 260                 }
 261
 262                 # Scale the configured load ratios according to the dynamic load (if the load monitor supports it)
 263                 $this->getLoadMonitor()->scaleLoads( $nonErrorLoads, $group, $wiki );
 264
 265                 $laggedSlaveMode = false;
 266
 267                 # No server found yet
 268                 $i = false;
 269                 $conn = false;
 270                 # First try quickly looking through the available servers for a server that
 271                 # meets our criteria
 272                 $currentLoads = $nonErrorLoads;
 273                 while ( count( $currentLoads ) ) {
 274                         if ( $this->mAllowLagged || $laggedSlaveMode ) {
 275                                 $i = ArrayUtils::pickRandom( $currentLoads );
 276                         } else {
 277                                 $i = false;
 278                                 if ( $this->mWaitForPos && $this->mWaitForPos->asOfTime() ) {
 279                                         # ChronologyProtecter causes mWaitForPos to be set via sessions.
 280                                         # This triggers doWait() after connect, so it's especially good to
 281                                         # avoid lagged servers so as to avoid just blocking in that method.
 282                                         $ago = microtime( true ) - $this->mWaitForPos->asOfTime();
 283                                         # Aim for <= 1 second of waiting (being too picky can backfire)
 284                                         $i = $this->getRandomNonLagged( $currentLoads, $wiki, $ago + 1 );
 285                                 }
 286                                 if ( $i === false ) {
 287                                         # Any server with less lag than it's 'max lag' param is preferable
 288                                         $i = $this->getRandomNonLagged( $currentLoads, $wiki );
 289                                 }
 290                                 if ( $i === false && count( $currentLoads ) != 0 ) {
 291                                         # All slaves lagged. Switch to read-only mode
 292                                         wfDebugLog( 'replication', "All slaves lagged. Switch to read-only mode" );
 293                                         $i = ArrayUtils::pickRandom( $currentLoads );
 294                                         $laggedSlaveMode = true;
 295                                 }
 296                         }
 297
 298                         if ( $i === false ) {
 299                                 # pickRandom() returned false
 300                                 # This is permanent and means the configuration or the load monitor
 301                                 # wants us to return false.
 302                                 wfDebugLog( 'connect', __METHOD__ . ": pickRandom() returned false" );
 303
 304                                 return false;
 305                         }
 306
 307                         $serverName = $this->getServerName( $i );
 308                         wfDebugLog( 'connect', __METHOD__ . ": Using reader #$i: $serverName..." );
 309
 310                         $conn = $this->openConnection( $i, $wiki );
 311                         if ( !$conn ) {
 312                                 wfDebugLog( 'connect', __METHOD__ . ": Failed connecting to $i/$wiki" );
 313                                 unset( $nonErrorLoads[$i] );
 314                                 unset( $currentLoads[$i] );
 315                                 $i = false;
 316                                 continue;
 317                         }
 318
 319                         // Decrement reference counter, we are finished with this connection.
 320                         // It will be incremented for the caller later.
 321                         if ( $wiki !== false ) {
 322                                 $this->reuseConnection( $conn );
 323                         }
 324
 325                         # Return this server
 326                         break;
 327                 }
 328
 329                 # If all servers were down, quit now
 330                 if ( !count( $nonErrorLoads ) ) {
 331                         wfDebugLog( 'connect', "All servers down" );
 332                 }
 333
 334                 if ( $i !== false ) {
 335                         # Slave connection successful
 336                         # Wait for the session master pos for a short time
 337                         if ( $this->mWaitForPos && $i > 0 ) {
 338                                 if ( !$this->doWait( $i ) ) {
 339                                         $this->mServers[$i]['slave pos'] = $conn->getSlavePos();
 340                                 }
 341                         }
 342                         if ( $this->mReadIndex <= 0 && $this->mLoads[$i] > 0 && $group === false ) {
 343                                 $this->mReadIndex = $i;
 344                                 # Record if the generic reader index is in "lagged slave" mode
 345                                 if ( $laggedSlaveMode ) {
 346                                         $this->laggedSlaveMode = true;
 347                                 }
 348                         }
 349                         $serverName = $this->getServerName( $i );
 350                         wfDebug( __METHOD__ . ": using server $serverName for group '$group'\n" );
 351                 }
 352
 353                 return $i;
 354         }
 355
 356         /**
 357          * Set the master wait position
 358          * If a DB_SLAVE connection has been opened already, waits
 359          * Otherwise sets a variable telling it to wait if such a connection is opened
 360          * @param DBMasterPos $pos
 361          */
 362         public function waitFor( $pos ) {
 363                 $this->mWaitForPos = $pos;
 364                 $i = $this->mReadIndex;
 365
 366                 if ( $i > 0 ) {
 367                         if ( !$this->doWait( $i ) ) {
 368                                 $this->mServers[$i]['slave pos'] = $this->getAnyOpenConnection( $i )->getSlavePos();
 369                                 $this->laggedSlaveMode = true;
 370                         }
 371                 }
 372         }
 373
 374         /**
 375          * Set the master wait position and wait for a "generic" slave to catch up to it
 376          *
 377          * This can be used a faster proxy for waitForAll()
 378          *
 379          * @param DBMasterPos $pos
 380          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 381          * @return bool Success (able to connect and no timeouts reached)
 382          * @since 1.26
 383          */
 384         public function waitForOne( $pos, $timeout = null ) {
 385                 $this->mWaitForPos = $pos;
 386
 387                 $i = $this->mReadIndex;
 388                 if ( $i <= 0 ) {
 389                         // Pick a generic slave if there isn't one yet
 390                         $readLoads = $this->mLoads;
 391                         unset( $readLoads[$this->getWriterIndex()] ); // slaves only
 392                         $readLoads = array_filter( $readLoads ); // with non-zero load
 393                         $i = ArrayUtils::pickRandom( $readLoads );
 394                 }
 395
 396                 if ( $i > 0 ) {
 397                         $ok = $this->doWait( $i, true, $timeout );
 398                 } else {
 399                         $ok = true; // no applicable loads
 400                 }
 401
 402                 return $ok;
 403         }
 404
 405         /**
 406          * Set the master wait position and wait for ALL slaves to catch up to it
 407          * @param DBMasterPos $pos
 408          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 409          * @return bool Success (able to connect and no timeouts reached)
 410          */
 411         public function waitForAll( $pos, $timeout = null ) {
 412                 $this->mWaitForPos = $pos;
 413                 $serverCount = count( $this->mServers );
 414
 415                 $ok = true;
 416                 for ( $i = 1; $i < $serverCount; $i++ ) {
 417                         if ( $this->mLoads[$i] > 0 ) {
 418                                 $ok = $this->doWait( $i, true, $timeout ) && $ok;
 419                         }
 420                 }
 421
 422                 return $ok;
 423         }
 424
 425         /**
 426          * Get any open connection to a given server index, local or foreign
 427          * Returns false if there is no connection open
 428          *
 429          * @param int $i
 430          * @return DatabaseBase|bool False on failure
 431          */
 432         public function getAnyOpenConnection( $i ) {
 433                 foreach ( $this->mConns as $conns ) {
 434                         if ( !empty( $conns[$i] ) ) {
 435                                 return reset( $conns[$i] );
 436                         }
 437                 }
 438
 439                 return false;
 440         }
 441
 442         /**
 443          * Wait for a given slave to catch up to the master pos stored in $this
 444          * @param int $index Server index
 445          * @param bool $open Check the server even if a new connection has to be made
 446          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 447          * @return bool
 448          */
 449         protected function doWait( $index, $open = false, $timeout = null ) {
 450                 $close = false; // close the connection afterwards
 451
 452                 // Check if we already know that the DB has reached this point
 453                 $server = $this->getServerName( $index );
 454                 $key = $this->srvCache->makeGlobalKey( __CLASS__, 'last-known-pos', $server );
 455                 /** @var DBMasterPos $knownReachedPos */
 456                 $knownReachedPos = $this->srvCache->get( $key );
 457                 if ( $knownReachedPos && $knownReachedPos->hasReached( $this->mWaitForPos ) ) {
 458                         wfDebugLog( 'replication', __METHOD__ .
 459                                 ": slave $server known to be caught up (pos >= $knownReachedPos).\n" );
 460                         return true;
 461                 }
 462
 463                 // Find a connection to wait on, creating one if needed and allowed
 464                 $conn = $this->getAnyOpenConnection( $index );
 465                 if ( !$conn ) {
 466                         if ( !$open ) {
 467                                 wfDebugLog( 'replication', __METHOD__ . ": no connection open for $server\n" );
 468
 469                                 return false;
 470                         } else {
 471                                 $conn = $this->openConnection( $index, '' );
 472                                 if ( !$conn ) {
 473                                         wfDebugLog( 'replication', __METHOD__ . ": failed to connect to $server\n" );
 474
 475                                         return false;
 476                                 }
 477                                 // Avoid connection spam in waitForAll() when connections
 478                                 // are made just for the sake of doing this lag check.
 479                                 $close = true;
 480                         }
 481                 }
 482
 483                 wfDebugLog( 'replication', __METHOD__ . ": Waiting for slave $server to catch up...\n" );
 484                 $timeout = $timeout ?: $this->mWaitTimeout;
 485                 $result = $conn->masterPosWait( $this->mWaitForPos, $timeout );
 486
 487                 if ( $result == -1 || is_null( $result ) ) {
 488                         // Timed out waiting for slave, use master instead
 489                         $msg = __METHOD__ . ": Timed out waiting on $server pos {$this->mWaitForPos}";
 490                         wfDebugLog( 'replication', "$msg\n" );
 491                         wfDebugLog( 'DBPerformance', "$msg:\n" . wfBacktrace( true ) );
 492                         $ok = false;
 493                 } else {
 494                         wfDebugLog( 'replication', __METHOD__ . ": Done\n" );
 495                         $ok = true;
 496                         // Remember that the DB reached this point
 497                         $this->srvCache->set( $key, $this->mWaitForPos, BagOStuff::TTL_DAY );
 498                 }
 499
 500                 if ( $close ) {
 501                         $this->closeConnection( $conn );
 502                 }
 503
 504                 return $ok;
 505         }
 506
 507         /**
 508          * Get a connection by index
 509          * This is the main entry point for this class.
 510          *
 511          * @param int $i Server index
 512          * @param array|string|bool $groups Query group(s), or false for the generic reader
 513          * @param string|bool $wiki Wiki ID, or false for the current wiki
 514          *
 515          * @throws MWException
 516          * @return DatabaseBase
 517          */
 518         public function getConnection( $i, $groups = array(), $wiki = false ) {
 519                 if ( $i === null || $i === false ) {
 520                         throw new MWException( 'Attempt to call ' . __METHOD__ .
 521                                 ' with invalid server index' );
 522                 }
 523
 524                 if ( $wiki === wfWikiID() ) {
 525                         $wiki = false;
 526                 }
 527
 528                 $groups = ( $groups === false || $groups === array() )
 529                         ? array( false ) // check one "group": the generic pool
 530                         : (array)$groups;
 531
 532                 $masterOnly = ( $i == DB_MASTER || $i == $this->getWriterIndex() );
 533                 $oldConnsOpened = $this->connsOpened; // connections open now
 534
 535                 if ( $i == DB_MASTER ) {
 536                         $i = $this->getWriterIndex();
 537                 } else {
 538                         # Try to find an available server in any the query groups (in order)
 539                         foreach ( $groups as $group ) {
 540                                 $groupIndex = $this->getReaderIndex( $group, $wiki );
 541                                 if ( $groupIndex !== false ) {
 542                                         $i = $groupIndex;
 543                                         break;
 544                                 }
 545                         }
 546                 }
 547
 548                 # Operation-based index
 549                 if ( $i == DB_SLAVE ) {
 550                         $this->mLastError = 'Unknown error'; // reset error string
 551                         # Try the general server pool if $groups are unavailable.
 552                         $i = in_array( false, $groups, true )
 553                                 ? false // don't bother with this if that is what was tried above
 554                                 : $this->getReaderIndex( false, $wiki );
 555                         # Couldn't find a working server in getReaderIndex()?
 556                         if ( $i === false ) {
 557                                 $this->mLastError = 'No working slave server: ' . $this->mLastError;
 558
 559                                 return $this->reportConnectionError();
 560                         }
 561                 }
 562
 563                 # Now we have an explicit index into the servers array
 564                 $conn = $this->openConnection( $i, $wiki );
 565                 if ( !$conn ) {
 566                         return $this->reportConnectionError();
 567                 }
 568
 569                 # Profile any new connections that happen
 570                 if ( $this->connsOpened > $oldConnsOpened ) {
 571                         $host = $conn->getServer();
 572                         $dbname = $conn->getDBname();
 573                         $trxProf = Profiler::instance()->getTransactionProfiler();
 574                         $trxProf->recordConnection( $host, $dbname, $masterOnly );
 575                 }
 576
 577                 if ( $masterOnly ) {
 578                         # Make master-requested DB handles inherit any read-only mode setting
 579                         $conn->setLBInfo( 'readOnlyReason', $this->getReadOnlyReason( $wiki ) );
 580                 }
 581
 582                 return $conn;
 583         }
 584
 585         /**
 586          * Mark a foreign connection as being available for reuse under a different
 587          * DB name or prefix. This mechanism is reference-counted, and must be called
 588          * the same number of times as getConnection() to work.
 589          *
 590          * @param DatabaseBase $conn
 591          * @throws MWException
 592          */
 593         public function reuseConnection( $conn ) {
 594                 $serverIndex = $conn->getLBInfo( 'serverIndex' );
 595                 $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
 596                 if ( $serverIndex === null || $refCount === null ) {
 597                         wfDebug( __METHOD__ . ": this connection was not opened as a foreign connection\n" );
 598                         /**
 599                          * This can happen in code like:
 600                          *   foreach ( $dbs as $db ) {
 601                          *     $conn = $lb->getConnection( DB_SLAVE, array(), $db );
 602                          *     ...
 603                          *     $lb->reuseConnection( $conn );
 604                          *   }
 605                          * When a connection to the local DB is opened in this way, reuseConnection()
 606                          * should be ignored
 607                          */
 608                         return;
 609                 }
 610
 611                 $dbName = $conn->getDBname();
 612                 $prefix = $conn->tablePrefix();
 613                 if ( strval( $prefix ) !== '' ) {
 614                         $wiki = "$dbName-$prefix";
 615                 } else {
 616                         $wiki = $dbName;
 617                 }
 618                 if ( $this->mConns['foreignUsed'][$serverIndex][$wiki] !== $conn ) {
 619                         throw new MWException( __METHOD__ . ": connection not found, has " .
 620                                 "the connection been freed already?" );
 621                 }
 622                 $conn->setLBInfo( 'foreignPoolRefCount', --$refCount );
 623                 if ( $refCount <= 0 ) {
 624                         $this->mConns['foreignFree'][$serverIndex][$wiki] = $conn;
 625                         unset( $this->mConns['foreignUsed'][$serverIndex][$wiki] );
 626                         wfDebug( __METHOD__ . ": freed connection $serverIndex/$wiki\n" );
 627                 } else {
 628                         wfDebug( __METHOD__ . ": reference count for $serverIndex/$wiki reduced to $refCount\n" );
 629                 }
 630         }
 631
 632         /**
 633          * Get a database connection handle reference
 634          *
 635          * The handle's methods wrap simply wrap those of a DatabaseBase handle
 636          *
 637          * @see LoadBalancer::getConnection() for parameter information
 638          *
 639          * @param int $db
 640          * @param array|string|bool $groups Query group(s), or false for the generic reader
 641          * @param string|bool $wiki Wiki ID, or false for the current wiki
 642          * @return DBConnRef
 643          */
 644         public function getConnectionRef( $db, $groups = array(), $wiki = false ) {
 645                 return new DBConnRef( $this, $this->getConnection( $db, $groups, $wiki ) );
 646         }
 647
 648         /**
 649          * Get a database connection handle reference without connecting yet
 650          *
 651          * The handle's methods wrap simply wrap those of a DatabaseBase handle
 652          *
 653          * @see LoadBalancer::getConnection() for parameter information
 654          *
 655          * @param int $db
 656          * @param array|string|bool $groups Query group(s), or false for the generic reader
 657          * @param string|bool $wiki Wiki ID, or false for the current wiki
 658          * @return DBConnRef
 659          */
 660         public function getLazyConnectionRef( $db, $groups = array(), $wiki = false ) {
 661                 return new DBConnRef( $this, array( $db, $groups, $wiki ) );
 662         }
 663
 664         /**
 665          * Open a connection to the server given by the specified index
 666          * Index must be an actual index into the array.
 667          * If the server is already open, returns it.
 668          *
 669          * On error, returns false, and the connection which caused the
 670          * error will be available via $this->mErrorConnection.
 671          *
 672          * @param int $i Server index
 673          * @param string|bool $wiki Wiki ID, or false for the current wiki
 674          * @return DatabaseBase
 675          *
 676          * @access private
 677          */
 678         public function openConnection( $i, $wiki = false ) {
 679                 if ( $wiki !== false ) {
 680                         $conn = $this->openForeignConnection( $i, $wiki );
 681                 } elseif ( isset( $this->mConns['local'][$i][0] ) ) {
 682                         $conn = $this->mConns['local'][$i][0];
 683                 } else {
 684                         $server = $this->mServers[$i];
 685                         $server['serverIndex'] = $i;
 686                         $conn = $this->reallyOpenConnection( $server, false );
 687                         $serverName = $this->getServerName( $i );
 688                         if ( $conn->isOpen() ) {
 689                                 wfDebug( "Connected to database $i at $serverName\n" );
 690                                 $this->mConns['local'][$i][0] = $conn;
 691                         } else {
 692                                 wfDebug( "Failed to connect to database $i at $serverName\n" );
 693                                 $this->mErrorConnection = $conn;
 694                                 $conn = false;
 695                         }
 696                 }
 697
 698                 if ( $conn && !$conn->isOpen() ) {
 699                         // Connection was made but later unrecoverably lost for some reason.
 700                         // Do not return a handle that will just throw exceptions on use,
 701                         // but let the calling code (e.g. getReaderIndex) try another server.
 702                         // See DatabaseMyslBase::ping() for how this can happen.
 703                         $this->mErrorConnection = $conn;
 704                         $conn = false;
 705                 }
 706
 707                 return $conn;
 708         }
 709
 710         /**
 711          * Open a connection to a foreign DB, or return one if it is already open.
 712          *
 713          * Increments a reference count on the returned connection which locks the
 714          * connection to the requested wiki. This reference count can be
 715          * decremented by calling reuseConnection().
 716          *
 717          * If a connection is open to the appropriate server already, but with the wrong
 718          * database, it will be switched to the right database and returned, as long as
 719          * it has been freed first with reuseConnection().
 720          *
 721          * On error, returns false, and the connection which caused the
 722          * error will be available via $this->mErrorConnection.
 723          *
 724          * @param int $i Server index
 725          * @param string $wiki Wiki ID to open
 726          * @return DatabaseBase
 727          */
 728         private function openForeignConnection( $i, $wiki ) {
 729                 list( $dbName, $prefix ) = wfSplitWikiID( $wiki );
 730                 if ( isset( $this->mConns['foreignUsed'][$i][$wiki] ) ) {
 731                         // Reuse an already-used connection
 732                         $conn = $this->mConns['foreignUsed'][$i][$wiki];
 733                         wfDebug( __METHOD__ . ": reusing connection $i/$wiki\n" );
 734                 } elseif ( isset( $this->mConns['foreignFree'][$i][$wiki] ) ) {
 735                         // Reuse a free connection for the same wiki
 736                         $conn = $this->mConns['foreignFree'][$i][$wiki];
 737                         unset( $this->mConns['foreignFree'][$i][$wiki] );
 738                         $this->mConns['foreignUsed'][$i][$wiki] = $conn;
 739                         wfDebug( __METHOD__ . ": reusing free connection $i/$wiki\n" );
 740                 } elseif ( !empty( $this->mConns['foreignFree'][$i] ) ) {
 741                         // Reuse a connection from another wiki
 742                         $conn = reset( $this->mConns['foreignFree'][$i] );
 743                         $oldWiki = key( $this->mConns['foreignFree'][$i] );
 744
 745                         // The empty string as a DB name means "don't care".
 746                         // DatabaseMysqlBase::open() already handle this on connection.
 747                         if ( $dbName !== '' && !$conn->selectDB( $dbName ) ) {
 748                                 $this->mLastError = "Error selecting database $dbName on server " .
 749                                         $conn->getServer() . " from client host " . wfHostname() . "\n";
 750                                 $this->mErrorConnection = $conn;
 751                                 $conn = false;
 752                         } else {
 753                                 $conn->tablePrefix( $prefix );
 754                                 unset( $this->mConns['foreignFree'][$i][$oldWiki] );
 755                                 $this->mConns['foreignUsed'][$i][$wiki] = $conn;
 756                                 wfDebug( __METHOD__ . ": reusing free connection from $oldWiki for $wiki\n" );
 757                         }
 758                 } else {
 759                         // Open a new connection
 760                         $server = $this->mServers[$i];
 761                         $server['serverIndex'] = $i;
 762                         $server['foreignPoolRefCount'] = 0;
 763                         $server['foreign'] = true;
 764                         $conn = $this->reallyOpenConnection( $server, $dbName );
 765                         if ( !$conn->isOpen() ) {
 766                                 wfDebug( __METHOD__ . ": error opening connection for $i/$wiki\n" );
 767                                 $this->mErrorConnection = $conn;
 768                                 $conn = false;
 769                         } else {
 770                                 $conn->tablePrefix( $prefix );
 771                                 $this->mConns['foreignUsed'][$i][$wiki] = $conn;
 772                                 wfDebug( __METHOD__ . ": opened new connection for $i/$wiki\n" );
 773                         }
 774                 }
 775
 776                 // Increment reference count
 777                 if ( $conn ) {
 778                         $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
 779                         $conn->setLBInfo( 'foreignPoolRefCount', $refCount + 1 );
 780                 }
 781
 782                 return $conn;
 783         }
 784
 785         /**
 786          * Test if the specified index represents an open connection
 787          *
 788          * @param int $index Server index
 789          * @access private
 790          * @return bool
 791          */
 792         private function isOpen( $index ) {
 793                 if ( !is_integer( $index ) ) {
 794                         return false;
 795                 }
 796
 797                 return (bool)$this->getAnyOpenConnection( $index );
 798         }
 799
 800         /**
 801          * Really opens a connection. Uncached.
 802          * Returns a Database object whether or not the connection was successful.
 803          * @access private
 804          *
 805          * @param array $server
 806          * @param bool $dbNameOverride
 807          * @throws MWException
 808          * @return DatabaseBase
 809          */
 810         protected function reallyOpenConnection( $server, $dbNameOverride = false ) {
 811                 if ( !is_array( $server ) ) {
 812                         throw new MWException( 'You must update your load-balancing configuration. ' .
 813                                 'See DefaultSettings.php entry for $wgDBservers.' );
 814                 }
 815
 816                 if ( $dbNameOverride !== false ) {
 817                         $server['dbname'] = $dbNameOverride;
 818                 }
 819
 820                 // Log when many connection are made on requests
 821                 if ( ++$this->connsOpened >= self::CONN_HELD_WARN_THRESHOLD ) {
 822                         $masterAddr = $this->getServerName( 0 );
 823                         wfDebugLog( 'DBPerformance', __METHOD__ . ": " .
 824                                 "{$this->connsOpened}+ connections made (master=$masterAddr)\n" .
 825                                 wfBacktrace( true ) );
 826                 }
 827
 828                 # Create object
 829                 try {
 830                         $db = DatabaseBase::factory( $server['type'], $server );
 831                 } catch ( DBConnectionError $e ) {
 832                         // FIXME: This is probably the ugliest thing I have ever done to
 833                         // PHP. I'm half-expecting it to segfault, just out of disgust. -- TS
 834                         $db = $e->db;
 835                 }
 836
 837                 $db->setLBInfo( $server );
 838
 839                 return $db;
 840         }
 841
 842         /**
 843          * @throws DBConnectionError
 844          * @return bool
 845          */
 846         private function reportConnectionError() {
 847                 $conn = $this->mErrorConnection; // The connection which caused the error
 848                 $context = array(
 849                         'method' => __METHOD__,
 850                         'last_error' => $this->mLastError,
 851                 );
 852
 853                 if ( !is_object( $conn ) ) {
 854                         // No last connection, probably due to all servers being too busy
 855                         wfLogDBError(
 856                                 "LB failure with no last connection. Connection error: {last_error}",
 857                                 $context
 858                         );
 859
 860                         // If all servers were busy, mLastError will contain something sensible
 861                         throw new DBConnectionError( null, $this->mLastError );
 862                 } else {
 863                         $context['db_server'] = $conn->getProperty( 'mServer' );
 864                         wfLogDBError(
 865                                 "Connection error: {last_error} ({db_server})",
 866                                 $context
 867                         );
 868
 869                         // throws DBConnectionError
 870                         $conn->reportConnectionError( "{$this->mLastError} ({$context['db_server']})" );
 871                 }
 872
 873                 return false; /* not reached */
 874         }
 875
 876         /**
 877          * @return int
 878          * @since 1.26
 879          */
 880         public function getWriterIndex() {
 881                 return 0;
 882         }
 883
 884         /**
 885          * Returns true if the specified index is a valid server index
 886          *
 887          * @param string $i
 888          * @return bool
 889          */
 890         public function haveIndex( $i ) {
 891                 return array_key_exists( $i, $this->mServers );
 892         }
 893
 894         /**
 895          * Returns true if the specified index is valid and has non-zero load
 896          *
 897          * @param string $i
 898          * @return bool
 899          */
 900         public function isNonZeroLoad( $i ) {
 901                 return array_key_exists( $i, $this->mServers ) && $this->mLoads[$i] != 0;
 902         }
 903
 904         /**
 905          * Get the number of defined servers (not the number of open connections)
 906          *
 907          * @return int
 908          */
 909         public function getServerCount() {
 910                 return count( $this->mServers );
 911         }
 912
 913         /**
 914          * Get the host name or IP address of the server with the specified index
 915          * Prefer a readable name if available.
 916          * @param string $i
 917          * @return string
 918          */
 919         public function getServerName( $i ) {
 920                 if ( isset( $this->mServers[$i]['hostName'] ) ) {
 921                         $name = $this->mServers[$i]['hostName'];
 922                 } elseif ( isset( $this->mServers[$i]['host'] ) ) {
 923                         $name = $this->mServers[$i]['host'];
 924                 } else {
 925                         $name = '';
 926                 }
 927
 928                 return ( $name != '' ) ? $name : 'localhost';
 929         }
 930
 931         /**
 932          * Return the server info structure for a given index, or false if the index is invalid.
 933          * @param int $i
 934          * @return array|bool
 935          */
 936         public function getServerInfo( $i ) {
 937                 if ( isset( $this->mServers[$i] ) ) {
 938                         return $this->mServers[$i];
 939                 } else {
 940                         return false;
 941                 }
 942         }
 943
 944         /**
 945          * Sets the server info structure for the given index. Entry at index $i
 946          * is created if it doesn't exist
 947          * @param int $i
 948          * @param array $serverInfo
 949          */
 950         public function setServerInfo( $i, array $serverInfo ) {
 951                 $this->mServers[$i] = $serverInfo;
 952         }
 953
 954         /**
 955          * Get the current master position for chronology control purposes
 956          * @return mixed
 957          */
 958         public function getMasterPos() {
 959                 # If this entire request was served from a slave without opening a connection to the
 960                 # master (however unlikely that may be), then we can fetch the position from the slave.
 961                 $masterConn = $this->getAnyOpenConnection( 0 );
 962                 if ( !$masterConn ) {
 963                         $serverCount = count( $this->mServers );
 964                         for ( $i = 1; $i < $serverCount; $i++ ) {
 965                                 $conn = $this->getAnyOpenConnection( $i );
 966                                 if ( $conn ) {
 967                                         wfDebug( "Master pos fetched from slave\n" );
 968
 969                                         return $conn->getSlavePos();
 970                                 }
 971                         }
 972                 } else {
 973                         wfDebug( "Master pos fetched from master\n" );
 974
 975                         return $masterConn->getMasterPos();
 976                 }
 977
 978                 return false;
 979         }
 980
 981         /**
 982          * Close all open connections
 983          */
 984         public function closeAll() {
 985                 foreach ( $this->mConns as $conns2 ) {
 986                         foreach ( $conns2 as $conns3 ) {
 987                                 /** @var DatabaseBase $conn */
 988                                 foreach ( $conns3 as $conn ) {
 989                                         $conn->close();
 990                                 }
 991                         }
 992                 }
 993                 $this->mConns = array(
 994                         'local' => array(),
 995                         'foreignFree' => array(),
 996                         'foreignUsed' => array(),
 997                 );
 998                 $this->connsOpened = 0;
 999         }
1000
1001         /**
1002          * Close a connection
1003          * Using this function makes sure the LoadBalancer knows the connection is closed.
1004          * If you use $conn->close() directly, the load balancer won't update its state.
1005          * @param DatabaseBase $conn
1006          */
1007         public function closeConnection( $conn ) {
1008                 $done = false;
1009                 foreach ( $this->mConns as $i1 => $conns2 ) {
1010                         foreach ( $conns2 as $i2 => $conns3 ) {
1011                                 foreach ( $conns3 as $i3 => $candidateConn ) {
1012                                         if ( $conn === $candidateConn ) {
1013                                                 $conn->close();
1014                                                 unset( $this->mConns[$i1][$i2][$i3] );
1015                                                 --$this->connsOpened;
1016                                                 $done = true;
1017                                                 break;
1018                                         }
1019                                 }
1020                         }
1021                 }
1022                 if ( !$done ) {
1023                         $conn->close();
1024                 }
1025         }
1026
1027         /**
1028          * Commit transactions on all open connections
1029          */
1030         public function commitAll() {
1031                 foreach ( $this->mConns as $conns2 ) {
1032                         foreach ( $conns2 as $conns3 ) {
1033                                 /** @var DatabaseBase[] $conns3 */
1034                                 foreach ( $conns3 as $conn ) {
1035                                         if ( $conn->trxLevel() ) {
1036                                                 $conn->commit( __METHOD__, 'flush' );
1037                                         }
1038                                 }
1039                         }
1040                 }
1041         }
1042
1043         /**
1044          *  Issue COMMIT only on master, only if queries were done on connection
1045          */
1046         public function commitMasterChanges() {
1047                 $masterIndex = $this->getWriterIndex();
1048                 foreach ( $this->mConns as $conns2 ) {
1049                         if ( empty( $conns2[$masterIndex] ) ) {
1050                                 continue;
1051                         }
1052                         /** @var DatabaseBase $conn */
1053                         foreach ( $conns2[$masterIndex] as $conn ) {
1054                                 if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
1055                                         $conn->commit( __METHOD__, 'flush' );
1056                                 }
1057                         }
1058                 }
1059         }
1060
1061         /**
1062          * Issue ROLLBACK only on master, only if queries were done on connection
1063          * @since 1.23
1064          */
1065         public function rollbackMasterChanges() {
1066                 $failedServers = array();
1067
1068                 $masterIndex = $this->getWriterIndex();
1069                 foreach ( $this->mConns as $conns2 ) {
1070                         if ( empty( $conns2[$masterIndex] ) ) {
1071                                 continue;
1072                         }
1073                         /** @var DatabaseBase $conn */
1074                         foreach ( $conns2[$masterIndex] as $conn ) {
1075                                 if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
1076                                         try {
1077                                                 $conn->rollback( __METHOD__, 'flush' );
1078                                         } catch ( DBError $e ) {
1079                                                 MWExceptionHandler::logException( $e );
1080                                                 $failedServers[] = $conn->getServer();
1081                                         }
1082                                 }
1083                         }
1084                 }
1085
1086                 if ( $failedServers ) {
1087                         throw new DBExpectedError( null, "Rollback failed on server(s) " .
1088                                 implode( ', ', array_unique( $failedServers ) ) );
1089                 }
1090         }
1091
1092         /**
1093          * @return bool Whether a master connection is already open
1094          * @since 1.24
1095          */
1096         public function hasMasterConnection() {
1097                 return $this->isOpen( $this->getWriterIndex() );
1098         }
1099
1100         /**
1101          * Determine if there are pending changes in a transaction by this thread
1102          * @since 1.23
1103          * @return bool
1104          */
1105         public function hasMasterChanges() {
1106                 $masterIndex = $this->getWriterIndex();
1107                 foreach ( $this->mConns as $conns2 ) {
1108                         if ( empty( $conns2[$masterIndex] ) ) {
1109                                 continue;
1110                         }
1111                         /** @var DatabaseBase $conn */
1112                         foreach ( $conns2[$masterIndex] as $conn ) {
1113                                 if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
1114                                         return true;
1115                                 }
1116                         }
1117                 }
1118                 return false;
1119         }
1120
1121         /**
1122          * Get the timestamp of the latest write query done by this thread
1123          * @since 1.25
1124          * @return float|bool UNIX timestamp or false
1125          */
1126         public function lastMasterChangeTimestamp() {
1127                 $lastTime = false;
1128                 $masterIndex = $this->getWriterIndex();
1129                 foreach ( $this->mConns as $conns2 ) {
1130                         if ( empty( $conns2[$masterIndex] ) ) {
1131                                 continue;
1132                         }
1133                         /** @var DatabaseBase $conn */
1134                         foreach ( $conns2[$masterIndex] as $conn ) {
1135                                 $lastTime = max( $lastTime, $conn->lastDoneWrites() );
1136                         }
1137                 }
1138                 return $lastTime;
1139         }
1140
1141         /**
1142          * Check if this load balancer object had any recent or still
1143          * pending writes issued against it by this PHP thread
1144          *
1145          * @param float $age How many seconds ago is "recent" [defaults to mWaitTimeout]
1146          * @return bool
1147          * @since 1.25
1148          */
1149         public function hasOrMadeRecentMasterChanges( $age = null ) {
1150                 $age = ( $age === null ) ? $this->mWaitTimeout : $age;
1151
1152                 return ( $this->hasMasterChanges()
1153                         || $this->lastMasterChangeTimestamp() > microtime( true ) - $age );
1154         }
1155
1156         /**
1157          * @param mixed $value
1158          * @return mixed
1159          */
1160         public function waitTimeout( $value = null ) {
1161                 return wfSetVar( $this->mWaitTimeout, $value );
1162         }
1163
1164         /**
1165          * @note This method will trigger a DB connection if not yet done
1166          *
1167          * @param string|bool $wiki Wiki ID, or false for the current wiki
1168          * @return bool Whether the generic connection for reads is highly "lagged"
1169          */
1170         public function getLaggedSlaveMode( $wiki = false ) {
1171                 // No-op if there is only one DB (also avoids recursion)
1172                 if ( !$this->laggedSlaveMode && $this->getServerCount() > 1 ) {
1173                         try {
1174                                 // See if laggedSlaveMode gets set
1175                                 $conn = $this->getConnection( DB_SLAVE, false, $wiki );
1176                                 $this->reuseConnection( $conn );
1177                         } catch ( DBConnectionError $e ) {
1178                                 // Avoid expensive re-connect attempts and failures
1179                                 $this->slavesDownMode = true;
1180                                 $this->laggedSlaveMode = true;
1181                         }
1182                 }
1183
1184                 return $this->laggedSlaveMode;
1185         }
1186
1187         /**
1188          * @note This method will never cause a new DB connection
1189          * @return bool Whether any generic connection used for reads was highly "lagged"
1190          * @since 1.27
1191          */
1192         public function laggedSlaveUsed() {
1193                 return $this->laggedSlaveMode;
1194         }
1195
1196         /**
1197          * @note This method may trigger a DB connection if not yet done
1198          * @param string|bool $wiki Wiki ID, or false for the current wiki
1199          * @return string|bool Reason the master is read-only or false if it is not
1200          * @since 1.27
1201          */
1202         public function getReadOnlyReason( $wiki = false ) {
1203                 if ( $this->readOnlyReason !== false ) {
1204                         return $this->readOnlyReason;
1205                 } elseif ( $this->getLaggedSlaveMode( $wiki ) ) {
1206                         if ( $this->slavesDownMode ) {
1207                                 return 'The database has been automatically locked ' .
1208                                         'until the slave database servers become available';
1209                         } else {
1210                                 return 'The database has been automatically locked ' .
1211                                         'while the slave database servers catch up to the master.';
1212                         }
1213                 }
1214
1215                 return false;
1216         }
1217
1218         /**
1219          * Disables/enables lag checks
1220          * @param null|bool $mode
1221          * @return bool
1222          */
1223         public function allowLagged( $mode = null ) {
1224                 if ( $mode === null ) {
1225                         return $this->mAllowLagged;
1226                 }
1227                 $this->mAllowLagged = $mode;
1228
1229                 return $this->mAllowLagged;
1230         }
1231
1232         /**
1233          * @return bool
1234          */
1235         public function pingAll() {
1236                 $success = true;
1237                 foreach ( $this->mConns as $conns2 ) {
1238                         foreach ( $conns2 as $conns3 ) {
1239                                 /** @var DatabaseBase[] $conns3 */
1240                                 foreach ( $conns3 as $conn ) {
1241                                         if ( !$conn->ping() ) {
1242                                                 $success = false;
1243                                         }
1244                                 }
1245                         }
1246                 }
1247
1248                 return $success;
1249         }
1250
1251         /**
1252          * Call a function with each open connection object
1253          * @param callable $callback
1254          * @param array $params
1255          */
1256         public function forEachOpenConnection( $callback, array $params = array() ) {
1257                 foreach ( $this->mConns as $conns2 ) {
1258                         foreach ( $conns2 as $conns3 ) {
1259                                 foreach ( $conns3 as $conn ) {
1260                                         $mergedParams = array_merge( array( $conn ), $params );
1261                                         call_user_func_array( $callback, $mergedParams );
1262                                 }
1263                         }
1264                 }
1265         }
1266
1267         /**
1268          * Get the hostname and lag time of the most-lagged slave
1269          *
1270          * This is useful for maintenance scripts that need to throttle their updates.
1271          * May attempt to open connections to slaves on the default DB. If there is
1272          * no lag, the maximum lag will be reported as -1.
1273          *
1274          * @param bool|string $wiki Wiki ID, or false for the default database
1275          * @return array ( host, max lag, index of max lagged host )
1276          */
1277         public function getMaxLag( $wiki = false ) {
1278                 $maxLag = -1;
1279                 $host = '';
1280                 $maxIndex = 0;
1281
1282                 if ( $this->getServerCount() <= 1 ) {
1283                         return array( $host, $maxLag, $maxIndex ); // no replication = no lag
1284                 }
1285
1286                 $lagTimes = $this->getLagTimes( $wiki );
1287                 foreach ( $lagTimes as $i => $lag ) {
1288                         if ( $lag > $maxLag ) {
1289                                 $maxLag = $lag;
1290                                 $host = $this->mServers[$i]['host'];
1291                                 $maxIndex = $i;
1292                         }
1293                 }
1294
1295                 return array( $host, $maxLag, $maxIndex );
1296         }
1297
1298         /**
1299          * Get an estimate of replication lag (in seconds) for each server
1300          *
1301          * Results are cached for a short time in memcached/process cache
1302          *
1303          * Values may be "false" if replication is too broken to estimate
1304          *
1305          * @param string|bool $wiki
1306          * @return int[] Map of (server index => float|int|bool)
1307          */
1308         public function getLagTimes( $wiki = false ) {
1309                 if ( $this->getServerCount() <= 1 ) {
1310                         return array( 0 => 0 ); // no replication = no lag
1311                 }
1312
1313                 # Send the request to the load monitor
1314                 return $this->getLoadMonitor()->getLagTimes( array_keys( $this->mServers ), $wiki );
1315         }
1316
1317         /**
1318          * Get the lag in seconds for a given connection, or zero if this load
1319          * balancer does not have replication enabled.
1320          *
1321          * This should be used in preference to Database::getLag() in cases where
1322          * replication may not be in use, since there is no way to determine if
1323          * replication is in use at the connection level without running
1324          * potentially restricted queries such as SHOW SLAVE STATUS. Using this
1325          * function instead of Database::getLag() avoids a fatal error in this
1326          * case on many installations.
1327          *
1328          * @param DatabaseBase $conn
1329          * @return int
1330          */
1331         public function safeGetLag( $conn ) {
1332                 if ( $this->getServerCount() == 1 ) {
1333                         return 0;
1334                 } else {
1335                         return $conn->getLag();
1336                 }
1337         }
1338
1339         /**
1340          * Clear the cache for slag lag delay times
1341          *
1342          * This is only used for testing
1343          */
1344         public function clearLagTimeCache() {
1345                 $this->getLoadMonitor()->clearCaches();
1346         }
1347 }