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                 # Return a random representative of the remainder
 212                 return ArrayUtils::pickRandom( $loads );
 213         }
 214
 215         /**
 216          * Get the index of the reader connection, which may be a slave
 217          * This takes into account load ratios and lag times. It should
 218          * always return a consistent index during a given invocation
 219          *
 220          * Side effect: opens connections to databases
 221          * @param string|bool $group Query group, or false for the generic reader
 222          * @param string|bool $wiki Wiki ID, or false for the current wiki
 223          * @throws MWException
 224          * @return bool|int|string
 225          */
 226         public function getReaderIndex( $group = false, $wiki = false ) {
 227                 global $wgDBtype;
 228
 229                 # @todo FIXME: For now, only go through all this for mysql databases
 230                 if ( $wgDBtype != 'mysql' ) {
 231                         return $this->getWriterIndex();
 232                 }
 233
 234                 if ( count( $this->mServers ) == 1 ) {
 235                         # Skip the load balancing if there's only one server
 236                         return 0;
 237                 } elseif ( $group === false && $this->mReadIndex >= 0 ) {
 238                         # Shortcut if generic reader exists already
 239                         return $this->mReadIndex;
 240                 }
 241
 242                 # Find the relevant load array
 243                 if ( $group !== false ) {
 244                         if ( isset( $this->mGroupLoads[$group] ) ) {
 245                                 $nonErrorLoads = $this->mGroupLoads[$group];
 246                         } else {
 247                                 # No loads for this group, return false and the caller can use some other group
 248                                 wfDebugLog( 'connect', __METHOD__ . ": no loads for group $group\n" );
 249
 250                                 return false;
 251                         }
 252                 } else {
 253                         $nonErrorLoads = $this->mLoads;
 254                 }
 255
 256                 if ( !count( $nonErrorLoads ) ) {
 257                         throw new MWException( "Empty server array given to LoadBalancer" );
 258                 }
 259
 260                 # Scale the configured load ratios according to the dynamic load (if the load monitor supports it)
 261                 $this->getLoadMonitor()->scaleLoads( $nonErrorLoads, $group, $wiki );
 262
 263                 $laggedSlaveMode = false;
 264
 265                 # No server found yet
 266                 $i = false;
 267                 $conn = false;
 268                 # First try quickly looking through the available servers for a server that
 269                 # meets our criteria
 270                 $currentLoads = $nonErrorLoads;
 271                 while ( count( $currentLoads ) ) {
 272                         if ( $this->mAllowLagged || $laggedSlaveMode ) {
 273                                 $i = ArrayUtils::pickRandom( $currentLoads );
 274                         } else {
 275                                 $i = false;
 276                                 if ( $this->mWaitForPos && $this->mWaitForPos->asOfTime() ) {
 277                                         # ChronologyProtecter causes mWaitForPos to be set via sessions.
 278                                         # This triggers doWait() after connect, so it's especially good to
 279                                         # avoid lagged servers so as to avoid just blocking in that method.
 280                                         $ago = microtime( true ) - $this->mWaitForPos->asOfTime();
 281                                         # Aim for <= 1 second of waiting (being too picky can backfire)
 282                                         $i = $this->getRandomNonLagged( $currentLoads, $wiki, $ago + 1 );
 283                                 }
 284                                 if ( $i === false ) {
 285                                         # Any server with less lag than it's 'max lag' param is preferable
 286                                         $i = $this->getRandomNonLagged( $currentLoads, $wiki );
 287                                 }
 288                                 if ( $i === false && count( $currentLoads ) != 0 ) {
 289                                         # All slaves lagged. Switch to read-only mode
 290                                         wfDebugLog( 'replication', "All slaves lagged. Switch to read-only mode" );
 291                                         $i = ArrayUtils::pickRandom( $currentLoads );
 292                                         $laggedSlaveMode = true;
 293                                 }
 294                         }
 295
 296                         if ( $i === false ) {
 297                                 # pickRandom() returned false
 298                                 # This is permanent and means the configuration or the load monitor
 299                                 # wants us to return false.
 300                                 wfDebugLog( 'connect', __METHOD__ . ": pickRandom() returned false" );
 301
 302                                 return false;
 303                         }
 304
 305                         $serverName = $this->getServerName( $i );
 306                         wfDebugLog( 'connect', __METHOD__ . ": Using reader #$i: $serverName..." );
 307
 308                         $conn = $this->openConnection( $i, $wiki );
 309                         if ( !$conn ) {
 310                                 wfDebugLog( 'connect', __METHOD__ . ": Failed connecting to $i/$wiki" );
 311                                 unset( $nonErrorLoads[$i] );
 312                                 unset( $currentLoads[$i] );
 313                                 $i = false;
 314                                 continue;
 315                         }
 316
 317                         // Decrement reference counter, we are finished with this connection.
 318                         // It will be incremented for the caller later.
 319                         if ( $wiki !== false ) {
 320                                 $this->reuseConnection( $conn );
 321                         }
 322
 323                         # Return this server
 324                         break;
 325                 }
 326
 327                 # If all servers were down, quit now
 328                 if ( !count( $nonErrorLoads ) ) {
 329                         wfDebugLog( 'connect', "All servers down" );
 330                 }
 331
 332                 if ( $i !== false ) {
 333                         # Slave connection successful
 334                         # Wait for the session master pos for a short time
 335                         if ( $this->mWaitForPos && $i > 0 ) {
 336                                 if ( !$this->doWait( $i ) ) {
 337                                         $this->mServers[$i]['slave pos'] = $conn->getSlavePos();
 338                                 }
 339                         }
 340                         if ( $this->mReadIndex <= 0 && $this->mLoads[$i] > 0 && $group === false ) {
 341                                 $this->mReadIndex = $i;
 342                                 # Record if the generic reader index is in "lagged slave" mode
 343                                 if ( $laggedSlaveMode ) {
 344                                         $this->laggedSlaveMode = true;
 345                                 }
 346                         }
 347                         $serverName = $this->getServerName( $i );
 348                         wfDebugLog( 'connect', __METHOD__ .
 349                                 ": using server $serverName for group '$group'\n" );
 350                 }
 351
 352                 return $i;
 353         }
 354
 355         /**
 356          * Set the master wait position
 357          * If a DB_SLAVE connection has been opened already, waits
 358          * Otherwise sets a variable telling it to wait if such a connection is opened
 359          * @param DBMasterPos $pos
 360          */
 361         public function waitFor( $pos ) {
 362                 $this->mWaitForPos = $pos;
 363                 $i = $this->mReadIndex;
 364
 365                 if ( $i > 0 ) {
 366                         if ( !$this->doWait( $i ) ) {
 367                                 $this->mServers[$i]['slave pos'] = $this->getAnyOpenConnection( $i )->getSlavePos();
 368                                 $this->laggedSlaveMode = true;
 369                         }
 370                 }
 371         }
 372
 373         /**
 374          * Set the master wait position and wait for a "generic" slave to catch up to it
 375          *
 376          * This can be used a faster proxy for waitForAll()
 377          *
 378          * @param DBMasterPos $pos
 379          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 380          * @return bool Success (able to connect and no timeouts reached)
 381          * @since 1.26
 382          */
 383         public function waitForOne( $pos, $timeout = null ) {
 384                 $this->mWaitForPos = $pos;
 385
 386                 $i = $this->mReadIndex;
 387                 if ( $i <= 0 ) {
 388                         // Pick a generic slave if there isn't one yet
 389                         $readLoads = $this->mLoads;
 390                         unset( $readLoads[$this->getWriterIndex()] ); // slaves only
 391                         $readLoads = array_filter( $readLoads ); // with non-zero load
 392                         $i = ArrayUtils::pickRandom( $readLoads );
 393                 }
 394
 395                 if ( $i > 0 ) {
 396                         $ok = $this->doWait( $i, true, $timeout );
 397                 } else {
 398                         $ok = true; // no applicable loads
 399                 }
 400
 401                 return $ok;
 402         }
 403
 404         /**
 405          * Set the master wait position and wait for ALL slaves to catch up to it
 406          * @param DBMasterPos $pos
 407          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 408          * @return bool Success (able to connect and no timeouts reached)
 409          */
 410         public function waitForAll( $pos, $timeout = null ) {
 411                 $this->mWaitForPos = $pos;
 412                 $serverCount = count( $this->mServers );
 413
 414                 $ok = true;
 415                 for ( $i = 1; $i < $serverCount; $i++ ) {
 416                         if ( $this->mLoads[$i] > 0 ) {
 417                                 $ok = $this->doWait( $i, true, $timeout ) && $ok;
 418                         }
 419                 }
 420
 421                 return $ok;
 422         }
 423
 424         /**
 425          * Get any open connection to a given server index, local or foreign
 426          * Returns false if there is no connection open
 427          *
 428          * @param int $i
 429          * @return DatabaseBase|bool False on failure
 430          */
 431         public function getAnyOpenConnection( $i ) {
 432                 foreach ( $this->mConns as $conns ) {
 433                         if ( !empty( $conns[$i] ) ) {
 434                                 return reset( $conns[$i] );
 435                         }
 436                 }
 437
 438                 return false;
 439         }
 440
 441         /**
 442          * Wait for a given slave to catch up to the master pos stored in $this
 443          * @param int $index Server index
 444          * @param bool $open Check the server even if a new connection has to be made
 445          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 446          * @return bool
 447          */
 448         protected function doWait( $index, $open = false, $timeout = null ) {
 449                 $close = false; // close the connection afterwards
 450
 451                 // Check if we already know that the DB has reached this point
 452                 $server = $this->getServerName( $index );
 453                 $key = $this->srvCache->makeGlobalKey( __CLASS__, 'last-known-pos', $server );
 454                 /** @var DBMasterPos $knownReachedPos */
 455                 $knownReachedPos = $this->srvCache->get( $key );
 456                 if ( $knownReachedPos && $knownReachedPos->hasReached( $this->mWaitForPos ) ) {
 457                         wfDebugLog( 'replication', __METHOD__ .
 458                                 ": slave $server known to be caught up (pos >= $knownReachedPos).\n" );
 459                         return true;
 460                 }
 461
 462                 // Find a connection to wait on, creating one if needed and allowed
 463                 $conn = $this->getAnyOpenConnection( $index );
 464                 if ( !$conn ) {
 465                         if ( !$open ) {
 466                                 wfDebugLog( 'replication', __METHOD__ . ": no connection open for $server\n" );
 467
 468                                 return false;
 469                         } else {
 470                                 $conn = $this->openConnection( $index, '' );
 471                                 if ( !$conn ) {
 472                                         wfDebugLog( 'replication', __METHOD__ . ": failed to connect to $server\n" );
 473
 474                                         return false;
 475                                 }
 476                                 // Avoid connection spam in waitForAll() when connections
 477                                 // are made just for the sake of doing this lag check.
 478                                 $close = true;
 479                         }
 480                 }
 481
 482                 wfDebugLog( 'replication', __METHOD__ . ": Waiting for slave $server to catch up...\n" );
 483                 $timeout = $timeout ?: $this->mWaitTimeout;
 484                 $result = $conn->masterPosWait( $this->mWaitForPos, $timeout );
 485
 486                 if ( $result == -1 || is_null( $result ) ) {
 487                         // Timed out waiting for slave, use master instead
 488                         $msg = __METHOD__ . ": Timed out waiting on $server pos {$this->mWaitForPos}";
 489                         wfDebugLog( 'replication', "$msg\n" );
 490                         wfDebugLog( 'DBPerformance', "$msg:\n" . wfBacktrace( true ) );
 491                         $ok = false;
 492                 } else {
 493                         wfDebugLog( 'replication', __METHOD__ . ": Done\n" );
 494                         $ok = true;
 495                         // Remember that the DB reached this point
 496                         $this->srvCache->set( $key, $this->mWaitForPos, BagOStuff::TTL_DAY );
 497                 }
 498
 499                 if ( $close ) {
 500                         $this->closeConnection( $conn );
 501                 }
 502
 503                 return $ok;
 504         }
 505
 506         /**
 507          * Get a connection by index
 508          * This is the main entry point for this class.
 509          *
 510          * @param int $i Server index
 511          * @param array|string|bool $groups Query group(s), or false for the generic reader
 512          * @param string|bool $wiki Wiki ID, or false for the current wiki
 513          *
 514          * @throws MWException
 515          * @return DatabaseBase
 516          */
 517         public function getConnection( $i, $groups = array(), $wiki = false ) {
 518                 if ( $i === null || $i === false ) {
 519                         throw new MWException( 'Attempt to call ' . __METHOD__ .
 520                                 ' with invalid server index' );
 521                 }
 522
 523                 if ( $wiki === wfWikiID() ) {
 524                         $wiki = false;
 525                 }
 526
 527                 $groups = ( $groups === false || $groups === array() )
 528                         ? array( false ) // check one "group": the generic pool
 529                         : (array)$groups;
 530
 531                 $masterOnly = ( $i == DB_MASTER || $i == $this->getWriterIndex() );
 532                 $oldConnsOpened = $this->connsOpened; // connections open now
 533
 534                 if ( $i == DB_MASTER ) {
 535                         $i = $this->getWriterIndex();
 536                 } else {
 537                         # Try to find an available server in any the query groups (in order)
 538                         foreach ( $groups as $group ) {
 539                                 $groupIndex = $this->getReaderIndex( $group, $wiki );
 540                                 if ( $groupIndex !== false ) {
 541                                         $i = $groupIndex;
 542                                         break;
 543                                 }
 544                         }
 545                 }
 546
 547                 # Operation-based index
 548                 if ( $i == DB_SLAVE ) {
 549                         $this->mLastError = 'Unknown error'; // reset error string
 550                         # Try the general server pool if $groups are unavailable.
 551                         $i = in_array( false, $groups, true )
 552                                 ? false // don't bother with this if that is what was tried above
 553                                 : $this->getReaderIndex( false, $wiki );
 554                         # Couldn't find a working server in getReaderIndex()?
 555                         if ( $i === false ) {
 556                                 $this->mLastError = 'No working slave server: ' . $this->mLastError;
 557
 558                                 return $this->reportConnectionError();
 559                         }
 560                 }
 561
 562                 # Now we have an explicit index into the servers array
 563                 $conn = $this->openConnection( $i, $wiki );
 564                 if ( !$conn ) {
 565                         return $this->reportConnectionError();
 566                 }
 567
 568                 # Profile any new connections that happen
 569                 if ( $this->connsOpened > $oldConnsOpened ) {
 570                         $host = $conn->getServer();
 571                         $dbname = $conn->getDBname();
 572                         $trxProf = Profiler::instance()->getTransactionProfiler();
 573                         $trxProf->recordConnection( $host, $dbname, $masterOnly );
 574                 }
 575
 576                 if ( $masterOnly ) {
 577                         # Make master-requested DB handles inherit any read-only mode setting
 578                         $conn->setLBInfo( 'readOnlyReason', $this->getReadOnlyReason( $wiki ) );
 579                 }
 580
 581                 return $conn;
 582         }
 583
 584         /**
 585          * Mark a foreign connection as being available for reuse under a different
 586          * DB name or prefix. This mechanism is reference-counted, and must be called
 587          * the same number of times as getConnection() to work.
 588          *
 589          * @param DatabaseBase $conn
 590          * @throws MWException
 591          */
 592         public function reuseConnection( $conn ) {
 593                 $serverIndex = $conn->getLBInfo( 'serverIndex' );
 594                 $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
 595                 if ( $serverIndex === null || $refCount === null ) {
 596                         wfDebug( __METHOD__ . ": this connection was not opened as a foreign connection\n" );
 597                         /**
 598                          * This can happen in code like:
 599                          *   foreach ( $dbs as $db ) {
 600                          *     $conn = $lb->getConnection( DB_SLAVE, array(), $db );
 601                          *     ...
 602                          *     $lb->reuseConnection( $conn );
 603                          *   }
 604                          * When a connection to the local DB is opened in this way, reuseConnection()
 605                          * should be ignored
 606                          */
 607                         return;
 608                 }
 609
 610                 $dbName = $conn->getDBname();
 611                 $prefix = $conn->tablePrefix();
 612                 if ( strval( $prefix ) !== '' ) {
 613                         $wiki = "$dbName-$prefix";
 614                 } else {
 615                         $wiki = $dbName;
 616                 }
 617                 if ( $this->mConns['foreignUsed'][$serverIndex][$wiki] !== $conn ) {
 618                         throw new MWException( __METHOD__ . ": connection not found, has " .
 619                                 "the connection been freed already?" );
 620                 }
 621                 $conn->setLBInfo( 'foreignPoolRefCount', --$refCount );
 622                 if ( $refCount <= 0 ) {
 623                         $this->mConns['foreignFree'][$serverIndex][$wiki] = $conn;
 624                         unset( $this->mConns['foreignUsed'][$serverIndex][$wiki] );
 625                         wfDebug( __METHOD__ . ": freed connection $serverIndex/$wiki\n" );
 626                 } else {
 627                         wfDebug( __METHOD__ . ": reference count for $serverIndex/$wiki reduced to $refCount\n" );
 628                 }
 629         }
 630
 631         /**
 632          * Get a database connection handle reference
 633          *
 634          * The handle's methods wrap simply wrap those of a DatabaseBase handle
 635          *
 636          * @see LoadBalancer::getConnection() for parameter information
 637          *
 638          * @param int $db
 639          * @param array|string|bool $groups Query group(s), or false for the generic reader
 640          * @param string|bool $wiki Wiki ID, or false for the current wiki
 641          * @return DBConnRef
 642          */
 643         public function getConnectionRef( $db, $groups = array(), $wiki = false ) {
 644                 return new DBConnRef( $this, $this->getConnection( $db, $groups, $wiki ) );
 645         }
 646
 647         /**
 648          * Get a database connection handle reference without connecting yet
 649          *
 650          * The handle's methods wrap simply wrap those of a DatabaseBase handle
 651          *
 652          * @see LoadBalancer::getConnection() for parameter information
 653          *
 654          * @param int $db
 655          * @param array|string|bool $groups Query group(s), or false for the generic reader
 656          * @param string|bool $wiki Wiki ID, or false for the current wiki
 657          * @return DBConnRef
 658          */
 659         public function getLazyConnectionRef( $db, $groups = array(), $wiki = false ) {
 660                 return new DBConnRef( $this, array( $db, $groups, $wiki ) );
 661         }
 662
 663         /**
 664          * Open a connection to the server given by the specified index
 665          * Index must be an actual index into the array.
 666          * If the server is already open, returns it.
 667          *
 668          * On error, returns false, and the connection which caused the
 669          * error will be available via $this->mErrorConnection.
 670          *
 671          * @param int $i Server index
 672          * @param string|bool $wiki Wiki ID, or false for the current wiki
 673          * @return DatabaseBase
 674          *
 675          * @access private
 676          */
 677         public function openConnection( $i, $wiki = false ) {
 678                 if ( $wiki !== false ) {
 679                         $conn = $this->openForeignConnection( $i, $wiki );
 680                 } elseif ( isset( $this->mConns['local'][$i][0] ) ) {
 681                         $conn = $this->mConns['local'][$i][0];
 682                 } else {
 683                         $server = $this->mServers[$i];
 684                         $server['serverIndex'] = $i;
 685                         $conn = $this->reallyOpenConnection( $server, false );
 686                         $serverName = $this->getServerName( $i );
 687                         if ( $conn->isOpen() ) {
 688                                 wfDebugLog( 'connect', "Connected to database $i at $serverName\n" );
 689                                 $this->mConns['local'][$i][0] = $conn;
 690                         } else {
 691                                 wfDebugLog( 'connect', "Failed to connect to database $i at $serverName\n" );
 692                                 $this->mErrorConnection = $conn;
 693                                 $conn = false;
 694                         }
 695                 }
 696
 697                 if ( $conn && !$conn->isOpen() ) {
 698                         // Connection was made but later unrecoverably lost for some reason.
 699                         // Do not return a handle that will just throw exceptions on use,
 700                         // but let the calling code (e.g. getReaderIndex) try another server.
 701                         // See DatabaseMyslBase::ping() for how this can happen.
 702                         $this->mErrorConnection = $conn;
 703                         $conn = false;
 704                 }
 705
 706                 return $conn;
 707         }
 708
 709         /**
 710          * Open a connection to a foreign DB, or return one if it is already open.
 711          *
 712          * Increments a reference count on the returned connection which locks the
 713          * connection to the requested wiki. This reference count can be
 714          * decremented by calling reuseConnection().
 715          *
 716          * If a connection is open to the appropriate server already, but with the wrong
 717          * database, it will be switched to the right database and returned, as long as
 718          * it has been freed first with reuseConnection().
 719          *
 720          * On error, returns false, and the connection which caused the
 721          * error will be available via $this->mErrorConnection.
 722          *
 723          * @param int $i Server index
 724          * @param string $wiki Wiki ID to open
 725          * @return DatabaseBase
 726          */
 727         private function openForeignConnection( $i, $wiki ) {
 728                 list( $dbName, $prefix ) = wfSplitWikiID( $wiki );
 729                 if ( isset( $this->mConns['foreignUsed'][$i][$wiki] ) ) {
 730                         // Reuse an already-used connection
 731                         $conn = $this->mConns['foreignUsed'][$i][$wiki];
 732                         wfDebug( __METHOD__ . ": reusing connection $i/$wiki\n" );
 733                 } elseif ( isset( $this->mConns['foreignFree'][$i][$wiki] ) ) {
 734                         // Reuse a free connection for the same wiki
 735                         $conn = $this->mConns['foreignFree'][$i][$wiki];
 736                         unset( $this->mConns['foreignFree'][$i][$wiki] );
 737                         $this->mConns['foreignUsed'][$i][$wiki] = $conn;
 738                         wfDebug( __METHOD__ . ": reusing free connection $i/$wiki\n" );
 739                 } elseif ( !empty( $this->mConns['foreignFree'][$i] ) ) {
 740                         // Reuse a connection from another wiki
 741                         $conn = reset( $this->mConns['foreignFree'][$i] );
 742                         $oldWiki = key( $this->mConns['foreignFree'][$i] );
 743
 744                         // The empty string as a DB name means "don't care".
 745                         // DatabaseMysqlBase::open() already handle this on connection.
 746                         if ( $dbName !== '' && !$conn->selectDB( $dbName ) ) {
 747                                 $this->mLastError = "Error selecting database $dbName on server " .
 748                                         $conn->getServer() . " from client host " . wfHostname() . "\n";
 749                                 $this->mErrorConnection = $conn;
 750                                 $conn = false;
 751                         } else {
 752                                 $conn->tablePrefix( $prefix );
 753                                 unset( $this->mConns['foreignFree'][$i][$oldWiki] );
 754                                 $this->mConns['foreignUsed'][$i][$wiki] = $conn;
 755                                 wfDebug( __METHOD__ . ": reusing free connection from $oldWiki for $wiki\n" );
 756                         }
 757                 } else {
 758                         // Open a new connection
 759                         $server = $this->mServers[$i];
 760                         $server['serverIndex'] = $i;
 761                         $server['foreignPoolRefCount'] = 0;
 762                         $server['foreign'] = true;
 763                         $conn = $this->reallyOpenConnection( $server, $dbName );
 764                         if ( !$conn->isOpen() ) {
 765                                 wfDebug( __METHOD__ . ": error opening connection for $i/$wiki\n" );
 766                                 $this->mErrorConnection = $conn;
 767                                 $conn = false;
 768                         } else {
 769                                 $conn->tablePrefix( $prefix );
 770                                 $this->mConns['foreignUsed'][$i][$wiki] = $conn;
 771                                 wfDebug( __METHOD__ . ": opened new connection for $i/$wiki\n" );
 772                         }
 773                 }
 774
 775                 // Increment reference count
 776                 if ( $conn ) {
 777                         $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
 778                         $conn->setLBInfo( 'foreignPoolRefCount', $refCount + 1 );
 779                 }
 780
 781                 return $conn;
 782         }
 783
 784         /**
 785          * Test if the specified index represents an open connection
 786          *
 787          * @param int $index Server index
 788          * @access private
 789          * @return bool
 790          */
 791         private function isOpen( $index ) {
 792                 if ( !is_integer( $index ) ) {
 793                         return false;
 794                 }
 795
 796                 return (bool)$this->getAnyOpenConnection( $index );
 797         }
 798
 799         /**
 800          * Really opens a connection. Uncached.
 801          * Returns a Database object whether or not the connection was successful.
 802          * @access private
 803          *
 804          * @param array $server
 805          * @param bool $dbNameOverride
 806          * @throws MWException
 807          * @return DatabaseBase
 808          */
 809         protected function reallyOpenConnection( $server, $dbNameOverride = false ) {
 810                 if ( !is_array( $server ) ) {
 811                         throw new MWException( 'You must update your load-balancing configuration. ' .
 812                                 'See DefaultSettings.php entry for $wgDBservers.' );
 813                 }
 814
 815                 if ( $dbNameOverride !== false ) {
 816                         $server['dbname'] = $dbNameOverride;
 817                 }
 818
 819                 // Let the handle know what the cluster master is (e.g. "db1052")
 820                 $masterName = $this->getServerName( 0 );
 821                 $server['clusterMasterHost'] = $masterName;
 822
 823                 // Log when many connection are made on requests
 824                 if ( ++$this->connsOpened >= self::CONN_HELD_WARN_THRESHOLD ) {
 825                         wfDebugLog( 'DBPerformance', __METHOD__ . ": " .
 826                                 "{$this->connsOpened}+ connections made (master=$masterName)\n" .
 827                                 wfBacktrace( true ) );
 828                 }
 829
 830                 # Create object
 831                 try {
 832                         $db = DatabaseBase::factory( $server['type'], $server );
 833                 } catch ( DBConnectionError $e ) {
 834                         // FIXME: This is probably the ugliest thing I have ever done to
 835                         // PHP. I'm half-expecting it to segfault, just out of disgust. -- TS
 836                         $db = $e->db;
 837                 }
 838
 839                 $db->setLBInfo( $server );
 840                 $db->setLazyMasterHandle(
 841                         $this->getLazyConnectionRef( DB_MASTER, array(), $db->getWikiID() )
 842                 );
 843
 844                 return $db;
 845         }
 846
 847         /**
 848          * @throws DBConnectionError
 849          * @return bool
 850          */
 851         private function reportConnectionError() {
 852                 $conn = $this->mErrorConnection; // The connection which caused the error
 853                 $context = array(
 854                         'method' => __METHOD__,
 855                         'last_error' => $this->mLastError,
 856                 );
 857
 858                 if ( !is_object( $conn ) ) {
 859                         // No last connection, probably due to all servers being too busy
 860                         wfLogDBError(
 861                                 "LB failure with no last connection. Connection error: {last_error}",
 862                                 $context
 863                         );
 864
 865                         // If all servers were busy, mLastError will contain something sensible
 866                         throw new DBConnectionError( null, $this->mLastError );
 867                 } else {
 868                         $context['db_server'] = $conn->getProperty( 'mServer' );
 869                         wfLogDBError(
 870                                 "Connection error: {last_error} ({db_server})",
 871                                 $context
 872                         );
 873
 874                         // throws DBConnectionError
 875                         $conn->reportConnectionError( "{$this->mLastError} ({$context['db_server']})" );
 876                 }
 877
 878                 return false; /* not reached */
 879         }
 880
 881         /**
 882          * @return int
 883          * @since 1.26
 884          */
 885         public function getWriterIndex() {
 886                 return 0;
 887         }
 888
 889         /**
 890          * Returns true if the specified index is a valid server index
 891          *
 892          * @param string $i
 893          * @return bool
 894          */
 895         public function haveIndex( $i ) {
 896                 return array_key_exists( $i, $this->mServers );
 897         }
 898
 899         /**
 900          * Returns true if the specified index is valid and has non-zero load
 901          *
 902          * @param string $i
 903          * @return bool
 904          */
 905         public function isNonZeroLoad( $i ) {
 906                 return array_key_exists( $i, $this->mServers ) && $this->mLoads[$i] != 0;
 907         }
 908
 909         /**
 910          * Get the number of defined servers (not the number of open connections)
 911          *
 912          * @return int
 913          */
 914         public function getServerCount() {
 915                 return count( $this->mServers );
 916         }
 917
 918         /**
 919          * Get the host name or IP address of the server with the specified index
 920          * Prefer a readable name if available.
 921          * @param string $i
 922          * @return string
 923          */
 924         public function getServerName( $i ) {
 925                 if ( isset( $this->mServers[$i]['hostName'] ) ) {
 926                         $name = $this->mServers[$i]['hostName'];
 927                 } elseif ( isset( $this->mServers[$i]['host'] ) ) {
 928                         $name = $this->mServers[$i]['host'];
 929                 } else {
 930                         $name = '';
 931                 }
 932
 933                 return ( $name != '' ) ? $name : 'localhost';
 934         }
 935
 936         /**
 937          * Return the server info structure for a given index, or false if the index is invalid.
 938          * @param int $i
 939          * @return array|bool
 940          */
 941         public function getServerInfo( $i ) {
 942                 if ( isset( $this->mServers[$i] ) ) {
 943                         return $this->mServers[$i];
 944                 } else {
 945                         return false;
 946                 }
 947         }
 948
 949         /**
 950          * Sets the server info structure for the given index. Entry at index $i
 951          * is created if it doesn't exist
 952          * @param int $i
 953          * @param array $serverInfo
 954          */
 955         public function setServerInfo( $i, array $serverInfo ) {
 956                 $this->mServers[$i] = $serverInfo;
 957         }
 958
 959         /**
 960          * Get the current master position for chronology control purposes
 961          * @return mixed
 962          */
 963         public function getMasterPos() {
 964                 # If this entire request was served from a slave without opening a connection to the
 965                 # master (however unlikely that may be), then we can fetch the position from the slave.
 966                 $masterConn = $this->getAnyOpenConnection( 0 );
 967                 if ( !$masterConn ) {
 968                         $serverCount = count( $this->mServers );
 969                         for ( $i = 1; $i < $serverCount; $i++ ) {
 970                                 $conn = $this->getAnyOpenConnection( $i );
 971                                 if ( $conn ) {
 972                                         return $conn->getSlavePos();
 973                                 }
 974                         }
 975                 } else {
 976                         return $masterConn->getMasterPos();
 977                 }
 978
 979                 return false;
 980         }
 981
 982         /**
 983          * Close all open connections
 984          */
 985         public function closeAll() {
 986                 foreach ( $this->mConns as $conns2 ) {
 987                         foreach ( $conns2 as $conns3 ) {
 988                                 /** @var DatabaseBase $conn */
 989                                 foreach ( $conns3 as $conn ) {
 990                                         $conn->close();
 991                                 }
 992                         }
 993                 }
 994                 $this->mConns = array(
 995                         'local' => array(),
 996                         'foreignFree' => array(),
 997                         'foreignUsed' => array(),
 998                 );
 999                 $this->connsOpened = 0;
1000         }
1001
1002         /**
1003          * Close a connection
1004          * Using this function makes sure the LoadBalancer knows the connection is closed.
1005          * If you use $conn->close() directly, the load balancer won't update its state.
1006          * @param DatabaseBase $conn
1007          */
1008         public function closeConnection( $conn ) {
1009                 $done = false;
1010                 foreach ( $this->mConns as $i1 => $conns2 ) {
1011                         foreach ( $conns2 as $i2 => $conns3 ) {
1012                                 foreach ( $conns3 as $i3 => $candidateConn ) {
1013                                         if ( $conn === $candidateConn ) {
1014                                                 $conn->close();
1015                                                 unset( $this->mConns[$i1][$i2][$i3] );
1016                                                 --$this->connsOpened;
1017                                                 $done = true;
1018                                                 break;
1019                                         }
1020                                 }
1021                         }
1022                 }
1023                 if ( !$done ) {
1024                         $conn->close();
1025                 }
1026         }
1027
1028         /**
1029          * Commit transactions on all open connections
1030          * @param string $fname Caller name
1031          */
1032         public function commitAll( $fname = __METHOD__ ) {
1033                 foreach ( $this->mConns as $conns2 ) {
1034                         foreach ( $conns2 as $conns3 ) {
1035                                 /** @var DatabaseBase[] $conns3 */
1036                                 foreach ( $conns3 as $conn ) {
1037                                         if ( $conn->trxLevel() ) {
1038                                                 $conn->commit( $fname, 'flush' );
1039                                         }
1040                                 }
1041                         }
1042                 }
1043         }
1044
1045         /**
1046          * Issue COMMIT only on master, only if queries were done on connection
1047          * @param string $fname Caller name
1048          */
1049         public function commitMasterChanges( $fname = __METHOD__ ) {
1050                 $masterIndex = $this->getWriterIndex();
1051                 foreach ( $this->mConns as $conns2 ) {
1052                         if ( empty( $conns2[$masterIndex] ) ) {
1053                                 continue;
1054                         }
1055                         /** @var DatabaseBase $conn */
1056                         foreach ( $conns2[$masterIndex] as $conn ) {
1057                                 if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
1058                                         $conn->commit( $fname, 'flush' );
1059                                 }
1060                         }
1061                 }
1062         }
1063
1064         /**
1065          * Issue ROLLBACK only on master, only if queries were done on connection
1066          * @param string $fname Caller name
1067          * @throws DBExpectedError
1068          * @since 1.23
1069          */
1070         public function rollbackMasterChanges( $fname = __METHOD__ ) {
1071                 $failedServers = array();
1072
1073                 $masterIndex = $this->getWriterIndex();
1074                 foreach ( $this->mConns as $conns2 ) {
1075                         if ( empty( $conns2[$masterIndex] ) ) {
1076                                 continue;
1077                         }
1078                         /** @var DatabaseBase $conn */
1079                         foreach ( $conns2[$masterIndex] as $conn ) {
1080                                 if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
1081                                         try {
1082                                                 $conn->rollback( $fname, 'flush' );
1083                                         } catch ( DBError $e ) {
1084                                                 MWExceptionHandler::logException( $e );
1085                                                 $failedServers[] = $conn->getServer();
1086                                         }
1087                                 }
1088                         }
1089                 }
1090
1091                 if ( $failedServers ) {
1092                         throw new DBExpectedError( null, "Rollback failed on server(s) " .
1093                                 implode( ', ', array_unique( $failedServers ) ) );
1094                 }
1095         }
1096
1097         /**
1098          * @return bool Whether a master connection is already open
1099          * @since 1.24
1100          */
1101         public function hasMasterConnection() {
1102                 return $this->isOpen( $this->getWriterIndex() );
1103         }
1104
1105         /**
1106          * Determine if there are pending changes in a transaction by this thread
1107          * @since 1.23
1108          * @return bool
1109          */
1110         public function hasMasterChanges() {
1111                 $masterIndex = $this->getWriterIndex();
1112                 foreach ( $this->mConns as $conns2 ) {
1113                         if ( empty( $conns2[$masterIndex] ) ) {
1114                                 continue;
1115                         }
1116                         /** @var DatabaseBase $conn */
1117                         foreach ( $conns2[$masterIndex] as $conn ) {
1118                                 if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
1119                                         return true;
1120                                 }
1121                         }
1122                 }
1123                 return false;
1124         }
1125
1126         /**
1127          * Get the timestamp of the latest write query done by this thread
1128          * @since 1.25
1129          * @return float|bool UNIX timestamp or false
1130          */
1131         public function lastMasterChangeTimestamp() {
1132                 $lastTime = false;
1133                 $masterIndex = $this->getWriterIndex();
1134                 foreach ( $this->mConns as $conns2 ) {
1135                         if ( empty( $conns2[$masterIndex] ) ) {
1136                                 continue;
1137                         }
1138                         /** @var DatabaseBase $conn */
1139                         foreach ( $conns2[$masterIndex] as $conn ) {
1140                                 $lastTime = max( $lastTime, $conn->lastDoneWrites() );
1141                         }
1142                 }
1143                 return $lastTime;
1144         }
1145
1146         /**
1147          * Check if this load balancer object had any recent or still
1148          * pending writes issued against it by this PHP thread
1149          *
1150          * @param float $age How many seconds ago is "recent" [defaults to mWaitTimeout]
1151          * @return bool
1152          * @since 1.25
1153          */
1154         public function hasOrMadeRecentMasterChanges( $age = null ) {
1155                 $age = ( $age === null ) ? $this->mWaitTimeout : $age;
1156
1157                 return ( $this->hasMasterChanges()
1158                         || $this->lastMasterChangeTimestamp() > microtime( true ) - $age );
1159         }
1160
1161         /**
1162          * @param mixed $value
1163          * @return mixed
1164          */
1165         public function waitTimeout( $value = null ) {
1166                 return wfSetVar( $this->mWaitTimeout, $value );
1167         }
1168
1169         /**
1170          * @note This method will trigger a DB connection if not yet done
1171          *
1172          * @param string|bool $wiki Wiki ID, or false for the current wiki
1173          * @return bool Whether the generic connection for reads is highly "lagged"
1174          */
1175         public function getLaggedSlaveMode( $wiki = false ) {
1176                 // No-op if there is only one DB (also avoids recursion)
1177                 if ( !$this->laggedSlaveMode && $this->getServerCount() > 1 ) {
1178                         try {
1179                                 // See if laggedSlaveMode gets set
1180                                 $conn = $this->getConnection( DB_SLAVE, false, $wiki );
1181                                 $this->reuseConnection( $conn );
1182                         } catch ( DBConnectionError $e ) {
1183                                 // Avoid expensive re-connect attempts and failures
1184                                 $this->slavesDownMode = true;
1185                                 $this->laggedSlaveMode = true;
1186                         }
1187                 }
1188
1189                 return $this->laggedSlaveMode;
1190         }
1191
1192         /**
1193          * @note This method will never cause a new DB connection
1194          * @return bool Whether any generic connection used for reads was highly "lagged"
1195          * @since 1.27
1196          */
1197         public function laggedSlaveUsed() {
1198                 return $this->laggedSlaveMode;
1199         }
1200
1201         /**
1202          * @note This method may trigger a DB connection if not yet done
1203          * @param string|bool $wiki Wiki ID, or false for the current wiki
1204          * @return string|bool Reason the master is read-only or false if it is not
1205          * @since 1.27
1206          */
1207         public function getReadOnlyReason( $wiki = false ) {
1208                 if ( $this->readOnlyReason !== false ) {
1209                         return $this->readOnlyReason;
1210                 } elseif ( $this->getLaggedSlaveMode( $wiki ) ) {
1211                         if ( $this->slavesDownMode ) {
1212                                 return 'The database has been automatically locked ' .
1213                                         'until the slave database servers become available';
1214                         } else {
1215                                 return 'The database has been automatically locked ' .
1216                                         'while the slave database servers catch up to the master.';
1217                         }
1218                 }
1219
1220                 return false;
1221         }
1222
1223         /**
1224          * Disables/enables lag checks
1225          * @param null|bool $mode
1226          * @return bool
1227          */
1228         public function allowLagged( $mode = null ) {
1229                 if ( $mode === null ) {
1230                         return $this->mAllowLagged;
1231                 }
1232                 $this->mAllowLagged = $mode;
1233
1234                 return $this->mAllowLagged;
1235         }
1236
1237         /**
1238          * @return bool
1239          */
1240         public function pingAll() {
1241                 $success = true;
1242                 foreach ( $this->mConns as $conns2 ) {
1243                         foreach ( $conns2 as $conns3 ) {
1244                                 /** @var DatabaseBase[] $conns3 */
1245                                 foreach ( $conns3 as $conn ) {
1246                                         if ( !$conn->ping() ) {
1247                                                 $success = false;
1248                                         }
1249                                 }
1250                         }
1251                 }
1252
1253                 return $success;
1254         }
1255
1256         /**
1257          * Call a function with each open connection object
1258          * @param callable $callback
1259          * @param array $params
1260          */
1261         public function forEachOpenConnection( $callback, array $params = array() ) {
1262                 foreach ( $this->mConns as $conns2 ) {
1263                         foreach ( $conns2 as $conns3 ) {
1264                                 foreach ( $conns3 as $conn ) {
1265                                         $mergedParams = array_merge( array( $conn ), $params );
1266                                         call_user_func_array( $callback, $mergedParams );
1267                                 }
1268                         }
1269                 }
1270         }
1271
1272         /**
1273          * Get the hostname and lag time of the most-lagged slave
1274          *
1275          * This is useful for maintenance scripts that need to throttle their updates.
1276          * May attempt to open connections to slaves on the default DB. If there is
1277          * no lag, the maximum lag will be reported as -1.
1278          *
1279          * @param bool|string $wiki Wiki ID, or false for the default database
1280          * @return array ( host, max lag, index of max lagged host )
1281          */
1282         public function getMaxLag( $wiki = false ) {
1283                 $maxLag = -1;
1284                 $host = '';
1285                 $maxIndex = 0;
1286
1287                 if ( $this->getServerCount() <= 1 ) {
1288                         return array( $host, $maxLag, $maxIndex ); // no replication = no lag
1289                 }
1290
1291                 $lagTimes = $this->getLagTimes( $wiki );
1292                 foreach ( $lagTimes as $i => $lag ) {
1293                         if ( $lag > $maxLag ) {
1294                                 $maxLag = $lag;
1295                                 $host = $this->mServers[$i]['host'];
1296                                 $maxIndex = $i;
1297                         }
1298                 }
1299
1300                 return array( $host, $maxLag, $maxIndex );
1301         }
1302
1303         /**
1304          * Get an estimate of replication lag (in seconds) for each server
1305          *
1306          * Results are cached for a short time in memcached/process cache
1307          *
1308          * Values may be "false" if replication is too broken to estimate
1309          *
1310          * @param string|bool $wiki
1311          * @return int[] Map of (server index => float|int|bool)
1312          */
1313         public function getLagTimes( $wiki = false ) {
1314                 if ( $this->getServerCount() <= 1 ) {
1315                         return array( 0 => 0 ); // no replication = no lag
1316                 }
1317
1318                 # Send the request to the load monitor
1319                 return $this->getLoadMonitor()->getLagTimes( array_keys( $this->mServers ), $wiki );
1320         }
1321
1322         /**
1323          * Get the lag in seconds for a given connection, or zero if this load
1324          * balancer does not have replication enabled.
1325          *
1326          * This should be used in preference to Database::getLag() in cases where
1327          * replication may not be in use, since there is no way to determine if
1328          * replication is in use at the connection level without running
1329          * potentially restricted queries such as SHOW SLAVE STATUS. Using this
1330          * function instead of Database::getLag() avoids a fatal error in this
1331          * case on many installations.
1332          *
1333          * @param DatabaseBase $conn
1334          * @return int
1335          */
1336         public function safeGetLag( $conn ) {
1337                 if ( $this->getServerCount() == 1 ) {
1338                         return 0;
1339                 } else {
1340                         return $conn->getLag();
1341                 }
1342         }
1343
1344         /**
1345          * Clear the cache for slag lag delay times
1346          *
1347          * This is only used for testing
1348          */
1349         public function clearLagTimeCache() {
1350                 $this->getLoadMonitor()->clearCaches();
1351         }
1352 }