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