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 = 30;
  72
  73         /**
  74          * @param array $params Array with keys:
  75          *  - servers : Required. Array of server info structures.
  76          *  - loadMonitor : Name of a class used to fetch server lag and load.
  77          *  - readOnlyReason : Reason the master DB is read-only if so [optional]
  78          * @throws MWException
  79          */
  80         public function __construct( array $params ) {
  81                 if ( !isset( $params['servers'] ) ) {
  82                         throw new MWException( __CLASS__ . ': missing servers parameter' );
  83                 }
  84                 $this->mServers = $params['servers'];
  85                 $this->mWaitTimeout = 10;
  86
  87                 $this->mReadIndex = -1;
  88                 $this->mWriteIndex = -1;
  89                 $this->mConns = array(
  90                         'local' => array(),
  91                         'foreignUsed' => array(),
  92                         'foreignFree' => array() );
  93                 $this->mLoads = array();
  94                 $this->mWaitForPos = false;
  95                 $this->mErrorConnection = false;
  96                 $this->mAllowLagged = false;
  97
  98                 if ( isset( $params['readOnlyReason'] ) && is_string( $params['readOnlyReason'] ) ) {
  99                         $this->readOnlyReason = $params['readOnlyReason'];
 100                 }
 101
 102                 if ( isset( $params['loadMonitor'] ) ) {
 103                         $this->mLoadMonitorClass = $params['loadMonitor'];
 104                 } else {
 105                         $master = reset( $params['servers'] );
 106                         if ( isset( $master['type'] ) && $master['type'] === 'mysql' ) {
 107                                 $this->mLoadMonitorClass = 'LoadMonitorMySQL';
 108                         } else {
 109                                 $this->mLoadMonitorClass = 'LoadMonitorNull';
 110                         }
 111                 }
 112
 113                 foreach ( $params['servers'] as $i => $server ) {
 114                         $this->mLoads[$i] = $server['load'];
 115                         if ( isset( $server['groupLoads'] ) ) {
 116                                 foreach ( $server['groupLoads'] as $group => $ratio ) {
 117                                         if ( !isset( $this->mGroupLoads[$group] ) ) {
 118                                                 $this->mGroupLoads[$group] = array();
 119                                         }
 120                                         $this->mGroupLoads[$group][$i] = $ratio;
 121                                 }
 122                         }
 123                 }
 124         }
 125
 126         /**
 127          * Get a LoadMonitor instance
 128          *
 129          * @return LoadMonitor
 130          */
 131         private function getLoadMonitor() {
 132                 if ( !isset( $this->mLoadMonitor ) ) {
 133                         $class = $this->mLoadMonitorClass;
 134                         $this->mLoadMonitor = new $class( $this );
 135                 }
 136
 137                 return $this->mLoadMonitor;
 138         }
 139
 140         /**
 141          * Get or set arbitrary data used by the parent object, usually an LBFactory
 142          * @param mixed $x
 143          * @return mixed
 144          */
 145         public function parentInfo( $x = null ) {
 146                 return wfSetVar( $this->mParentInfo, $x );
 147         }
 148
 149         /**
 150          * Given an array of non-normalised probabilities, this function will select
 151          * an element and return the appropriate key
 152          *
 153          * @deprecated since 1.21, use ArrayUtils::pickRandom()
 154          *
 155          * @param array $weights
 156          * @return bool|int|string
 157          */
 158         public function pickRandom( array $weights ) {
 159                 return ArrayUtils::pickRandom( $weights );
 160         }
 161
 162         /**
 163          * @param array $loads
 164          * @param bool|string $wiki Wiki to get non-lagged for
 165          * @param int $maxLag Restrict the maximum allowed lag to this many seconds
 166          * @return bool|int|string
 167          */
 168         private function getRandomNonLagged( array $loads, $wiki = false, $maxLag = self::MAX_LAG ) {
 169                 $lags = $this->getLagTimes( $wiki );
 170
 171                 # Unset excessively lagged servers
 172                 foreach ( $lags as $i => $lag ) {
 173                         if ( $i != 0 ) {
 174                                 $maxServerLag = $maxLag;
 175                                 if ( isset( $this->mServers[$i]['max lag'] ) ) {
 176                                         $maxServerLag = min( $maxServerLag, $this->mServers[$i]['max lag'] );
 177                                 }
 178                                 if ( $lag === false ) {
 179                                         wfDebugLog( 'replication', "Server #$i is not replicating" );
 180                                         unset( $loads[$i] );
 181                                 } elseif ( $lag > $maxServerLag ) {
 182                                         wfDebugLog( 'replication', "Server #$i is excessively lagged ($lag seconds)" );
 183                                         unset( $loads[$i] );
 184                                 }
 185                         }
 186                 }
 187
 188                 # Find out if all the slaves with non-zero load are lagged
 189                 $sum = 0;
 190                 foreach ( $loads as $load ) {
 191                         $sum += $load;
 192                 }
 193                 if ( $sum == 0 ) {
 194                         # No appropriate DB servers except maybe the master and some slaves with zero load
 195                         # Do NOT use the master
 196                         # Instead, this function will return false, triggering read-only mode,
 197                         # and a lagged slave will be used instead.
 198                         return false;
 199                 }
 200
 201                 if ( count( $loads ) == 0 ) {
 202                         return false;
 203                 }
 204
 205                 # wfDebugLog( 'connect', var_export( $loads, true ) );
 206
 207                 # Return a random representative of the remainder
 208                 return ArrayUtils::pickRandom( $loads );
 209         }
 210
 211         /**
 212          * Get the index of the reader connection, which may be a slave
 213          * This takes into account load ratios and lag times. It should
 214          * always return a consistent index during a given invocation
 215          *
 216          * Side effect: opens connections to databases
 217          * @param string|bool $group Query group, or false for the generic reader
 218          * @param string|bool $wiki Wiki ID, or false for the current wiki
 219          * @throws MWException
 220          * @return bool|int|string
 221          */
 222         public function getReaderIndex( $group = false, $wiki = false ) {
 223                 global $wgDBtype;
 224
 225                 # @todo FIXME: For now, only go through all this for mysql databases
 226                 if ( $wgDBtype != 'mysql' ) {
 227                         return $this->getWriterIndex();
 228                 }
 229
 230                 if ( count( $this->mServers ) == 1 ) {
 231                         # Skip the load balancing if there's only one server
 232                         return 0;
 233                 } elseif ( $group === false && $this->mReadIndex >= 0 ) {
 234                         # Shortcut if generic reader exists already
 235                         return $this->mReadIndex;
 236                 }
 237
 238                 # Find the relevant load array
 239                 if ( $group !== false ) {
 240                         if ( isset( $this->mGroupLoads[$group] ) ) {
 241                                 $nonErrorLoads = $this->mGroupLoads[$group];
 242                         } else {
 243                                 # No loads for this group, return false and the caller can use some other group
 244                                 wfDebug( __METHOD__ . ": no loads for group $group\n" );
 245
 246                                 return false;
 247                         }
 248                 } else {
 249                         $nonErrorLoads = $this->mLoads;
 250                 }
 251
 252                 if ( !count( $nonErrorLoads ) ) {
 253                         throw new MWException( "Empty server array given to LoadBalancer" );
 254                 }
 255
 256                 # Scale the configured load ratios according to the dynamic load (if the load monitor supports it)
 257                 $this->getLoadMonitor()->scaleLoads( $nonErrorLoads, $group, $wiki );
 258
 259                 $laggedSlaveMode = false;
 260
 261                 # No server found yet
 262                 $i = false;
 263                 $conn = false;
 264                 # First try quickly looking through the available servers for a server that
 265                 # meets our criteria
 266                 $currentLoads = $nonErrorLoads;
 267                 while ( count( $currentLoads ) ) {
 268                         if ( $this->mAllowLagged || $laggedSlaveMode ) {
 269                                 $i = ArrayUtils::pickRandom( $currentLoads );
 270                         } else {
 271                                 $i = false;
 272                                 if ( $this->mWaitForPos && $this->mWaitForPos->asOfTime() ) {
 273                                         # ChronologyProtecter causes mWaitForPos to be set via sessions.
 274                                         # This triggers doWait() after connect, so it's especially good to
 275                                         # avoid lagged servers so as to avoid just blocking in that method.
 276                                         $ago = microtime( true ) - $this->mWaitForPos->asOfTime();
 277                                         # Aim for <= 1 second of waiting (being too picky can backfire)
 278                                         $i = $this->getRandomNonLagged( $currentLoads, $wiki, $ago + 1 );
 279                                 }
 280                                 if ( $i === false ) {
 281                                         # Any server with less lag than it's 'max lag' param is preferable
 282                                         $i = $this->getRandomNonLagged( $currentLoads, $wiki );
 283                                 }
 284                                 if ( $i === false && count( $currentLoads ) != 0 ) {
 285                                         # All slaves lagged. Switch to read-only mode
 286                                         wfDebugLog( 'replication', "All slaves lagged. Switch to read-only mode" );
 287                                         $i = ArrayUtils::pickRandom( $currentLoads );
 288                                         $laggedSlaveMode = true;
 289                                 }
 290                         }
 291
 292                         if ( $i === false ) {
 293                                 # pickRandom() returned false
 294                                 # This is permanent and means the configuration or the load monitor
 295                                 # wants us to return false.
 296                                 wfDebugLog( 'connect', __METHOD__ . ": pickRandom() returned false" );
 297
 298                                 return false;
 299                         }
 300
 301                         $serverName = $this->getServerName( $i );
 302                         wfDebugLog( 'connect', __METHOD__ . ": Using reader #$i: $serverName..." );
 303
 304                         $conn = $this->openConnection( $i, $wiki );
 305                         if ( !$conn ) {
 306                                 wfDebugLog( 'connect', __METHOD__ . ": Failed connecting to $i/$wiki" );
 307                                 unset( $nonErrorLoads[$i] );
 308                                 unset( $currentLoads[$i] );
 309                                 $i = false;
 310                                 continue;
 311                         }
 312
 313                         // Decrement reference counter, we are finished with this connection.
 314                         // It will be incremented for the caller later.
 315                         if ( $wiki !== false ) {
 316                                 $this->reuseConnection( $conn );
 317                         }
 318
 319                         # Return this server
 320                         break;
 321                 }
 322
 323                 # If all servers were down, quit now
 324                 if ( !count( $nonErrorLoads ) ) {
 325                         wfDebugLog( 'connect', "All servers down" );
 326                 }
 327
 328                 if ( $i !== false ) {
 329                         # Slave connection successful
 330                         # Wait for the session master pos for a short time
 331                         if ( $this->mWaitForPos && $i > 0 ) {
 332                                 if ( !$this->doWait( $i ) ) {
 333                                         $this->mServers[$i]['slave pos'] = $conn->getSlavePos();
 334                                 }
 335                         }
 336                         if ( $this->mReadIndex <= 0 && $this->mLoads[$i] > 0 && $group === false ) {
 337                                 $this->mReadIndex = $i;
 338                                 # Record if the generic reader index is in "lagged slave" mode
 339                                 if ( $laggedSlaveMode ) {
 340                                         $this->laggedSlaveMode = true;
 341                                 }
 342                         }
 343                         $serverName = $this->getServerName( $i );
 344                         wfDebug( __METHOD__ . ": using server $serverName for group '$group'\n" );
 345                 }
 346
 347                 return $i;
 348         }
 349
 350         /**
 351          * Set the master wait position
 352          * If a DB_SLAVE connection has been opened already, waits
 353          * Otherwise sets a variable telling it to wait if such a connection is opened
 354          * @param DBMasterPos $pos
 355          */
 356         public function waitFor( $pos ) {
 357                 $this->mWaitForPos = $pos;
 358                 $i = $this->mReadIndex;
 359
 360                 if ( $i > 0 ) {
 361                         if ( !$this->doWait( $i ) ) {
 362                                 $this->mServers[$i]['slave pos'] = $this->getAnyOpenConnection( $i )->getSlavePos();
 363                                 $this->laggedSlaveMode = true;
 364                         }
 365                 }
 366         }
 367
 368         /**
 369          * Set the master wait position and wait for a "generic" slave to catch up to it
 370          *
 371          * This can be used a faster proxy for waitForAll()
 372          *
 373          * @param DBMasterPos $pos
 374          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 375          * @return bool Success (able to connect and no timeouts reached)
 376          * @since 1.26
 377          */
 378         public function waitForOne( $pos, $timeout = null ) {
 379                 $this->mWaitForPos = $pos;
 380
 381                 $i = $this->mReadIndex;
 382                 if ( $i <= 0 ) {
 383                         // Pick a generic slave if there isn't one yet
 384                         $readLoads = $this->mLoads;
 385                         unset( $readLoads[$this->getWriterIndex()] ); // slaves only
 386                         $readLoads = array_filter( $readLoads ); // with non-zero load
 387                         $i = ArrayUtils::pickRandom( $readLoads );
 388                 }
 389
 390                 if ( $i > 0 ) {
 391                         $ok = $this->doWait( $i, true, $timeout );
 392                 } else {
 393                         $ok = true; // no applicable loads
 394                 }
 395
 396                 return $ok;
 397         }
 398
 399         /**
 400          * Set the master wait position and wait for ALL slaves to catch up to it
 401          * @param DBMasterPos $pos
 402          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 403          * @return bool Success (able to connect and no timeouts reached)
 404          */
 405         public function waitForAll( $pos, $timeout = null ) {
 406                 $this->mWaitForPos = $pos;
 407                 $serverCount = count( $this->mServers );
 408
 409                 $ok = true;
 410                 for ( $i = 1; $i < $serverCount; $i++ ) {
 411                         if ( $this->mLoads[$i] > 0 ) {
 412                                 $ok = $this->doWait( $i, true, $timeout ) && $ok;
 413                         }
 414                 }
 415
 416                 return $ok;
 417         }
 418
 419         /**
 420          * Get any open connection to a given server index, local or foreign
 421          * Returns false if there is no connection open
 422          *
 423          * @param int $i
 424          * @return DatabaseBase|bool False on failure
 425          */
 426         public function getAnyOpenConnection( $i ) {
 427                 foreach ( $this->mConns as $conns ) {
 428                         if ( !empty( $conns[$i] ) ) {
 429                                 return reset( $conns[$i] );
 430                         }
 431                 }
 432
 433                 return false;
 434         }
 435
 436         /**
 437          * Wait for a given slave to catch up to the master pos stored in $this
 438          * @param int $index Server index
 439          * @param bool $open Check the server even if a new connection has to be made
 440          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 441          * @return bool
 442          */
 443         protected function doWait( $index, $open = false, $timeout = null ) {
 444                 $close = false; // close the connection afterwards
 445
 446                 # Find a connection to wait on, creating one if needed and allowed
 447                 $conn = $this->getAnyOpenConnection( $index );
 448                 if ( !$conn ) {
 449                         if ( !$open ) {
 450                                 wfDebug( __METHOD__ . ": no connection open\n" );
 451
 452                                 return false;
 453                         } else {
 454                                 $conn = $this->openConnection( $index, '' );
 455                                 if ( !$conn ) {
 456                                         wfDebug( __METHOD__ . ": failed to open connection\n" );
 457
 458                                         return false;
 459                                 }
 460                                 // Avoid connection spam in waitForAll() when connections
 461                                 // are made just for the sake of doing this lag check.
 462                                 $close = true;
 463                         }
 464                 }
 465
 466                 wfDebug( __METHOD__ . ": Waiting for slave #$index to catch up...\n" );
 467                 $timeout = $timeout ?: $this->mWaitTimeout;
 468                 $result = $conn->masterPosWait( $this->mWaitForPos, $timeout );
 469
 470                 if ( $result == -1 || is_null( $result ) ) {
 471                         # Timed out waiting for slave, use master instead
 472                         $server = $server = $this->getServerName( $index );
 473                         $msg = __METHOD__ . ": Timed out waiting on $server pos {$this->mWaitForPos}";
 474                         wfDebug( "$msg\n" );
 475                         wfDebugLog( 'DBPerformance', "$msg:\n" . wfBacktrace( true ) );
 476                         $ok = false;
 477                 } else {
 478                         wfDebug( __METHOD__ . ": Done\n" );
 479                         $ok = true;
 480                 }
 481
 482                 if ( $close ) {
 483                         $this->closeConnection( $conn );
 484                 }
 485
 486                 return $ok;
 487         }
 488
 489         /**
 490          * Get a connection by index
 491          * This is the main entry point for this class.
 492          *
 493          * @param int $i Server index
 494          * @param array|string|bool $groups Query group(s), or false for the generic reader
 495          * @param string|bool $wiki Wiki ID, or false for the current wiki
 496          *
 497          * @throws MWException
 498          * @return DatabaseBase
 499          */
 500         public function getConnection( $i, $groups = array(), $wiki = false ) {
 501                 if ( $i === null || $i === false ) {
 502                         throw new MWException( 'Attempt to call ' . __METHOD__ .
 503                                 ' with invalid server index' );
 504                 }
 505
 506                 if ( $wiki === wfWikiID() ) {
 507                         $wiki = false;
 508                 }
 509
 510                 $groups = ( $groups === false || $groups === array() )
 511                         ? array( false ) // check one "group": the generic pool
 512                         : (array)$groups;
 513
 514                 $masterOnly = ( $i == DB_MASTER || $i == $this->getWriterIndex() );
 515                 $oldConnsOpened = $this->connsOpened; // connections open now
 516
 517                 if ( $i == DB_MASTER ) {
 518                         $i = $this->getWriterIndex();
 519                 } else {
 520                         # Try to find an available server in any the query groups (in order)
 521                         foreach ( $groups as $group ) {
 522                                 $groupIndex = $this->getReaderIndex( $group, $wiki );
 523                                 if ( $groupIndex !== false ) {
 524                                         $i = $groupIndex;
 525                                         break;
 526                                 }
 527                         }
 528                 }
 529
 530                 # Operation-based index
 531                 if ( $i == DB_SLAVE ) {
 532                         $this->mLastError = 'Unknown error'; // reset error string
 533                         # Try the general server pool if $groups are unavailable.
 534                         $i = in_array( false, $groups, true )
 535                                 ? false // don't bother with this if that is what was tried above
 536                                 : $this->getReaderIndex( false, $wiki );
 537                         # Couldn't find a working server in getReaderIndex()?
 538                         if ( $i === false ) {
 539                                 $this->mLastError = 'No working slave server: ' . $this->mLastError;
 540
 541                                 return $this->reportConnectionError();
 542                         }
 543                 }
 544
 545                 # Now we have an explicit index into the servers array
 546                 $conn = $this->openConnection( $i, $wiki );
 547                 if ( !$conn ) {
 548                         return $this->reportConnectionError();
 549                 }
 550
 551                 # Profile any new connections that happen
 552                 if ( $this->connsOpened > $oldConnsOpened ) {
 553                         $host = $conn->getServer();
 554                         $dbname = $conn->getDBname();
 555                         $trxProf = Profiler::instance()->getTransactionProfiler();
 556                         $trxProf->recordConnection( $host, $dbname, $masterOnly );
 557                 }
 558
 559                 if ( $masterOnly ) {
 560                         # Make master-requested DB handles inherit any read-only mode setting
 561                         $conn->setLBInfo( 'readOnlyReason', $this->getReadOnlyReason( $wiki ) );
 562                 }
 563
 564                 return $conn;
 565         }
 566
 567         /**
 568          * Mark a foreign connection as being available for reuse under a different
 569          * DB name or prefix. This mechanism is reference-counted, and must be called
 570          * the same number of times as getConnection() to work.
 571          *
 572          * @param DatabaseBase $conn
 573          * @throws MWException
 574          */
 575         public function reuseConnection( $conn ) {
 576                 $serverIndex = $conn->getLBInfo( 'serverIndex' );
 577                 $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
 578                 if ( $serverIndex === null || $refCount === null ) {
 579                         wfDebug( __METHOD__ . ": this connection was not opened as a foreign connection\n" );
 580
 581                         /**
 582                          * This can happen in code like:
 583                          *   foreach ( $dbs as $db ) {
 584                          *     $conn = $lb->getConnection( DB_SLAVE, array(), $db );
 585                          *     ...
 586                          *     $lb->reuseConnection( $conn );
 587                          *   }
 588                          * When a connection to the local DB is opened in this way, reuseConnection()
 589                          * should be ignored
 590                          */
 591
 592                         return;
 593                 }
 594
 595                 $dbName = $conn->getDBname();
 596                 $prefix = $conn->tablePrefix();
 597                 if ( strval( $prefix ) !== '' ) {
 598                         $wiki = "$dbName-$prefix";
 599                 } else {
 600                         $wiki = $dbName;
 601                 }
 602                 if ( $this->mConns['foreignUsed'][$serverIndex][$wiki] !== $conn ) {
 603                         throw new MWException( __METHOD__ . ": connection not found, has " .
 604                                 "the connection been freed already?" );
 605                 }
 606                 $conn->setLBInfo( 'foreignPoolRefCount', --$refCount );
 607                 if ( $refCount <= 0 ) {
 608                         $this->mConns['foreignFree'][$serverIndex][$wiki] = $conn;
 609                         unset( $this->mConns['foreignUsed'][$serverIndex][$wiki] );
 610                         wfDebug( __METHOD__ . ": freed connection $serverIndex/$wiki\n" );
 611                 } else {
 612                         wfDebug( __METHOD__ . ": reference count for $serverIndex/$wiki reduced to $refCount\n" );
 613                 }
 614         }
 615
 616         /**
 617          * Get a database connection handle reference
 618          *
 619          * The handle's methods wrap simply wrap those of a DatabaseBase handle
 620          *
 621          * @see LoadBalancer::getConnection() for parameter information
 622          *
 623          * @param int $db
 624          * @param array|string|bool $groups Query group(s), or false for the generic reader
 625          * @param string|bool $wiki Wiki ID, or false for the current wiki
 626          * @return DBConnRef
 627          */
 628         public function getConnectionRef( $db, $groups = array(), $wiki = false ) {
 629                 return new DBConnRef( $this, $this->getConnection( $db, $groups, $wiki ) );
 630         }
 631
 632         /**
 633          * Get a database connection handle reference without connecting yet
 634          *
 635          * The handle's methods wrap simply wrap those of a DatabaseBase handle
 636          *
 637          * @see LoadBalancer::getConnection() for parameter information
 638          *
 639          * @param int $db
 640          * @param array|string|bool $groups Query group(s), or false for the generic reader
 641          * @param string|bool $wiki Wiki ID, or false for the current wiki
 642          * @return DBConnRef
 643          */
 644         public function getLazyConnectionRef( $db, $groups = array(), $wiki = false ) {
 645                 return new DBConnRef( $this, array( $db, $groups, $wiki ) );
 646         }
 647
 648         /**
 649          * Open a connection to the server given by the specified index
 650          * Index must be an actual index into the array.
 651          * If the server is already open, returns it.
 652          *
 653          * On error, returns false, and the connection which caused the
 654          * error will be available via $this->mErrorConnection.
 655          *
 656          * @param int $i Server index
 657          * @param string|bool $wiki Wiki ID, or false for the current wiki
 658          * @return DatabaseBase
 659          *
 660          * @access private
 661          */
 662         public function openConnection( $i, $wiki = false ) {
 663                 if ( $wiki !== false ) {
 664                         $conn = $this->openForeignConnection( $i, $wiki );
 665                 } elseif ( isset( $this->mConns['local'][$i][0] ) ) {
 666                         $conn = $this->mConns['local'][$i][0];
 667                 } else {
 668                         $server = $this->mServers[$i];
 669                         $server['serverIndex'] = $i;
 670                         $conn = $this->reallyOpenConnection( $server, false );
 671                         $serverName = $this->getServerName( $i );
 672                         if ( $conn->isOpen() ) {
 673                                 wfDebug( "Connected to database $i at $serverName\n" );
 674                                 $this->mConns['local'][$i][0] = $conn;
 675                         } else {
 676                                 wfDebug( "Failed to connect to database $i at $serverName\n" );
 677                                 $this->mErrorConnection = $conn;
 678                                 $conn = false;
 679                         }
 680                 }
 681
 682                 if ( $conn && !$conn->isOpen() ) {
 683                         // Connection was made but later unrecoverably lost for some reason.
 684                         // Do not return a handle that will just throw exceptions on use,
 685                         // but let the calling code (e.g. getReaderIndex) try another server.
 686                         // See DatabaseMyslBase::ping() for how this can happen.
 687                         $this->mErrorConnection = $conn;
 688                         $conn = false;
 689                 }
 690
 691                 return $conn;
 692         }
 693
 694         /**
 695          * Open a connection to a foreign DB, or return one if it is already open.
 696          *
 697          * Increments a reference count on the returned connection which locks the
 698          * connection to the requested wiki. This reference count can be
 699          * decremented by calling reuseConnection().
 700          *
 701          * If a connection is open to the appropriate server already, but with the wrong
 702          * database, it will be switched to the right database and returned, as long as
 703          * it has been freed first with reuseConnection().
 704          *
 705          * On error, returns false, and the connection which caused the
 706          * error will be available via $this->mErrorConnection.
 707          *
 708          * @param int $i Server index
 709          * @param string $wiki Wiki ID to open
 710          * @return DatabaseBase
 711          */
 712         private function openForeignConnection( $i, $wiki ) {
 713                 list( $dbName, $prefix ) = wfSplitWikiID( $wiki );
 714                 if ( isset( $this->mConns['foreignUsed'][$i][$wiki] ) ) {
 715                         // Reuse an already-used connection
 716                         $conn = $this->mConns['foreignUsed'][$i][$wiki];
 717                         wfDebug( __METHOD__ . ": reusing connection $i/$wiki\n" );
 718                 } elseif ( isset( $this->mConns['foreignFree'][$i][$wiki] ) ) {
 719                         // Reuse a free connection for the same wiki
 720                         $conn = $this->mConns['foreignFree'][$i][$wiki];
 721                         unset( $this->mConns['foreignFree'][$i][$wiki] );
 722                         $this->mConns['foreignUsed'][$i][$wiki] = $conn;
 723                         wfDebug( __METHOD__ . ": reusing free connection $i/$wiki\n" );
 724                 } elseif ( !empty( $this->mConns['foreignFree'][$i] ) ) {
 725                         // Reuse a connection from another wiki
 726                         $conn = reset( $this->mConns['foreignFree'][$i] );
 727                         $oldWiki = key( $this->mConns['foreignFree'][$i] );
 728
 729                         // The empty string as a DB name means "don't care".
 730                         // DatabaseMysqlBase::open() already handle this on connection.
 731                         if ( $dbName !== '' && !$conn->selectDB( $dbName ) ) {
 732                                 $this->mLastError = "Error selecting database $dbName on server " .
 733                                         $conn->getServer() . " from client host " . wfHostname() . "\n";
 734                                 $this->mErrorConnection = $conn;
 735                                 $conn = false;
 736                         } else {
 737                                 $conn->tablePrefix( $prefix );
 738                                 unset( $this->mConns['foreignFree'][$i][$oldWiki] );
 739                                 $this->mConns['foreignUsed'][$i][$wiki] = $conn;
 740                                 wfDebug( __METHOD__ . ": reusing free connection from $oldWiki for $wiki\n" );
 741                         }
 742                 } else {
 743                         // Open a new connection
 744                         $server = $this->mServers[$i];
 745                         $server['serverIndex'] = $i;
 746                         $server['foreignPoolRefCount'] = 0;
 747                         $server['foreign'] = true;
 748                         $conn = $this->reallyOpenConnection( $server, $dbName );
 749                         if ( !$conn->isOpen() ) {
 750                                 wfDebug( __METHOD__ . ": error opening connection for $i/$wiki\n" );
 751                                 $this->mErrorConnection = $conn;
 752                                 $conn = false;
 753                         } else {
 754                                 $conn->tablePrefix( $prefix );
 755                                 $this->mConns['foreignUsed'][$i][$wiki] = $conn;
 756                                 wfDebug( __METHOD__ . ": opened new connection for $i/$wiki\n" );
 757                         }
 758                 }
 759
 760                 // Increment reference count
 761                 if ( $conn ) {
 762                         $refCount = $conn->getLBInfo( 'foreignPoolRefCount' );
 763                         $conn->setLBInfo( 'foreignPoolRefCount', $refCount + 1 );
 764                 }
 765
 766                 return $conn;
 767         }
 768
 769         /**
 770          * Test if the specified index represents an open connection
 771          *
 772          * @param int $index Server index
 773          * @access private
 774          * @return bool
 775          */
 776         private function isOpen( $index ) {
 777                 if ( !is_integer( $index ) ) {
 778                         return false;
 779                 }
 780
 781                 return (bool)$this->getAnyOpenConnection( $index );
 782         }
 783
 784         /**
 785          * Really opens a connection. Uncached.
 786          * Returns a Database object whether or not the connection was successful.
 787          * @access private
 788          *
 789          * @param array $server
 790          * @param bool $dbNameOverride
 791          * @throws MWException
 792          * @return DatabaseBase
 793          */
 794         protected function reallyOpenConnection( $server, $dbNameOverride = false ) {
 795                 if ( !is_array( $server ) ) {
 796                         throw new MWException( 'You must update your load-balancing configuration. ' .
 797                                 'See DefaultSettings.php entry for $wgDBservers.' );
 798                 }
 799
 800                 if ( $dbNameOverride !== false ) {
 801                         $server['dbname'] = $dbNameOverride;
 802                 }
 803
 804                 // Log when many connection are made on requests
 805                 if ( ++$this->connsOpened >= self::CONN_HELD_WARN_THRESHOLD ) {
 806                         $masterAddr = $this->getServerName( 0 );
 807                         wfDebugLog( 'DBPerformance', __METHOD__ . ": " .
 808                                 "{$this->connsOpened}+ connections made (master=$masterAddr)\n" .
 809                                 wfBacktrace( true ) );
 810                 }
 811
 812                 # Create object
 813                 try {
 814                         $db = DatabaseBase::factory( $server['type'], $server );
 815                 } catch ( DBConnectionError $e ) {
 816                         // FIXME: This is probably the ugliest thing I have ever done to
 817                         // PHP. I'm half-expecting it to segfault, just out of disgust. -- TS
 818                         $db = $e->db;
 819                 }
 820
 821                 $db->setLBInfo( $server );
 822
 823                 return $db;
 824         }
 825
 826         /**
 827          * @throws DBConnectionError
 828          * @return bool
 829          */
 830         private function reportConnectionError() {
 831                 $conn = $this->mErrorConnection; // The connection which caused the error
 832                 $context = array(
 833                         'method' => __METHOD__,
 834                         'last_error' => $this->mLastError,
 835                 );
 836
 837                 if ( !is_object( $conn ) ) {
 838                         // No last connection, probably due to all servers being too busy
 839                         wfLogDBError(
 840                                 "LB failure with no last connection. Connection error: {last_error}",
 841                                 $context
 842                         );
 843
 844                         // If all servers were busy, mLastError will contain something sensible
 845                         throw new DBConnectionError( null, $this->mLastError );
 846                 } else {
 847                         $context['db_server'] = $conn->getProperty( 'mServer' );
 848                         wfLogDBError(
 849                                 "Connection error: {last_error} ({db_server})",
 850                                 $context
 851                         );
 852
 853                         // throws DBConnectionError
 854                         $conn->reportConnectionError( "{$this->mLastError} ({$context['db_server']})" );
 855                 }
 856
 857                 return false; /* not reached */
 858         }
 859
 860         /**
 861          * @return int
 862          * @since 1.26
 863          */
 864         public function getWriterIndex() {
 865                 return 0;
 866         }
 867
 868         /**
 869          * Returns true if the specified index is a valid server index
 870          *
 871          * @param string $i
 872          * @return bool
 873          */
 874         public function haveIndex( $i ) {
 875                 return array_key_exists( $i, $this->mServers );
 876         }
 877
 878         /**
 879          * Returns true if the specified index is valid and has non-zero load
 880          *
 881          * @param string $i
 882          * @return bool
 883          */
 884         public function isNonZeroLoad( $i ) {
 885                 return array_key_exists( $i, $this->mServers ) && $this->mLoads[$i] != 0;
 886         }
 887
 888         /**
 889          * Get the number of defined servers (not the number of open connections)
 890          *
 891          * @return int
 892          */
 893         public function getServerCount() {
 894                 return count( $this->mServers );
 895         }
 896
 897         /**
 898          * Get the host name or IP address of the server with the specified index
 899          * Prefer a readable name if available.
 900          * @param string $i
 901          * @return string
 902          */
 903         public function getServerName( $i ) {
 904                 if ( isset( $this->mServers[$i]['hostName'] ) ) {
 905                         $name = $this->mServers[$i]['hostName'];
 906                 } elseif ( isset( $this->mServers[$i]['host'] ) ) {
 907                         $name = $this->mServers[$i]['host'];
 908                 } else {
 909                         $name = '';
 910                 }
 911
 912                 return ( $name != '' ) ? $name : 'localhost';
 913         }
 914
 915         /**
 916          * Return the server info structure for a given index, or false if the index is invalid.
 917          * @param int $i
 918          * @return array|bool
 919          */
 920         public function getServerInfo( $i ) {
 921                 if ( isset( $this->mServers[$i] ) ) {
 922                         return $this->mServers[$i];
 923                 } else {
 924                         return false;
 925                 }
 926         }
 927
 928         /**
 929          * Sets the server info structure for the given index. Entry at index $i
 930          * is created if it doesn't exist
 931          * @param int $i
 932          * @param array $serverInfo
 933          */
 934         public function setServerInfo( $i, array $serverInfo ) {
 935                 $this->mServers[$i] = $serverInfo;
 936         }
 937
 938         /**
 939          * Get the current master position for chronology control purposes
 940          * @return mixed
 941          */
 942         public function getMasterPos() {
 943                 # If this entire request was served from a slave without opening a connection to the
 944                 # master (however unlikely that may be), then we can fetch the position from the slave.
 945                 $masterConn = $this->getAnyOpenConnection( 0 );
 946                 if ( !$masterConn ) {
 947                         $serverCount = count( $this->mServers );
 948                         for ( $i = 1; $i < $serverCount; $i++ ) {
 949                                 $conn = $this->getAnyOpenConnection( $i );
 950                                 if ( $conn ) {
 951                                         wfDebug( "Master pos fetched from slave\n" );
 952
 953                                         return $conn->getSlavePos();
 954                                 }
 955                         }
 956                 } else {
 957                         wfDebug( "Master pos fetched from master\n" );
 958
 959                         return $masterConn->getMasterPos();
 960                 }
 961
 962                 return false;
 963         }
 964
 965         /**
 966          * Close all open connections
 967          */
 968         public function closeAll() {
 969                 foreach ( $this->mConns as $conns2 ) {
 970                         foreach ( $conns2 as $conns3 ) {
 971                                 /** @var DatabaseBase $conn */
 972                                 foreach ( $conns3 as $conn ) {
 973                                         $conn->close();
 974                                 }
 975                         }
 976                 }
 977                 $this->mConns = array(
 978                         'local' => array(),
 979                         'foreignFree' => array(),
 980                         'foreignUsed' => array(),
 981                 );
 982                 $this->connsOpened = 0;
 983         }
 984
 985         /**
 986          * Close a connection
 987          * Using this function makes sure the LoadBalancer knows the connection is closed.
 988          * If you use $conn->close() directly, the load balancer won't update its state.
 989          * @param DatabaseBase $conn
 990          */
 991         public function closeConnection( $conn ) {
 992                 $done = false;
 993                 foreach ( $this->mConns as $i1 => $conns2 ) {
 994                         foreach ( $conns2 as $i2 => $conns3 ) {
 995                                 foreach ( $conns3 as $i3 => $candidateConn ) {
 996                                         if ( $conn === $candidateConn ) {
 997                                                 $conn->close();
 998                                                 unset( $this->mConns[$i1][$i2][$i3] );
 999                                                 --$this->connsOpened;
1000                                                 $done = true;
1001                                                 break;
1002                                         }
1003                                 }
1004                         }
1005                 }
1006                 if ( !$done ) {
1007                         $conn->close();
1008                 }
1009         }
1010
1011         /**
1012          * Commit transactions on all open connections
1013          */
1014         public function commitAll() {
1015                 foreach ( $this->mConns as $conns2 ) {
1016                         foreach ( $conns2 as $conns3 ) {
1017                                 /** @var DatabaseBase[] $conns3 */
1018                                 foreach ( $conns3 as $conn ) {
1019                                         if ( $conn->trxLevel() ) {
1020                                                 $conn->commit( __METHOD__, 'flush' );
1021                                         }
1022                                 }
1023                         }
1024                 }
1025         }
1026
1027         /**
1028          *  Issue COMMIT only on master, only if queries were done on connection
1029          */
1030         public function commitMasterChanges() {
1031                 $masterIndex = $this->getWriterIndex();
1032                 foreach ( $this->mConns as $conns2 ) {
1033                         if ( empty( $conns2[$masterIndex] ) ) {
1034                                 continue;
1035                         }
1036                         /** @var DatabaseBase $conn */
1037                         foreach ( $conns2[$masterIndex] as $conn ) {
1038                                 if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
1039                                         $conn->commit( __METHOD__, 'flush' );
1040                                 }
1041                         }
1042                 }
1043         }
1044
1045         /**
1046          * Issue ROLLBACK only on master, only if queries were done on connection
1047          * @since 1.23
1048          */
1049         public function rollbackMasterChanges() {
1050                 $failedServers = array();
1051
1052                 $masterIndex = $this->getWriterIndex();
1053                 foreach ( $this->mConns as $conns2 ) {
1054                         if ( empty( $conns2[$masterIndex] ) ) {
1055                                 continue;
1056                         }
1057                         /** @var DatabaseBase $conn */
1058                         foreach ( $conns2[$masterIndex] as $conn ) {
1059                                 if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
1060                                         try {
1061                                                 $conn->rollback( __METHOD__, 'flush' );
1062                                         } catch ( DBError $e ) {
1063                                                 MWExceptionHandler::logException( $e );
1064                                                 $failedServers[] = $conn->getServer();
1065                                         }
1066                                 }
1067                         }
1068                 }
1069
1070                 if ( $failedServers ) {
1071                         throw new DBExpectedError( null, "Rollback failed on server(s) " .
1072                                 implode( ', ', array_unique( $failedServers ) ) );
1073                 }
1074         }
1075
1076         /**
1077          * @return bool Whether a master connection is already open
1078          * @since 1.24
1079          */
1080         public function hasMasterConnection() {
1081                 return $this->isOpen( $this->getWriterIndex() );
1082         }
1083
1084         /**
1085          * Determine if there are pending changes in a transaction by this thread
1086          * @since 1.23
1087          * @return bool
1088          */
1089         public function hasMasterChanges() {
1090                 $masterIndex = $this->getWriterIndex();
1091                 foreach ( $this->mConns as $conns2 ) {
1092                         if ( empty( $conns2[$masterIndex] ) ) {
1093                                 continue;
1094                         }
1095                         /** @var DatabaseBase $conn */
1096                         foreach ( $conns2[$masterIndex] as $conn ) {
1097                                 if ( $conn->trxLevel() && $conn->writesOrCallbacksPending() ) {
1098                                         return true;
1099                                 }
1100                         }
1101                 }
1102                 return false;
1103         }
1104
1105         /**
1106          * Get the timestamp of the latest write query done by this thread
1107          * @since 1.25
1108          * @return float|bool UNIX timestamp or false
1109          */
1110         public function lastMasterChangeTimestamp() {
1111                 $lastTime = false;
1112                 $masterIndex = $this->getWriterIndex();
1113                 foreach ( $this->mConns as $conns2 ) {
1114                         if ( empty( $conns2[$masterIndex] ) ) {
1115                                 continue;
1116                         }
1117                         /** @var DatabaseBase $conn */
1118                         foreach ( $conns2[$masterIndex] as $conn ) {
1119                                 $lastTime = max( $lastTime, $conn->lastDoneWrites() );
1120                         }
1121                 }
1122                 return $lastTime;
1123         }
1124
1125         /**
1126          * Check if this load balancer object had any recent or still
1127          * pending writes issued against it by this PHP thread
1128          *
1129          * @param float $age How many seconds ago is "recent" [defaults to mWaitTimeout]
1130          * @return bool
1131          * @since 1.25
1132          */
1133         public function hasOrMadeRecentMasterChanges( $age = null ) {
1134                 $age = ( $age === null ) ? $this->mWaitTimeout : $age;
1135
1136                 return ( $this->hasMasterChanges()
1137                         || $this->lastMasterChangeTimestamp() > microtime( true ) - $age );
1138         }
1139
1140         /**
1141          * @param mixed $value
1142          * @return mixed
1143          */
1144         public function waitTimeout( $value = null ) {
1145                 return wfSetVar( $this->mWaitTimeout, $value );
1146         }
1147
1148         /**
1149          * @note This method will trigger a DB connection if not yet done
1150          *
1151          * @param string|bool $wiki Wiki ID, or false for the current wiki
1152          * @return bool Whether the generic connection for reads is highly "lagged"
1153          */
1154         public function getLaggedSlaveMode( $wiki = false ) {
1155                 // No-op if there is only one DB (also avoids recursion)
1156                 if ( !$this->laggedSlaveMode && $this->getServerCount() > 1 ) {
1157                         try {
1158                                 // See if laggedSlaveMode gets set
1159                                 $this->getConnection( DB_SLAVE, false, $wiki );
1160                         } catch ( DBConnectionError $e ) {
1161                                 // Avoid expensive re-connect attempts and failures
1162                                 $this->slavesDownMode = true;
1163                                 $this->laggedSlaveMode = true;
1164                         }
1165                 }
1166
1167                 return $this->laggedSlaveMode;
1168         }
1169
1170         /**
1171          * @note This method will never cause a new DB connection
1172          * @return bool Whether any generic connection used for reads was highly "lagged"
1173          * @since 1.27
1174          */
1175         public function laggedSlaveUsed() {
1176                 return $this->laggedSlaveMode;
1177         }
1178
1179         /**
1180          * @note This method may trigger a DB connection if not yet done
1181          * @param string|bool $wiki Wiki ID, or false for the current wiki
1182          * @return string|bool Reason the master is read-only or false if it is not
1183          * @since 1.27
1184          */
1185         public function getReadOnlyReason( $wiki = false ) {
1186                 if ( $this->readOnlyReason !== false ) {
1187                         return $this->readOnlyReason;
1188                 } elseif ( $this->getLaggedSlaveMode( $wiki ) ) {
1189                         if ( $this->slavesDownMode ) {
1190                                 return 'The database has been automatically locked ' .
1191                                         'until the slave database servers become available';
1192                         } else {
1193                                 return 'The database has been automatically locked ' .
1194                                         'while the slave database servers catch up to the master.';
1195                         }
1196                 }
1197
1198                 return false;
1199         }
1200
1201         /**
1202          * Disables/enables lag checks
1203          * @param null|bool $mode
1204          * @return bool
1205          */
1206         public function allowLagged( $mode = null ) {
1207                 if ( $mode === null ) {
1208                         return $this->mAllowLagged;
1209                 }
1210                 $this->mAllowLagged = $mode;
1211
1212                 return $this->mAllowLagged;
1213         }
1214
1215         /**
1216          * @return bool
1217          */
1218         public function pingAll() {
1219                 $success = true;
1220                 foreach ( $this->mConns as $conns2 ) {
1221                         foreach ( $conns2 as $conns3 ) {
1222                                 /** @var DatabaseBase[] $conns3 */
1223                                 foreach ( $conns3 as $conn ) {
1224                                         if ( !$conn->ping() ) {
1225                                                 $success = false;
1226                                         }
1227                                 }
1228                         }
1229                 }
1230
1231                 return $success;
1232         }
1233
1234         /**
1235          * Call a function with each open connection object
1236          * @param callable $callback
1237          * @param array $params
1238          */
1239         public function forEachOpenConnection( $callback, array $params = array() ) {
1240                 foreach ( $this->mConns as $conns2 ) {
1241                         foreach ( $conns2 as $conns3 ) {
1242                                 foreach ( $conns3 as $conn ) {
1243                                         $mergedParams = array_merge( array( $conn ), $params );
1244                                         call_user_func_array( $callback, $mergedParams );
1245                                 }
1246                         }
1247                 }
1248         }
1249
1250         /**
1251          * Get the hostname and lag time of the most-lagged slave
1252          *
1253          * This is useful for maintenance scripts that need to throttle their updates.
1254          * May attempt to open connections to slaves on the default DB. If there is
1255          * no lag, the maximum lag will be reported as -1.
1256          *
1257          * @param bool|string $wiki Wiki ID, or false for the default database
1258          * @return array ( host, max lag, index of max lagged host )
1259          */
1260         public function getMaxLag( $wiki = false ) {
1261                 $maxLag = -1;
1262                 $host = '';
1263                 $maxIndex = 0;
1264
1265                 if ( $this->getServerCount() <= 1 ) {
1266                         return array( $host, $maxLag, $maxIndex ); // no replication = no lag
1267                 }
1268
1269                 $lagTimes = $this->getLagTimes( $wiki );
1270                 foreach ( $lagTimes as $i => $lag ) {
1271                         if ( $lag > $maxLag ) {
1272                                 $maxLag = $lag;
1273                                 $host = $this->mServers[$i]['host'];
1274                                 $maxIndex = $i;
1275                         }
1276                 }
1277
1278                 return array( $host, $maxLag, $maxIndex );
1279         }
1280
1281         /**
1282          * Get an estimate of replication lag (in seconds) for each server
1283          *
1284          * Results are cached for a short time in memcached/process cache
1285          *
1286          * Values may be "false" if replication is too broken to estimate
1287          *
1288          * @param string|bool $wiki
1289          * @return int[] Map of (server index => float|int|bool)
1290          */
1291         public function getLagTimes( $wiki = false ) {
1292                 if ( $this->getServerCount() <= 1 ) {
1293                         return array( 0 => 0 ); // no replication = no lag
1294                 }
1295
1296                 # Send the request to the load monitor
1297                 return $this->getLoadMonitor()->getLagTimes( array_keys( $this->mServers ), $wiki );
1298         }
1299
1300         /**
1301          * Get the lag in seconds for a given connection, or zero if this load
1302          * balancer does not have replication enabled.
1303          *
1304          * This should be used in preference to Database::getLag() in cases where
1305          * replication may not be in use, since there is no way to determine if
1306          * replication is in use at the connection level without running
1307          * potentially restricted queries such as SHOW SLAVE STATUS. Using this
1308          * function instead of Database::getLag() avoids a fatal error in this
1309          * case on many installations.
1310          *
1311          * @param DatabaseBase $conn
1312          * @return int
1313          */
1314         public function safeGetLag( $conn ) {
1315                 if ( $this->getServerCount() == 1 ) {
1316                         return 0;
1317                 } else {
1318                         return $conn->getLag();
1319                 }
1320         }
1321
1322         /**
1323          * Clear the cache for slag lag delay times
1324          *
1325          * This is only used for testing
1326          */
1327         public function clearLagTimeCache() {
1328                 $this->getLoadMonitor()->clearCaches();
1329         }
1330 }