includes/objectcache/SqlBagOStuff.php

   1 <?php
   2 /**
   3  * Object caching using a SQL database.
   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 Cache
  22  */
  23
  24 /**
  25  * Class to store objects in the database
  26  *
  27  * @ingroup Cache
  28  */
  29 class SqlBagOStuff extends BagOStuff {
  30         /**
  31          * @var LoadBalancer
  32          */
  33         var $lb;
  34
  35         var $serverInfos;
  36         var $serverNames;
  37         var $numServers;
  38         var $conns;
  39         var $lastExpireAll = 0;
  40         var $purgePeriod = 100;
  41         var $shards = 1;
  42         var $tableName = 'objectcache';
  43
  44         protected $connFailureTimes = array(); // UNIX timestamps
  45         protected $connFailureErrors = array(); // exceptions
  46
  47         /**
  48          * Constructor. Parameters are:
  49          *   - server:      A server info structure in the format required by each
  50          *                  element in $wgDBServers.
  51          *
  52          *   - servers:     An array of server info structures describing a set of
  53          *                  database servers to distribute keys to. If this is
  54          *                  specified, the "server" option will be ignored.
  55          *
  56          *   - purgePeriod: The average number of object cache requests in between
  57          *                  garbage collection operations, where expired entries
  58          *                  are removed from the database. Or in other words, the
  59          *                  reciprocal of the probability of purging on any given
  60          *                  request. If this is set to zero, purging will never be
  61          *                  done.
  62          *
  63          *   - tableName:   The table name to use, default is "objectcache".
  64          *
  65          *   - shards:      The number of tables to use for data storage on each server.
  66          *                  If this is more than 1, table names will be formed in the style
  67          *                  objectcacheNNN where NNN is the shard index, between 0 and
  68          *                  shards-1. The number of digits will be the minimum number
  69          *                  required to hold the largest shard index. Data will be
  70          *                  distributed across all tables by key hash. This is for
  71          *                  MySQL bugs 61735 and 61736.
  72          *
  73          * @param $params array
  74          */
  75         public function __construct( $params ) {
  76                 if ( isset( $params['servers'] ) ) {
  77                         $this->serverInfos = $params['servers'];
  78                         $this->numServers = count( $this->serverInfos );
  79                         $this->serverNames = array();
  80                         foreach ( $this->serverInfos as $i => $info ) {
  81                                 $this->serverNames[$i] = isset( $info['host'] ) ? $info['host'] : "#$i";
  82                         }
  83                 } elseif ( isset( $params['server'] ) ) {
  84                         $this->serverInfos = array( $params['server'] );
  85                         $this->numServers = count( $this->serverInfos );
  86                 } else {
  87                         $this->serverInfos = false;
  88                         $this->numServers = 1;
  89                 }
  90                 if ( isset( $params['purgePeriod'] ) ) {
  91                         $this->purgePeriod = intval( $params['purgePeriod'] );
  92                 }
  93                 if ( isset( $params['tableName'] ) ) {
  94                         $this->tableName = $params['tableName'];
  95                 }
  96                 if ( isset( $params['shards'] ) ) {
  97                         $this->shards = intval( $params['shards'] );
  98                 }
  99         }
 100
 101         /**
 102          * Get a connection to the specified database
 103          *
 104          * @param $serverIndex integer
 105          * @return DatabaseBase
 106          */
 107         protected function getDB( $serverIndex ) {
 108                 global $wgDebugDBTransactions;
 109
 110                 if ( !isset( $this->conns[$serverIndex] ) ) {
 111                         if ( $serverIndex >= $this->numServers ) {
 112                                 throw new MWException( __METHOD__ . ": Invalid server index \"$serverIndex\"" );
 113                         }
 114
 115                         # Don't keep timing out trying to connect for each call if the DB is down
 116                         if ( isset( $this->connFailureErrors[$serverIndex] )
 117                                 && ( time() - $this->connFailureTimes[$serverIndex] ) < 60 )
 118                         {
 119                                 throw $this->connFailureErrors[$serverIndex];
 120                         }
 121
 122                         # If server connection info was given, use that
 123                         if ( $this->serverInfos ) {
 124                                 if ( $wgDebugDBTransactions ) {
 125                                         wfDebug( "Using provided serverInfo for SqlBagOStuff\n" );
 126                                 }
 127                                 $info = $this->serverInfos[$serverIndex];
 128                                 $type = isset( $info['type'] ) ? $info['type'] : 'mysql';
 129                                 $host = isset( $info['host'] ) ? $info['host'] : '[unknown]';
 130                                 wfDebug( __CLASS__.": connecting to $host\n" );
 131                                 $db = DatabaseBase::factory( $type, $info );
 132                                 $db->clearFlag( DBO_TRX );
 133                         } else {
 134                                 /*
 135                                  * We must keep a separate connection to MySQL in order to avoid deadlocks
 136                                  * However, SQLite has an opposite behaviour. And PostgreSQL needs to know
 137                                  * if we are in transaction or no
 138                                  */
 139                                 if ( wfGetDB( DB_MASTER )->getType() == 'mysql' ) {
 140                                         $this->lb = wfGetLBFactory()->newMainLB();
 141                                         $db = $this->lb->getConnection( DB_MASTER );
 142                                         $db->clearFlag( DBO_TRX ); // auto-commit mode
 143                                 } else {
 144                                         $db = wfGetDB( DB_MASTER );
 145                                 }
 146                         }
 147                         if ( $wgDebugDBTransactions ) {
 148                                 wfDebug( sprintf( "Connection %s will be used for SqlBagOStuff\n", $db ) );
 149                         }
 150                         $this->conns[$serverIndex] = $db;
 151                 }
 152
 153                 return $this->conns[$serverIndex];
 154         }
 155
 156         /**
 157          * Get the server index and table name for a given key
 158          * @param $key string
 159          * @return Array: server index and table name
 160          */
 161         protected function getTableByKey( $key ) {
 162                 if ( $this->shards > 1 ) {
 163                         $hash = hexdec( substr( md5( $key ), 0, 8 ) ) & 0x7fffffff;
 164                         $tableIndex = $hash % $this->shards;
 165                 } else {
 166                         $tableIndex = 0;
 167                 }
 168                 if ( $this->numServers > 1 ) {
 169                         $sortedServers = $this->serverNames;
 170                         ArrayUtils::consistentHashSort( $sortedServers, $key );
 171                         reset( $sortedServers );
 172                         $serverIndex = key( $sortedServers );
 173                 } else {
 174                         $serverIndex = 0;
 175                 }
 176                 return array( $serverIndex, $this->getTableNameByShard( $tableIndex ) );
 177         }
 178
 179         /**
 180          * Get the table name for a given shard index
 181          * @param $index int
 182          * @return string
 183          */
 184         protected function getTableNameByShard( $index ) {
 185                 if ( $this->shards > 1 ) {
 186                         $decimals = strlen( $this->shards - 1 );
 187                         return $this->tableName .
 188                                 sprintf( "%0{$decimals}d", $index );
 189                 } else {
 190                         return $this->tableName;
 191                 }
 192         }
 193
 194         /**
 195          * @param $key string
 196          * @return mixed
 197          */
 198         public function get( $key ) {
 199                 $values = $this->getMulti( array( $key ) );
 200                 return array_key_exists( $key, $values ) ? $values[$key] : false;
 201         }
 202
 203         /**
 204          * @param $keys array
 205          * @return Array
 206          */
 207         public function getMulti( array $keys ) {
 208                 $values = array(); // array of (key => value)
 209
 210                 $keysByTable = array();
 211                 foreach ( $keys as $key ) {
 212                         list( $serverIndex, $tableName ) = $this->getTableByKey( $key );
 213                         $keysByTable[$serverIndex][$tableName][] = $key;
 214                 }
 215
 216                 $this->garbageCollect(); // expire old entries if any
 217
 218                 $dataRows = array();
 219                 foreach ( $keysByTable as $serverIndex => $serverKeys ) {
 220                         $db = $this->getDB( $serverIndex );
 221                         try {
 222                                 foreach ( $serverKeys as $tableName => $tableKeys ) {
 223                                         $res = $db->select( $tableName,
 224                                                 array( 'keyname', 'value', 'exptime' ),
 225                                                 array( 'keyname' => $tableKeys ),
 226                                                 __METHOD__ );
 227                                         foreach ( $res as $row ) {
 228                                                 $row->serverIndex = $serverIndex;
 229                                                 $row->tableName = $tableName;
 230                                                 $dataRows[$row->keyname] = $row;
 231                                         }
 232                                 }
 233                         } catch ( DBError $e ) {
 234                                 $this->handleReadError( $e, $serverIndex );
 235                         }
 236                 }
 237
 238                 foreach ( $keys as $key ) {
 239                         if ( isset( $dataRows[$key] ) ) { // HIT?
 240                                 $row = $dataRows[$key];
 241                                 $this->debug( "get: retrieved data; expiry time is " . $row->exptime );
 242                                 $db = $this->getDB( $row->serverIndex );
 243                                 if ( $this->isExpired( $db, $row->exptime ) ) { // MISS
 244                                         $this->debug( "get: key has expired, deleting" );
 245                                         try {
 246                                                 $db->begin( __METHOD__ );
 247                                                 # Put the expiry time in the WHERE condition to avoid deleting a
 248                                                 # newly-inserted value
 249                                                 $db->delete( $row->tableName,
 250                                                         array( 'keyname' => $key, 'exptime' => $row->exptime ),
 251                                                         __METHOD__ );
 252                                                 $db->commit( __METHOD__ );
 253                                         } catch ( DBQueryError $e ) {
 254                                                 $this->handleWriteError( $e, $row->serverIndex );
 255                                         }
 256                                         $values[$key] = false;
 257                                 } else { // HIT
 258                                         $values[$key] = $this->unserialize( $db->decodeBlob( $row->value ) );
 259                                 }
 260                         } else { // MISS
 261                                 $values[$key] = false;
 262                                 $this->debug( 'get: no matching rows' );
 263                         }
 264                 }
 265
 266                 return $values;
 267         }
 268
 269         /**
 270          * @param $key string
 271          * @param $value mixed
 272          * @param $exptime int
 273          * @return bool
 274          */
 275         public function set( $key, $value, $exptime = 0 ) {
 276                 list( $serverIndex, $tableName ) = $this->getTableByKey( $key );
 277                 try {
 278                         $db = $this->getDB( $serverIndex );
 279                         $exptime = intval( $exptime );
 280
 281                         if ( $exptime < 0 ) {
 282                                 $exptime = 0;
 283                         }
 284
 285                         if ( $exptime == 0 ) {
 286                                 $encExpiry = $this->getMaxDateTime( $db );
 287                         } else {
 288                                 if ( $exptime < 3.16e8 ) { # ~10 years
 289                                         $exptime += time();
 290                                 }
 291
 292                                 $encExpiry = $db->timestamp( $exptime );
 293                         }
 294                         $db->begin( __METHOD__ );
 295                         // (bug 24425) use a replace if the db supports it instead of
 296                         // delete/insert to avoid clashes with conflicting keynames
 297                         $db->replace(
 298                                 $tableName,
 299                                 array( 'keyname' ),
 300                                 array(
 301                                         'keyname' => $key,
 302                                         'value' => $db->encodeBlob( $this->serialize( $value ) ),
 303                                         'exptime' => $encExpiry
 304                                 ), __METHOD__ );
 305                         $db->commit( __METHOD__ );
 306                 } catch ( DBError $e ) {
 307                         $this->handleWriteError( $e, $serverIndex );
 308                         return false;
 309                 }
 310
 311                 return true;
 312         }
 313
 314         /**
 315          * @param $key string
 316          * @param $time int
 317          * @return bool
 318          */
 319         public function delete( $key, $time = 0 ) {
 320                 list( $serverIndex, $tableName ) = $this->getTableByKey( $key );
 321                 try {
 322                         $db = $this->getDB( $serverIndex );
 323                         $db->begin( __METHOD__ );
 324                         $db->delete(
 325                                 $tableName,
 326                                 array( 'keyname' => $key ),
 327                                 __METHOD__ );
 328                         $db->commit( __METHOD__ );
 329                 } catch ( DBError $e ) {
 330                         $this->handleWriteError( $e, $serverIndex );
 331                         return false;
 332                 }
 333
 334                 return true;
 335         }
 336
 337         /**
 338          * @param $key string
 339          * @param $step int
 340          * @return int|null
 341          */
 342         public function incr( $key, $step = 1 ) {
 343                 list( $serverIndex, $tableName ) = $this->getTableByKey( $key );
 344                 try {
 345                         $db = $this->getDB( $serverIndex );
 346                         $step = intval( $step );
 347                         $db->begin( __METHOD__ );
 348                         $row = $db->selectRow(
 349                                 $tableName,
 350                                 array( 'value', 'exptime' ),
 351                                 array( 'keyname' => $key ),
 352                                 __METHOD__,
 353                                 array( 'FOR UPDATE' ) );
 354                         if ( $row === false ) {
 355                                 // Missing
 356                                 $db->commit( __METHOD__ );
 357
 358                                 return null;
 359                         }
 360                         $db->delete( $tableName, array( 'keyname' => $key ), __METHOD__ );
 361                         if ( $this->isExpired( $db, $row->exptime ) ) {
 362                                 // Expired, do not reinsert
 363                                 $db->commit( __METHOD__ );
 364
 365                                 return null;
 366                         }
 367
 368                         $oldValue = intval( $this->unserialize( $db->decodeBlob( $row->value ) ) );
 369                         $newValue = $oldValue + $step;
 370                         $db->insert( $tableName,
 371                                 array(
 372                                         'keyname' => $key,
 373                                         'value' => $db->encodeBlob( $this->serialize( $newValue ) ),
 374                                         'exptime' => $row->exptime
 375                                 ), __METHOD__, 'IGNORE' );
 376
 377                         if ( $db->affectedRows() == 0 ) {
 378                                 // Race condition. See bug 28611
 379                                 $newValue = null;
 380                         }
 381                         $db->commit( __METHOD__ );
 382                 } catch ( DBError $e ) {
 383                         $this->handleWriteError( $e, $serverIndex );
 384                         return null;
 385                 }
 386
 387                 return $newValue;
 388         }
 389
 390         /**
 391          * @return Array
 392          */
 393         public function keys() {
 394                 $result = array();
 395
 396                 for ( $serverIndex = 0; $serverIndex < $this->numServers; $serverIndex++ ) {
 397                         try {
 398                                 $db = $this->getDB( $serverIndex );
 399                                 for ( $i = 0; $i < $this->shards; $i++ ) {
 400                                         $res = $db->select( $this->getTableNameByShard( $i ),
 401                                                 array( 'keyname' ), false, __METHOD__ );
 402                                         foreach ( $res as $row ) {
 403                                                 $result[] = $row->keyname;
 404                                         }
 405                                 }
 406                         } catch ( DBError $e ) {
 407                                 $this->handleReadError( $e, $serverIndex );
 408                         }
 409                 }
 410                 return $result;
 411         }
 412
 413         /**
 414          * @param $exptime string
 415          * @return bool
 416          */
 417         protected function isExpired( $db, $exptime ) {
 418                 return $exptime != $this->getMaxDateTime( $db ) && wfTimestamp( TS_UNIX, $exptime ) < time();
 419         }
 420
 421         /**
 422          * @return string
 423          */
 424         protected function getMaxDateTime( $db ) {
 425                 if ( time() > 0x7fffffff ) {
 426                         return $db->timestamp( 1 << 62 );
 427                 } else {
 428                         return $db->timestamp( 0x7fffffff );
 429                 }
 430         }
 431
 432         protected function garbageCollect() {
 433                 if ( !$this->purgePeriod ) {
 434                         // Disabled
 435                         return;
 436                 }
 437                 // Only purge on one in every $this->purgePeriod requests.
 438                 if ( $this->purgePeriod !== 1 && mt_rand( 0, $this->purgePeriod - 1 ) ) {
 439                         return;
 440                 }
 441                 $now = time();
 442                 // Avoid repeating the delete within a few seconds
 443                 if ( $now > ( $this->lastExpireAll + 1 ) ) {
 444                         $this->lastExpireAll = $now;
 445                         $this->expireAll();
 446                 }
 447         }
 448
 449         public function expireAll() {
 450                 $this->deleteObjectsExpiringBefore( wfTimestampNow() );
 451         }
 452
 453         /**
 454          * Delete objects from the database which expire before a certain date.
 455          * @param $timestamp string
 456          * @param $progressCallback bool|callback
 457          * @return bool
 458          */
 459         public function deleteObjectsExpiringBefore( $timestamp, $progressCallback = false ) {
 460                 for ( $serverIndex = 0; $serverIndex < $this->numServers; $serverIndex++ ) {
 461                         try {
 462                                 $db = $this->getDB( $serverIndex );
 463                                 $dbTimestamp = $db->timestamp( $timestamp );
 464                                 $totalSeconds = false;
 465                                 $baseConds = array( 'exptime < ' . $db->addQuotes( $dbTimestamp ) );
 466                                 for ( $i = 0; $i < $this->shards; $i++ ) {
 467                                         $maxExpTime = false;
 468                                         while ( true ) {
 469                                                 $conds = $baseConds;
 470                                                 if ( $maxExpTime !== false ) {
 471                                                         $conds[] = 'exptime > ' . $db->addQuotes( $maxExpTime );
 472                                                 }
 473                                                 $rows = $db->select(
 474                                                         $this->getTableNameByShard( $i ),
 475                                                         array( 'keyname', 'exptime' ),
 476                                                         $conds,
 477                                                         __METHOD__,
 478                                                         array( 'LIMIT' => 100, 'ORDER BY' => 'exptime' ) );
 479                                                 if ( !$rows->numRows() ) {
 480                                                         break;
 481                                                 }
 482                                                 $keys = array();
 483                                                 $row = $rows->current();
 484                                                 $minExpTime = $row->exptime;
 485                                                 if ( $totalSeconds === false ) {
 486                                                         $totalSeconds = wfTimestamp( TS_UNIX, $timestamp )
 487                                                                 - wfTimestamp( TS_UNIX, $minExpTime );
 488                                                 }
 489                                                 foreach ( $rows as $row ) {
 490                                                         $keys[] = $row->keyname;
 491                                                         $maxExpTime = $row->exptime;
 492                                                 }
 493
 494                                                 $db->begin( __METHOD__ );
 495                                                 $db->delete(
 496                                                         $this->getTableNameByShard( $i ),
 497                                                         array(
 498                                                                 'exptime >= ' . $db->addQuotes( $minExpTime ),
 499                                                                 'exptime < ' . $db->addQuotes( $dbTimestamp ),
 500                                                                 'keyname' => $keys
 501                                                         ),
 502                                                         __METHOD__ );
 503                                                 $db->commit( __METHOD__ );
 504
 505                                                 if ( $progressCallback ) {
 506                                                         if ( intval( $totalSeconds ) === 0 ) {
 507                                                                 $percent = 0;
 508                                                         } else {
 509                                                                 $remainingSeconds = wfTimestamp( TS_UNIX, $timestamp )
 510                                                                         - wfTimestamp( TS_UNIX, $maxExpTime );
 511                                                                 if ( $remainingSeconds > $totalSeconds ) {
 512                                                                         $totalSeconds = $remainingSeconds;
 513                                                                 }
 514                                                                 $percent = ( $i + $remainingSeconds / $totalSeconds )
 515                                                                         / $this->shards * 100;
 516                                                         }
 517                                                         $percent = ( $percent / $this->numServers )
 518                                                                 + ( $serverIndex / $this->numServers * 100 );
 519                                                         call_user_func( $progressCallback, $percent );
 520                                                 }
 521                                         }
 522                                 }
 523                         } catch ( DBError $e ) {
 524                                 $this->handleWriteError( $e, $serverIndex );
 525                                 return false;
 526                         }
 527                 }
 528                 return true;
 529         }
 530
 531         public function deleteAll() {
 532                 for ( $serverIndex = 0; $serverIndex < $this->numServers; $serverIndex++ ) {
 533                         try {
 534                                 $db = $this->getDB( $serverIndex );
 535                                 for ( $i = 0; $i < $this->shards; $i++ ) {
 536                                         $db->begin( __METHOD__ );
 537                                         $db->delete( $this->getTableNameByShard( $i ), '*', __METHOD__ );
 538                                         $db->commit( __METHOD__ );
 539                                 }
 540                         } catch ( DBError $e ) {
 541                                 $this->handleWriteError( $e, $serverIndex );
 542                                 return false;
 543                         }
 544                 }
 545                 return true;
 546         }
 547
 548         /**
 549          * Serialize an object and, if possible, compress the representation.
 550          * On typical message and page data, this can provide a 3X decrease
 551          * in storage requirements.
 552          *
 553          * @param $data mixed
 554          * @return string
 555          */
 556         protected function serialize( &$data ) {
 557                 $serial = serialize( $data );
 558
 559                 if ( function_exists( 'gzdeflate' ) ) {
 560                         return gzdeflate( $serial );
 561                 } else {
 562                         return $serial;
 563                 }
 564         }
 565
 566         /**
 567          * Unserialize and, if necessary, decompress an object.
 568          * @param $serial string
 569          * @return mixed
 570          */
 571         protected function unserialize( $serial ) {
 572                 if ( function_exists( 'gzinflate' ) ) {
 573                         wfSuppressWarnings();
 574                         $decomp = gzinflate( $serial );
 575                         wfRestoreWarnings();
 576
 577                         if ( false !== $decomp ) {
 578                                 $serial = $decomp;
 579                         }
 580                 }
 581
 582                 $ret = unserialize( $serial );
 583
 584                 return $ret;
 585         }
 586
 587         /**
 588          * Handle a DBError which occurred during a read operation.
 589          */
 590         protected function handleReadError( DBError $exception, $serverIndex ) {
 591                 if ( $exception instanceof DBConnectionError ) {
 592                         $this->markServerDown( $exception, $serverIndex );
 593                 }
 594                 wfDebugLog( 'SQLBagOStuff', "DBError: {$exception->getMessage()}" );
 595                 if ( $exception instanceof DBConnectionError ) {
 596                         wfDebug( __METHOD__ . ": ignoring connection error\n" );
 597                 } else {
 598                         wfDebug( __METHOD__ . ": ignoring query error\n" );
 599                 }
 600         }
 601
 602         /**
 603          * Handle a DBQueryError which occurred during a write operation.
 604          */
 605         protected function handleWriteError( DBError $exception, $serverIndex ) {
 606                 if ( $exception instanceof DBConnectionError ) {
 607                         $this->markServerDown( $exception, $serverIndex );
 608                 }
 609                 if ( $exception->db && $exception->db->wasReadOnlyError() ) {
 610                         try {
 611                                 $exception->db->rollback( __METHOD__ );
 612                         } catch ( DBError $e ) {}
 613                 }
 614                 wfDebugLog( 'SQLBagOStuff', "DBError: {$exception->getMessage()}" );
 615                 if ( $exception instanceof DBConnectionError ) {
 616                         wfDebug( __METHOD__ . ": ignoring connection error\n" );
 617                 } else {
 618                         wfDebug( __METHOD__ . ": ignoring query error\n" );
 619                 }
 620         }
 621
 622         /**
 623          * Mark a server down due to a DBConnectionError exception
 624          */
 625         protected function markServerDown( $exception, $serverIndex ) {
 626                 if ( isset( $this->connFailureTimes[$serverIndex] ) ) {
 627                         if ( time() - $this->connFailureTimes[$serverIndex] >= 60 ) {
 628                                 unset( $this->connFailureTimes[$serverIndex] );
 629                                 unset( $this->connFailureErrors[$serverIndex] );
 630                         } else {
 631                                 wfDebug( __METHOD__.": Server #$serverIndex already down\n" );
 632                                 return;
 633                         }
 634                 }
 635                 $now = time();
 636                 wfDebug( __METHOD__.": Server #$serverIndex down until " . ( $now + 60 ) . "\n" );
 637                 $this->connFailureTimes[$serverIndex] = $now;
 638                 $this->connFailureErrors[$serverIndex] = $exception;
 639         }
 640
 641         /**
 642          * Create shard tables. For use from eval.php.
 643          */
 644         public function createTables() {
 645                 for ( $serverIndex = 0; $serverIndex < $this->numServers; $serverIndex++ ) {
 646                         $db = $this->getDB( $serverIndex );
 647                         if ( $db->getType() !== 'mysql'
 648                                 || version_compare( $db->getServerVersion(), '4.1.0', '<' ) )
 649                         {
 650                                 throw new MWException( __METHOD__ . ' is not supported on this DB server' );
 651                         }
 652
 653                         for ( $i = 0; $i < $this->shards; $i++ ) {
 654                                 $db->begin( __METHOD__ );
 655                                 $db->query(
 656                                         'CREATE TABLE ' . $db->tableName( $this->getTableNameByShard( $i ) ) .
 657                                         ' LIKE ' . $db->tableName( 'objectcache' ),
 658                                         __METHOD__ );
 659                                 $db->commit( __METHOD__ );
 660                         }
 661                 }
 662         }
 663 }
 664
 665 /**
 666  * Backwards compatibility alias
 667  */
 668 class MediaWikiBagOStuff extends SqlBagOStuff { }
 669