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