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[mediawiki.git] / includes / objectcache / SqlBagOStuff.php

<?php
/**
 * Object caching using a SQL database.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 * @ingroup Cache
 */

/**
 * Class to store objects in the database
 *
 * @ingroup Cache
 */
class SqlBagOStuff extends BagOStuff {
        /** @var LoadBalancer */
        protected $lb;

        protected $serverInfos;

        /** @var array */
        protected $serverNames;

        /** @var int */
        protected $numServers;

        /** @var array */
        protected $conns;

        /** @var int */
        protected $lastExpireAll = 0;

        /** @var int */
        protected $purgePeriod = 100;

        /** @var int */
        protected $shards = 1;

        /** @var string */
        protected $tableName = 'objectcache';

        /** @var array UNIX timestamps */
        protected $connFailureTimes = array();

        /** @var array Exceptions */
        protected $connFailureErrors = array();

        /**
         * Constructor. Parameters are:
         *   - server:      A server info structure in the format required by each
         *                  element in $wgDBServers.
         *
         *   - servers:     An array of server info structures describing a set of
         *                  database servers to distribute keys to. If this is
         *                  specified, the "server" option will be ignored.
         *
         *   - purgePeriod: The average number of object cache requests in between
         *                  garbage collection operations, where expired entries
         *                  are removed from the database. Or in other words, the
         *                  reciprocal of the probability of purging on any given
         *                  request. If this is set to zero, purging will never be
         *                  done.
         *
         *   - tableName:   The table name to use, default is "objectcache".
         *
         *   - shards:      The number of tables to use for data storage on each server.
         *                  If this is more than 1, table names will be formed in the style
         *                  objectcacheNNN where NNN is the shard index, between 0 and
         *                  shards-1. The number of digits will be the minimum number
         *                  required to hold the largest shard index. Data will be
         *                  distributed across all tables by key hash. This is for
         *                  MySQL bugs 61735 and 61736.
         *
         * @param array $params
         */
        public function __construct( $params ) {
                parent::__construct( $params );
                if ( isset( $params['servers'] ) ) {
                        $this->serverInfos = $params['servers'];
                        $this->numServers = count( $this->serverInfos );
                        $this->serverNames = array();
                        foreach ( $this->serverInfos as $i => $info ) {
                                $this->serverNames[$i] = isset( $info['host'] ) ? $info['host'] : "#$i";
                        }
                } elseif ( isset( $params['server'] ) ) {
                        $this->serverInfos = array( $params['server'] );
                        $this->numServers = count( $this->serverInfos );
                } else {
                        $this->serverInfos = false;
                        $this->numServers = 1;
                }
                if ( isset( $params['purgePeriod'] ) ) {
                        $this->purgePeriod = intval( $params['purgePeriod'] );
                }
                if ( isset( $params['tableName'] ) ) {
                        $this->tableName = $params['tableName'];
                }
                if ( isset( $params['shards'] ) ) {
                        $this->shards = intval( $params['shards'] );
                }
        }

        /**
         * Get a connection to the specified database
         *
         * @param int $serverIndex
         * @return DatabaseBase
         * @throws MWException
         */
        protected function getDB( $serverIndex ) {
                global $wgDebugDBTransactions;

                if ( !isset( $this->conns[$serverIndex] ) ) {
                        if ( $serverIndex >= $this->numServers ) {
                                throw new MWException( __METHOD__ . ": Invalid server index \"$serverIndex\"" );
                        }

                        # Don't keep timing out trying to connect for each call if the DB is down
                        if ( isset( $this->connFailureErrors[$serverIndex] )
                                && ( time() - $this->connFailureTimes[$serverIndex] ) < 60
                        ) {
                                throw $this->connFailureErrors[$serverIndex];
                        }

                        # If server connection info was given, use that
                        if ( $this->serverInfos ) {
                                if ( $wgDebugDBTransactions ) {
                                        $this->logger->debug( "Using provided serverInfo for SqlBagOStuff" );
                                }
                                $info = $this->serverInfos[$serverIndex];
                                $type = isset( $info['type'] ) ? $info['type'] : 'mysql';
                                $host = isset( $info['host'] ) ? $info['host'] : '[unknown]';
                                $this->logger->debug( __CLASS__ . ": connecting to $host" );
                                // Use a blank trx profiler to ignore expections as this is a cache
                                $info['trxProfiler'] = new TransactionProfiler();
                                $db = DatabaseBase::factory( $type, $info );
                                $db->clearFlag( DBO_TRX );
                        } else {
                                /*
                                 * We must keep a separate connection to MySQL in order to avoid deadlocks
                                 * However, SQLite has an opposite behavior. And PostgreSQL needs to know
                                 * if we are in transaction or no
                                 */
                                if ( wfGetDB( DB_MASTER )->getType() == 'mysql' ) {
                                        $this->lb = wfGetLBFactory()->newMainLB();
                                        $db = $this->lb->getConnection( DB_MASTER );
                                        $db->clearFlag( DBO_TRX ); // auto-commit mode
                                } else {
                                        $db = wfGetDB( DB_MASTER );
                                }
                        }
                        if ( $wgDebugDBTransactions ) {
                                $this->logger->debug( sprintf( "Connection %s will be used for SqlBagOStuff", $db ) );
                        }
                        $this->conns[$serverIndex] = $db;
                }

                return $this->conns[$serverIndex];
        }

        /**
         * Get the server index and table name for a given key
         * @param string $key
         * @return array Server index and table name
         */
        protected function getTableByKey( $key ) {
                if ( $this->shards > 1 ) {
                        $hash = hexdec( substr( md5( $key ), 0, 8 ) ) & 0x7fffffff;
                        $tableIndex = $hash % $this->shards;
                } else {
                        $tableIndex = 0;
                }
                if ( $this->numServers > 1 ) {
                        $sortedServers = $this->serverNames;
                        ArrayUtils::consistentHashSort( $sortedServers, $key );
                        reset( $sortedServers );
                        $serverIndex = key( $sortedServers );
                } else {
                        $serverIndex = 0;
                }
                return array( $serverIndex, $this->getTableNameByShard( $tableIndex ) );
        }

        /**
         * Get the table name for a given shard index
         * @param int $index
         * @return string
         */
        protected function getTableNameByShard( $index ) {
                if ( $this->shards > 1 ) {
                        $decimals = strlen( $this->shards - 1 );
                        return $this->tableName .
                                sprintf( "%0{$decimals}d", $index );
                } else {
                        return $this->tableName;
                }
        }

        /**
         * @param string $key
         * @param mixed $casToken [optional]
         * @return mixed
         */
        public function get( $key, &$casToken = null ) {
                $values = $this->getMulti( array( $key ) );
                if ( array_key_exists( $key, $values ) ) {
                        $casToken = $values[$key];
                        return $values[$key];
                }
                return false;
        }

        /**
         * @param array $keys
         * @return array
         */
        public function getMulti( array $keys ) {
                $values = array(); // array of (key => value)

                $keysByTable = array();
                foreach ( $keys as $key ) {
                        list( $serverIndex, $tableName ) = $this->getTableByKey( $key );
                        $keysByTable[$serverIndex][$tableName][] = $key;
                }

                $this->garbageCollect(); // expire old entries if any

                $dataRows = array();
                foreach ( $keysByTable as $serverIndex => $serverKeys ) {
                        try {
                                $db = $this->getDB( $serverIndex );
                                foreach ( $serverKeys as $tableName => $tableKeys ) {
                                        $res = $db->select( $tableName,
                                                array( 'keyname', 'value', 'exptime' ),
                                                array( 'keyname' => $tableKeys ),
                                                __METHOD__,
                                                // Approximate write-on-the-fly BagOStuff API via blocking.
                                                // This approximation fails if a ROLLBACK happens (which is rare).
                                                // We do not want to flush the TRX as that can break callers.
                                                $db->trxLevel() ? array( 'LOCK IN SHARE MODE' ) : array()
                                        );
                                        if ( $res === false ) {
                                                continue;
                                        }
                                        foreach ( $res as $row ) {
                                                $row->serverIndex = $serverIndex;
                                                $row->tableName = $tableName;
                                                $dataRows[$row->keyname] = $row;
                                        }
                                }
                        } catch ( DBError $e ) {
                                $this->handleReadError( $e, $serverIndex );
                        }
                }

                foreach ( $keys as $key ) {
                        if ( isset( $dataRows[$key] ) ) { // HIT?
                                $row = $dataRows[$key];
                                $this->debug( "get: retrieved data; expiry time is " . $row->exptime );
                                try {
                                        $db = $this->getDB( $row->serverIndex );
                                        if ( $this->isExpired( $db, $row->exptime ) ) { // MISS
                                                $this->debug( "get: key has expired, deleting" );
                                                # Put the expiry time in the WHERE condition to avoid deleting a
                                                # newly-inserted value
                                                $db->delete( $row->tableName,
                                                        array( 'keyname' => $key, 'exptime' => $row->exptime ),
                                                        __METHOD__ );
                                        } else { // HIT
                                                $values[$key] = $this->unserialize( $db->decodeBlob( $row->value ) );
                                        }
                                } catch ( DBQueryError $e ) {
                                        $this->handleWriteError( $e, $row->serverIndex );
                                }
                        } else { // MISS
                                $this->debug( 'get: no matching rows' );
                        }
                }

                return $values;
        }

        /**
         * @param array $data
         * @param int $expiry
         * @return bool
         */
        public function setMulti( array $data, $expiry = 0 ) {
                $keysByTable = array();
                foreach ( $data as $key => $value ) {
                        list( $serverIndex, $tableName ) = $this->getTableByKey( $key );
                        $keysByTable[$serverIndex][$tableName][] = $key;
                }

                $this->garbageCollect(); // expire old entries if any

                $result = true;
                $exptime = (int)$expiry;
                foreach ( $keysByTable as $serverIndex => $serverKeys ) {
                        try {
                                $db = $this->getDB( $serverIndex );
                        } catch ( DBError $e ) {
                                $this->handleWriteError( $e, $serverIndex );
                                $result = false;
                                continue;
                        }

                        if ( $exptime < 0 ) {
                                $exptime = 0;
                        }

                        if ( $exptime == 0 ) {
                                $encExpiry = $this->getMaxDateTime( $db );
                        } else {
                                $exptime = $this->convertExpiry( $exptime );
                                $encExpiry = $db->timestamp( $exptime );
                        }
                        foreach ( $serverKeys as $tableName => $tableKeys ) {
                                $rows = array();
                                foreach ( $tableKeys as $key ) {
                                        $rows[] = array(
                                                'keyname' => $key,
                                                'value' => $db->encodeBlob( $this->serialize( $data[$key] ) ),
                                                'exptime' => $encExpiry,
                                        );
                                }

                                try {
                                        $db->replace(
                                                $tableName,
                                                array( 'keyname' ),
                                                $rows,
                                                __METHOD__
                                        );
                                } catch ( DBError $e ) {
                                        $this->handleWriteError( $e, $serverIndex );
                                        $result = false;
                                }

                        }

                }

                return $result;
        }



        /**
         * @param string $key
         * @param mixed $value
         * @param int $exptime
         * @return bool
         */
        public function set( $key, $value, $exptime = 0 ) {
                list( $serverIndex, $tableName ) = $this->getTableByKey( $key );
                try {
                        $db = $this->getDB( $serverIndex );
                        $exptime = intval( $exptime );

                        if ( $exptime < 0 ) {
                                $exptime = 0;
                        }

                        if ( $exptime == 0 ) {
                                $encExpiry = $this->getMaxDateTime( $db );
                        } else {
                                $exptime = $this->convertExpiry( $exptime );
                                $encExpiry = $db->timestamp( $exptime );
                        }
                        // (bug 24425) use a replace if the db supports it instead of
                        // delete/insert to avoid clashes with conflicting keynames
                        $db->replace(
                                $tableName,
                                array( 'keyname' ),
                                array(
                                        'keyname' => $key,
                                        'value' => $db->encodeBlob( $this->serialize( $value ) ),
                                        'exptime' => $encExpiry
                                ), __METHOD__ );
                } catch ( DBError $e ) {
                        $this->handleWriteError( $e, $serverIndex );
                        return false;
                }

                return true;
        }

        /**
         * @param mixed $casToken
         * @param string $key
         * @param mixed $value
         * @param int $exptime
         * @return bool
         */
        protected function cas( $casToken, $key, $value, $exptime = 0 ) {
                list( $serverIndex, $tableName ) = $this->getTableByKey( $key );
                try {
                        $db = $this->getDB( $serverIndex );
                        $exptime = intval( $exptime );

                        if ( $exptime < 0 ) {
                                $exptime = 0;
                        }

                        if ( $exptime == 0 ) {
                                $encExpiry = $this->getMaxDateTime( $db );
                        } else {
                                $exptime = $this->convertExpiry( $exptime );
                                $encExpiry = $db->timestamp( $exptime );
                        }
                        // (bug 24425) use a replace if the db supports it instead of
                        // delete/insert to avoid clashes with conflicting keynames
                        $db->update(
                                $tableName,
                                array(
                                        'keyname' => $key,
                                        'value' => $db->encodeBlob( $this->serialize( $value ) ),
                                        'exptime' => $encExpiry
                                ),
                                array(
                                        'keyname' => $key,
                                        'value' => $db->encodeBlob( $this->serialize( $casToken ) )
                                ),
                                __METHOD__
                        );
                } catch ( DBQueryError $e ) {
                        $this->handleWriteError( $e, $serverIndex );

                        return false;
                }

                return (bool)$db->affectedRows();
        }

        /**
         * @param string $key
         * @return bool
         */
        public function delete( $key ) {
                list( $serverIndex, $tableName ) = $this->getTableByKey( $key );
                try {
                        $db = $this->getDB( $serverIndex );
                        $db->delete(
                                $tableName,
                                array( 'keyname' => $key ),
                                __METHOD__ );
                } catch ( DBError $e ) {
                        $this->handleWriteError( $e, $serverIndex );
                        return false;
                }

                return true;
        }

        /**
         * @param string $key
         * @param int $step
         * @return int|null
         */
        public function incr( $key, $step = 1 ) {
                list( $serverIndex, $tableName ) = $this->getTableByKey( $key );
                try {
                        $db = $this->getDB( $serverIndex );
                        $step = intval( $step );
                        $row = $db->selectRow(
                                $tableName,
                                array( 'value', 'exptime' ),
                                array( 'keyname' => $key ),
                                __METHOD__,
                                array( 'FOR UPDATE' ) );
                        if ( $row === false ) {
                                // Missing

                                return null;
                        }
                        $db->delete( $tableName, array( 'keyname' => $key ), __METHOD__ );
                        if ( $this->isExpired( $db, $row->exptime ) ) {
                                // Expired, do not reinsert

                                return null;
                        }

                        $oldValue = intval( $this->unserialize( $db->decodeBlob( $row->value ) ) );
                        $newValue = $oldValue + $step;
                        $db->insert( $tableName,
                                array(
                                        'keyname' => $key,
                                        'value' => $db->encodeBlob( $this->serialize( $newValue ) ),
                                        'exptime' => $row->exptime
                                ), __METHOD__, 'IGNORE' );

                        if ( $db->affectedRows() == 0 ) {
                                // Race condition. See bug 28611
                                $newValue = null;
                        }
                } catch ( DBError $e ) {
                        $this->handleWriteError( $e, $serverIndex );
                        return null;
                }

                return $newValue;
        }

        public function merge( $key, $callback, $exptime = 0, $attempts = 10 ) {
                if ( !is_callable( $callback ) ) {
                        throw new Exception( "Got invalid callback." );
                }

                return $this->mergeViaCas( $key, $callback, $exptime, $attempts );
        }

        /**
         * @param DatabaseBase $db
         * @param string $exptime
         * @return bool
         */
        protected function isExpired( $db, $exptime ) {
                return $exptime != $this->getMaxDateTime( $db ) && wfTimestamp( TS_UNIX, $exptime ) < time();
        }

        /**
         * @param DatabaseBase $db
         * @return string
         */
        protected function getMaxDateTime( $db ) {
                if ( time() > 0x7fffffff ) {
                        return $db->timestamp( 1 << 62 );
                } else {
                        return $db->timestamp( 0x7fffffff );
                }
        }

        protected function garbageCollect() {
                if ( !$this->purgePeriod ) {
                        // Disabled
                        return;
                }
                // Only purge on one in every $this->purgePeriod requests.
                if ( $this->purgePeriod !== 1 && mt_rand( 0, $this->purgePeriod - 1 ) ) {
                        return;
                }
                $now = time();
                // Avoid repeating the delete within a few seconds
                if ( $now > ( $this->lastExpireAll + 1 ) ) {
                        $this->lastExpireAll = $now;
                        $this->expireAll();
                }
        }

        public function expireAll() {
                $this->deleteObjectsExpiringBefore( wfTimestampNow() );
        }

        /**
         * Delete objects from the database which expire before a certain date.
         * @param string $timestamp
         * @param bool|callable $progressCallback
         * @return bool
         */
        public function deleteObjectsExpiringBefore( $timestamp, $progressCallback = false ) {
                for ( $serverIndex = 0; $serverIndex < $this->numServers; $serverIndex++ ) {
                        try {
                                $db = $this->getDB( $serverIndex );
                                $dbTimestamp = $db->timestamp( $timestamp );
                                $totalSeconds = false;
                                $baseConds = array( 'exptime < ' . $db->addQuotes( $dbTimestamp ) );
                                for ( $i = 0; $i < $this->shards; $i++ ) {
                                        $maxExpTime = false;
                                        while ( true ) {
                                                $conds = $baseConds;
                                                if ( $maxExpTime !== false ) {
                                                        $conds[] = 'exptime > ' . $db->addQuotes( $maxExpTime );
                                                }
                                                $rows = $db->select(
                                                        $this->getTableNameByShard( $i ),
                                                        array( 'keyname', 'exptime' ),
                                                        $conds,
                                                        __METHOD__,
                                                        array( 'LIMIT' => 100, 'ORDER BY' => 'exptime' ) );
                                                if ( $rows === false || !$rows->numRows() ) {
                                                        break;
                                                }
                                                $keys = array();
                                                $row = $rows->current();
                                                $minExpTime = $row->exptime;
                                                if ( $totalSeconds === false ) {
                                                        $totalSeconds = wfTimestamp( TS_UNIX, $timestamp )
                                                                - wfTimestamp( TS_UNIX, $minExpTime );
                                                }
                                                foreach ( $rows as $row ) {
                                                        $keys[] = $row->keyname;
                                                        $maxExpTime = $row->exptime;
                                                }

                                                $db->delete(
                                                        $this->getTableNameByShard( $i ),
                                                        array(
                                                                'exptime >= ' . $db->addQuotes( $minExpTime ),
                                                                'exptime < ' . $db->addQuotes( $dbTimestamp ),
                                                                'keyname' => $keys
                                                        ),
                                                        __METHOD__ );

                                                if ( $progressCallback ) {
                                                        if ( intval( $totalSeconds ) === 0 ) {
                                                                $percent = 0;
                                                        } else {
                                                                $remainingSeconds = wfTimestamp( TS_UNIX, $timestamp )
                                                                        - wfTimestamp( TS_UNIX, $maxExpTime );
                                                                if ( $remainingSeconds > $totalSeconds ) {
                                                                        $totalSeconds = $remainingSeconds;
                                                                }
                                                                $processedSeconds = $totalSeconds - $remainingSeconds;
                                                                $percent = ( $i + $processedSeconds / $totalSeconds )
                                                                        / $this->shards * 100;
                                                        }
                                                        $percent = ( $percent / $this->numServers )
                                                                + ( $serverIndex / $this->numServers * 100 );
                                                        call_user_func( $progressCallback, $percent );
                                                }
                                        }
                                }
                        } catch ( DBError $e ) {
                                $this->handleWriteError( $e, $serverIndex );
                                return false;
                        }
                }
                return true;
        }

        /**
         * Delete content of shard tables in every server.
         * Return true if the operation is successful, false otherwise.
         * @return bool
         */
        public function deleteAll() {
                for ( $serverIndex = 0; $serverIndex < $this->numServers; $serverIndex++ ) {
                        try {
                                $db = $this->getDB( $serverIndex );
                                for ( $i = 0; $i < $this->shards; $i++ ) {
                                        $db->delete( $this->getTableNameByShard( $i ), '*', __METHOD__ );
                                }
                        } catch ( DBError $e ) {
                                $this->handleWriteError( $e, $serverIndex );
                                return false;
                        }
                }
                return true;
        }

        /**
         * Serialize an object and, if possible, compress the representation.
         * On typical message and page data, this can provide a 3X decrease
         * in storage requirements.
         *
         * @param mixed $data
         * @return string
         */
        protected function serialize( &$data ) {
                $serial = serialize( $data );

                if ( function_exists( 'gzdeflate' ) ) {
                        return gzdeflate( $serial );
                } else {
                        return $serial;
                }
        }

        /**
         * Unserialize and, if necessary, decompress an object.
         * @param string $serial
         * @return mixed
         */
        protected function unserialize( $serial ) {
                if ( function_exists( 'gzinflate' ) ) {
                        wfSuppressWarnings();
                        $decomp = gzinflate( $serial );
                        wfRestoreWarnings();

                        if ( false !== $decomp ) {
                                $serial = $decomp;
                        }
                }

                $ret = unserialize( $serial );

                return $ret;
        }

        /**
         * Handle a DBError which occurred during a read operation.
         *
         * @param DBError $exception
         * @param int $serverIndex
         */
        protected function handleReadError( DBError $exception, $serverIndex ) {
                if ( $exception instanceof DBConnectionError ) {
                        $this->markServerDown( $exception, $serverIndex );
                }
                $this->logger->error( "DBError: {$exception->getMessage()}" );
                if ( $exception instanceof DBConnectionError ) {
                        $this->setLastError( BagOStuff::ERR_UNREACHABLE );
                        $this->logger->debug( __METHOD__ . ": ignoring connection error" );
                } else {
                        $this->setLastError( BagOStuff::ERR_UNEXPECTED );
                        $this->logger->debug( __METHOD__ . ": ignoring query error" );
                }
        }

        /**
         * Handle a DBQueryError which occurred during a write operation.
         *
         * @param DBError $exception
         * @param int $serverIndex
         */
        protected function handleWriteError( DBError $exception, $serverIndex ) {
                if ( $exception instanceof DBConnectionError ) {
                        $this->markServerDown( $exception, $serverIndex );
                }
                if ( $exception->db && $exception->db->wasReadOnlyError() ) {
                        if ( $exception->db->trxLevel() ) {
                                try {
                                        $exception->db->rollback( __METHOD__ );
                                } catch ( DBError $e ) {
                                }
                        }
                }

                $this->logger->error( "DBError: {$exception->getMessage()}" );
                if ( $exception instanceof DBConnectionError ) {
                        $this->setLastError( BagOStuff::ERR_UNREACHABLE );
                        $this->logger->debug( __METHOD__ . ": ignoring connection error" );
                } else {
                        $this->setLastError( BagOStuff::ERR_UNEXPECTED );
                        $this->logger->debug( __METHOD__ . ": ignoring query error" );
                }
        }

        /**
         * Mark a server down due to a DBConnectionError exception
         *
         * @param DBError $exception
         * @param int $serverIndex
         */
        protected function markServerDown( $exception, $serverIndex ) {
                if ( isset( $this->connFailureTimes[$serverIndex] ) ) {
                        if ( time() - $this->connFailureTimes[$serverIndex] >= 60 ) {
                                unset( $this->connFailureTimes[$serverIndex] );
                                unset( $this->connFailureErrors[$serverIndex] );
                        } else {
                                $this->logger->debug( __METHOD__ . ": Server #$serverIndex already down" );
                                return;
                        }
                }
                $now = time();
                $this->logger->info( __METHOD__ . ": Server #$serverIndex down until " . ( $now + 60 ) );
                $this->connFailureTimes[$serverIndex] = $now;
                $this->connFailureErrors[$serverIndex] = $exception;
        }

        /**
         * Create shard tables. For use from eval.php.
         */
        public function createTables() {
                for ( $serverIndex = 0; $serverIndex < $this->numServers; $serverIndex++ ) {
                        $db = $this->getDB( $serverIndex );
                        if ( $db->getType() !== 'mysql' ) {
                                throw new MWException( __METHOD__ . ' is not supported on this DB server' );
                        }

                        for ( $i = 0; $i < $this->shards; $i++ ) {
                                $db->query(
                                        'CREATE TABLE ' . $db->tableName( $this->getTableNameByShard( $i ) ) .
                                        ' LIKE ' . $db->tableName( 'objectcache' ),
                                        __METHOD__ );
                        }
                }
        }
}