[JobQueue] Optimized redis queue to use Lua scripts.

[mediawiki.git] / includes / job / JobQueueRedis.php

<?php
/**
 * Redis-backed job queue code.
 *
 * 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
 * @author Aaron Schulz
 */

/**
 * Class to handle job queues stored in Redis
 *
 * This is faster, less resource intensive, queue that JobQueueDB.
 * All data for a queue using this class is placed into one redis server.
 *
 * There are seven main redis keys used to track jobs:
 *   - l-unclaimed  : A list of job IDs used for push/pop
 *   - z-claimed    : A sorted set of (job ID, UNIX timestamp as score) used for job retries
 *   - z-abandoned  : A sorted set of (job ID, UNIX timestamp as score) used for broken jobs
 *   - h-idBySha1   : A hash of (SHA1 => job ID) for unclaimed jobs used for de-duplication
 *   - h-sha1Byid   : A hash of (job ID => SHA1) for unclaimed jobs used for de-duplication
 *   - h-attempts   : A hash of (job ID => attempt count) used for job claiming/retries
 *   - h-data       : A hash of (job ID => serialized blobs) for job storage
 * Any given job ID can be in only one of l-unclaimed, z-claimed, and z-abandoned.
 * If an ID appears in any of those lists, it should have a h-data entry for its ID.
 * If a job has a non-empty SHA1 de-duplication value and its ID is in l-unclaimed,
 * then there should be no other such jobs. Every h-idBySha1 entry has an h-sha1Byid
 * entry and every h-sha1Byid must refer to an ID that is l-unclaimed. If a job has its
 * ID in z-claimed or z-abandoned, then it must also have an h-attempts entry for its ID.
 *
 * Additionally, "rootjob:* keys to track "root jobs" used for additional de-duplication.
 * Aside from root job keys, all keys have no expiry, and are only removed when jobs are run.
 * All the keys are prefixed with the relevant wiki ID information.
 *
 * This class requires Redis 2.6 as it makes use Lua scripts for fast atomic operations.
 * Additionally, it should be noted that redis has different persistence modes, such
 * as rdb snapshots, journaling, and no persistent. Appropriate configuration should be
 * made on the servers based on what queues are using it and what tolerance they have.
 *
 * @ingroup JobQueue
 * @since 1.21
 */
class JobQueueRedis extends JobQueue {
        /** @var RedisConnectionPool */
        protected $redisPool;

        protected $server; // string; server address

        const ROOTJOB_TTL = 1209600; // integer; seconds to remember root jobs (14 days)
        const MAX_AGE_PRUNE = 604800; // integer; seconds a job can live once claimed (7 days)

        protected $key; // string; key to prefix the queue keys with (used for testing)

        /**
         * @params include:
         *   - redisConfig : An array of parameters to RedisConnectionPool::__construct().
         *                   Note that the serializer option is ignored "none" is always used.
         *   - redisServer : A hostname/port combination or the absolute path of a UNIX socket.
         *                   If a hostname is specified but no port, the standard port number
         *                   6379 will be used. Required.
         * @param array $params
         */
        public function __construct( array $params ) {
                parent::__construct( $params );
                $params['redisConfig']['serializer'] = 'none'; // make it easy to use Lua
                $this->server = $params['redisServer'];
                $this->redisPool = RedisConnectionPool::singleton( $params['redisConfig'] );
        }

        protected function supportedOrders() {
                return array( 'timestamp', 'fifo' );
        }

        protected function optimalOrder() {
                return 'fifo';
        }

        /**
         * @see JobQueue::doIsEmpty()
         * @return bool
         * @throws MWException
         */
        protected function doIsEmpty() {
                return $this->doGetSize() == 0;
        }

        /**
         * @see JobQueue::doGetSize()
         * @return integer
         * @throws MWException
         */
        protected function doGetSize() {
                $conn = $this->getConnection();
                try {
                        return $conn->lSize( $this->getQueueKey( 'l-unclaimed' ) );
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $this->server, $conn, $e );
                }
        }

        /**
         * @see JobQueue::doGetAcquiredCount()
         * @return integer
         * @throws MWException
         */
        protected function doGetAcquiredCount() {
                if ( $this->claimTTL <= 0 ) {
                        return 0; // no acknowledgements
                }
                $conn = $this->getConnection();
                try {
                        $conn->multi( Redis::PIPELINE );
                        $conn->zSize( $this->getQueueKey( 'z-claimed' ) );
                        $conn->zSize( $this->getQueueKey( 'z-abandoned' ) );
                        return array_sum( $conn->exec() );
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $this->server, $conn, $e );
                }
        }

        /**
         * @see JobQueue::doBatchPush()
         * @param array $jobs
         * @param $flags
         * @return bool
         * @throws MWException
         */
        protected function doBatchPush( array $jobs, $flags ) {
                // Convert the jobs into field maps (de-duplicated against each other)
                $items = array(); // (job ID => job fields map)
                foreach ( $jobs as $job ) {
                        $item = $this->getNewJobFields( $job );
                        if ( strlen( $item['sha1'] ) ) { // hash identifier => de-duplicate
                                $items[$item['sha1']] = $item;
                        } else {
                                $items[$item['uuid']] = $item;
                        }
                }
                // Convert the field maps into serialized blobs
                $tuples = array();
                foreach ( $items as $item ) {
                        $tuples[] = array( $item['uuid'], $item['sha1'], serialize( $item ) );
                }

                if ( !count( $tuples ) ) {
                        return true; // nothing to do
                }

                $conn = $this->getConnection();
                try {
                        // Actually push the non-duplicate jobs into the queue...
                        if ( $flags & self::QoS_Atomic ) {
                                $batches = array( $tuples ); // all or nothing
                        } else {
                                $batches = array_chunk( $tuples, 500 ); // avoid tying up the server
                        }
                        $failed = 0;
                        $pushed = 0;
                        foreach ( $batches as $tupleBatch ) {
                                $added = $this->pushBlobs( $conn, $tupleBatch );
                                if ( is_int( $added ) ) {
                                        $pushed += $added;
                                } else {
                                        $failed += count( $tupleBatch );
                                }
                        }
                        if ( $failed > 0 ) {
                                wfDebugLog( 'JobQueueRedis', "Could not insert {$failed} {$this->type} job(s)." );
                                return false;
                        }
                        wfIncrStats( 'job-insert', count( $tuples ) );
                        wfIncrStats( 'job-insert-duplicate', count( $tuples ) - $failed - $pushed );
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $this->server, $conn, $e );
                }

                return true;
        }

        /**
         * @param RedisConnRef $conn
         * @param array $tuples List of tuples of (job ID, job SHA1 or '', serialized blob)
         * @return integer Number of jobs inserted (duplicates are ignored)
         * @throws RedisException
         */
        protected function pushBlobs( RedisConnRef $conn, array $tuples ) {
                $args = array(); // ([id, sha1, blob [, id, sha1, blob ... ] ] )
                foreach ( $tuples as $tuple ) {
                        $args[] = $tuple[0]; // id
                        $args[] = $tuple[1]; // sha1
                        $args[] = $tuple[2]; // blob
                }
                static $script =
<<<LUA
                if #ARGV % 3 ~= 0 then return redis.error_reply('Unmatched arguments') end
                local pushed = 0
                for i = 1,#ARGV,3 do
                        local id,sha1,blob = ARGV[i],ARGV[i+1],ARGV[i+2]
                        if sha1 == '' or redis.call('hExists',KEYS[3],sha1) == 0 then
                                redis.call('lPush',KEYS[1],id)
                                if sha1 ~= '' then
                                        redis.call('hSet',KEYS[2],id,sha1)
                                        redis.call('hSet',KEYS[3],sha1,id)
                                end
                                redis.call('hSet',KEYS[4],id,blob)
                                pushed = pushed + 1
                        end
                end
                return pushed
LUA;
                return $this->redisEval( $conn, $script,
                        array_merge(
                                array(
                                        $this->getQueueKey( 'l-unclaimed' ), # KEYS[1]
                                        $this->getQueueKey( 'h-sha1ById' ), # KEYS[2]
                                        $this->getQueueKey( 'h-idBySha1' ), # KEYS[3]
                                        $this->getQueueKey( 'h-data' ), # KEYS[4]
                                ),
                                $args
                        ),
                        4 # number of first argument(s) that are keys
                );
        }

        /**
         * @see JobQueue::doPop()
         * @return Job|bool
         * @throws MWException
         */
        protected function doPop() {
                $job = false;

                $conn = $this->getConnection();
                try {
                        do {
                                if ( $this->claimTTL > 0 ) {
                                        // Keep the claimed job list down for high-traffic queues
                                        if ( mt_rand( 0, 99 ) == 0 ) {
                                                $this->recycleAndDeleteStaleJobs();
                                        }
                                        $blob = $this->popAndAcquireBlob( $conn );
                                } else {
                                        $blob = $this->popAndDeleteBlob( $conn );
                                }
                                if ( $blob === false ) {
                                        break; // no jobs; nothing to do
                                }

                                wfIncrStats( 'job-pop' );
                                $item = unserialize( $blob );
                                if ( $item === false ) {
                                        wfDebugLog( 'JobQueueRedis', "Could not unserialize {$this->type} job." );
                                        continue;
                                }

                                // If $item is invalid, recycleAndDeleteStaleJobs() will cleanup as needed
                                $job = $this->getJobFromFields( $item ); // may be false
                        } while ( !$job ); // job may be false if invalid
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $this->server, $conn, $e );
                }

                // Flag this job as an old duplicate based on its "root" job...
                try {
                        if ( $job && $this->isRootJobOldDuplicate( $job ) ) {
                                wfIncrStats( 'job-pop-duplicate' );
                                return DuplicateJob::newFromJob( $job ); // convert to a no-op
                        }
                } catch ( MWException $e ) {} // don't lose jobs over this

                return $job;
        }

        /**
         * @param RedisConnRef $conn
         * @return array serialized string or false
         * @throws RedisException
         */
        protected function popAndDeleteBlob( RedisConnRef $conn ) {
                static $script =
<<<LUA
                -- Pop an item off the queue
                local id = redis.call('rpop',KEYS[1])
                if not id then return false end
                -- Get the job data and remove it
                local item = redis.call('hGet',KEYS[4],id)
                redis.call('hDel',KEYS[4],id)
                -- Allow new duplicates of this job
                local sha1 = redis.call('hGet',KEYS[2],id)
                if sha1 then redis.call('hDel',KEYS[3],sha1) end
                redis.call('hDel',KEYS[2],id)
                -- Return the job data
                return item
LUA;
                return $this->redisEval( $conn, $script,
                        array(
                                $this->getQueueKey( 'l-unclaimed' ), # KEYS[1]
                                $this->getQueueKey( 'h-sha1ById' ), # KEYS[2]
                                $this->getQueueKey( 'h-idBySha1' ), # KEYS[3]
                                $this->getQueueKey( 'h-data' ), # KEYS[4]
                        ),
                        4 # number of first argument(s) that are keys
                );
        }

        /**
         * @param RedisConnRef $conn
         * @return array serialized string or false
         * @throws RedisException
         */
        protected function popAndAcquireBlob( RedisConnRef $conn ) {
                static $script =
<<<LUA
                -- Pop an item off the queue
                local id = redis.call('rPop',KEYS[1])
                if not id then return false end
                -- Allow new duplicates of this job
                local sha1 = redis.call('hGet',KEYS[2],id)
                if sha1 then redis.call('hDel',KEYS[3],sha1) end
                redis.call('hDel',KEYS[2],id)
                -- Mark the jobs as claimed and return it
                redis.call('zAdd',KEYS[4],ARGV[1],id)
                redis.call('hIncrBy',KEYS[5],id,1)
                return redis.call('hGet',KEYS[6],id)
LUA;
                return $this->redisEval( $conn, $script,
                        array(
                                $this->getQueueKey( 'l-unclaimed' ), # KEYS[1]
                                $this->getQueueKey( 'h-sha1ById' ), # KEYS[2]
                                $this->getQueueKey( 'h-idBySha1' ), # KEYS[3]
                                $this->getQueueKey( 'z-claimed' ), # KEYS[4]
                                $this->getQueueKey( 'h-attempts' ), # KEYS[5]
                                $this->getQueueKey( 'h-data' ), # KEYS[6]
                                time(), # ARGV[1] (injected to be replication-safe)
                        ),
                        6 # number of first argument(s) that are keys
                );
        }

        /**
         * @see JobQueue::doAck()
         * @param Job $job
         * @return Job|bool
         * @throws MWException
         */
        protected function doAck( Job $job ) {
                if ( $this->claimTTL > 0 ) {
                        $conn = $this->getConnection();
                        try {
                                // Get the exact field map this Job came from, regardless of whether
                                // the job was transformed into a DuplicateJob or anything of the sort.
                                $item = $job->metadata['sourceFields'];

                                static $script =
<<<LUA
                                -- Unmark the job as claimed
                                redis.call('zRem',KEYS[1],ARGV[1])
                                redis.call('hDel',KEYS[2],ARGV[1])
                                -- Delete the job data itself
                                return redis.call('hDel',KEYS[3],ARGV[1])
LUA;
                                $res = $this->redisEval( $conn, $script,
                                        array(
                                                $this->getQueueKey( 'z-claimed' ), # KEYS[1]
                                                $this->getQueueKey( 'h-attempts' ), # KEYS[2]
                                                $this->getQueueKey( 'h-data' ), # KEYS[3]
                                                $item['uuid'] # ARGV[1]
                                        ),
                                        3 # number of first argument(s) that are keys
                                );

                                if ( !$res ) {
                                        wfDebugLog( 'JobQueueRedis', "Could not acknowledge {$this->type} job." );
                                        return false;
                                }
                        } catch ( RedisException $e ) {
                                $this->throwRedisException( $this->server, $conn, $e );
                        }
                }
                return true;
        }

        /**
         * @see JobQueue::doDeduplicateRootJob()
         * @param Job $job
         * @return bool
         * @throws MWException
         */
        protected function doDeduplicateRootJob( Job $job ) {
                $params = $job->getParams();
                if ( !isset( $params['rootJobSignature'] ) ) {
                        throw new MWException( "Cannot register root job; missing 'rootJobSignature'." );
                } elseif ( !isset( $params['rootJobTimestamp'] ) ) {
                        throw new MWException( "Cannot register root job; missing 'rootJobTimestamp'." );
                }
                $key = $this->getRootJobKey( $params['rootJobSignature'] );

                $conn = $this->getConnection();
                try {
                        $timestamp = $conn->get( $key ); // current last timestamp of this job
                        if ( $timestamp && $timestamp >= $params['rootJobTimestamp'] ) {
                                return true; // a newer version of this root job was enqueued
                        }
                        // Update the timestamp of the last root job started at the location...
                        return $conn->set( $key, $params['rootJobTimestamp'], self::ROOTJOB_TTL ); // 2 weeks
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $this->server, $conn, $e );
                }
        }

        /**
         * Check if the "root" job of a given job has been superseded by a newer one
         *
         * @param $job Job
         * @return bool
         * @throws MWException
         */
        protected function isRootJobOldDuplicate( Job $job ) {
                $params = $job->getParams();
                if ( !isset( $params['rootJobSignature'] ) ) {
                        return false; // job has no de-deplication info
                } elseif ( !isset( $params['rootJobTimestamp'] ) ) {
                        wfDebugLog( 'JobQueueRedis', "Cannot check root job; missing 'rootJobTimestamp'." );
                        return false;
                }

                $conn = $this->getConnection();
                try {
                        // Get the last time this root job was enqueued
                        $timestamp = $conn->get( $this->getRootJobKey( $params['rootJobSignature'] ) );
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $this->server, $conn, $e );
                }

                // Check if a new root job was started at the location after this one's...
                return ( $timestamp && $timestamp > $params['rootJobTimestamp'] );
        }

        /**
         * @see JobQueue::getAllQueuedJobs()
         * @return Iterator
         */
        public function getAllQueuedJobs() {
                $conn = $this->getConnection();
                if ( !$conn ) {
                        throw new MWException( "Unable to connect to redis server." );
                }
                try {
                        $that = $this;
                        return new MappedIterator(
                                $conn->lRange( $this->getQueueKey( 'l-unclaimed' ), 0, -1 ),
                                function( $uid ) use ( $that, $conn ) {
                                        return $that->getJobFromUidInternal( $uid, $conn );
                                }
                        );
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $this->server, $conn, $e );
                }
        }

        /**
         * This function should not be called outside RedisJobQueue
         *
         * @param $uid string
         * @param $conn RedisConnRef
         * @return Job
         * @throws MWException
         */
        public function getJobFromUidInternal( $uid, RedisConnRef $conn ) {
                try {
                        $item = unserialize( $conn->hGet( $this->getQueueKey( 'h-data' ), $uid ) );
                        if ( !is_array( $item ) ) { // this shouldn't happen
                                throw new MWException( "Could not find job with ID '$uid'." );
                        }
                        $title = Title::makeTitle( $item['namespace'], $item['title'] );
                        $job = Job::factory( $item['type'], $title, $item['params'] );
                        $job->metadata['sourceFields'] = $item;
                        return $job;
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $this->server, $conn, $e );
                }
        }

        /**
         * Recycle or destroy any jobs that have been claimed for too long
         *
         * @return integer Number of jobs recycled/deleted
         * @throws MWException
         */
        public function recycleAndDeleteStaleJobs() {
                if ( $this->claimTTL <= 0 ) { // sanity
                        throw new MWException( "Cannot recycle jobs since acknowledgements are disabled." );
                }
                $count = 0;
                // For each job item that can be retried, we need to add it back to the
                // main queue and remove it from the list of currenty claimed job items.
                // For those that cannot, they are marked as dead and kept around for
                // investigation and manual job restoration but are eventually deleted.
                $conn = $this->getConnection();
                try {
                        $now = time();
                        static $script =
<<<LUA
                        local released,abandoned,pruned = 0,0,0
                        -- Get all non-dead jobs that have an expired claim on them.
                        -- The score for each item is the last claim timestamp (UNIX).
                        local staleClaims = redis.call('zRangeByScore',KEYS[1],0,ARGV[1],'WITHSCORES')
                        for id,timestamp in ipairs(staleClaims) do
                                local attempts = redis.call('hGet',KEYS[2],id)
                                if attempts < ARGV[3] then
                                        -- Claim expired and retries left: re-enqueue the job
                                        redis.call('lPush',KEYS[3],id)
                                        redis.call('hIncrBy',KEYS[2],id,1)
                                        released = released + 1
                                else
                                        -- Claim expired and no retries left: mark the job as dead
                                        redis.call('zAdd',KEYS[5],timestamp,id)
                                        abandoned = abandoned + 1
                                end
                                redis.call('zRem',KEYS[1],id)
                        end
                        -- Get all of the dead jobs that have been marked as dead for too long.
                        -- The score for each item is the last claim timestamp (UNIX).
                        local deadClaims = redis.call('zRangeByScore',KEYS[5],0,ARGV[2],'WITHSCORES')
                        for id,timestamp in ipairs(deadClaims) do
                                -- Stale and out of retries: remove any traces of the job
                                redis.call('zRem',KEYS[5],id)
                                redis.call('hDel',KEYS[2],id)
                                redis.call('hDel',KEYS[4],id)
                                pruned = pruned + 1
                        end
                        return {released,abandoned,pruned}
LUA;
                        $res = $this->redisEval( $conn, $script,
                                array(
                                        $this->getQueueKey( 'z-claimed' ), # KEYS[1]
                                        $this->getQueueKey( 'h-attempts' ), # KEYS[2]
                                        $this->getQueueKey( 'l-unclaimed' ), # KEYS[3]
                                        $this->getQueueKey( 'h-data' ), # KEYS[4]
                                        $this->getQueueKey( 'z-abandoned' ), # KEYS[5]
                                        $now - $this->claimTTL, # ARGV[1]
                                        $now - self::MAX_AGE_PRUNE, # ARGV[2]
                                        $this->maxTries # ARGV[3]
                                ),
                                5 # number of first argument(s) that are keys
                        );
                        if ( $res ) {
                                list( $released, $abandoned, $pruned ) = $res;
                                $count += $released + $pruned;
                                wfIncrStats( 'job-recycle', count( $released ) );
                        }
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $this->server, $conn, $e );
                }

                return $count;
        }

        /**
         * @return Array
         */
        protected function doGetPeriodicTasks() {
                if ( $this->claimTTL > 0 ) {
                        return array(
                                'recycleAndDeleteStaleJobs' => array(
                                        'callback' => array( $this, 'recycleAndDeleteStaleJobs' ),
                                        'period'   => ceil( $this->claimTTL / 2 )
                                )
                        );
                } else {
                        return array();
                }
        }

        /**
         * @param RedisConnRef $conn
         * @param string $script
         * @param array $params
         * @param integer $numKeys
         * @return mixed
         */
        protected function redisEval( RedisConnRef $conn, $script, array $params, $numKeys ) {
                $res = $conn->evalSha( sha1( $script ), $params, $numKeys );
                if ( $res === false && $conn->getLastError() != '' ) { // not in script cache?
                        wfDebugLog( 'JobQueueRedis', "Lua script error: " . $conn->getLastError() );
                        $res = $conn->eval( $script, $params, $numKeys );
                }
                return $res;
        }

        /**
         * @param $job Job
         * @return array
         */
        protected function getNewJobFields( Job $job ) {
                return array(
                        // Fields that describe the nature of the job
                        'type'      => $job->getType(),
                        'namespace' => $job->getTitle()->getNamespace(),
                        'title'     => $job->getTitle()->getDBkey(),
                        'params'    => $job->getParams(),
                        // Additional job metadata
                        'uuid'      => UIDGenerator::newRawUUIDv4( UIDGenerator::QUICK_RAND ),
                        'sha1'      => $job->ignoreDuplicates()
                                ? wfBaseConvert( sha1( serialize( $job->getDeduplicationInfo() ) ), 16, 36, 31 )
                                : '',
                        'timestamp' => time() // UNIX timestamp
                );
        }

        /**
         * @param $fields array
         * @return Job|bool
         */
        protected function getJobFromFields( array $fields ) {
                $title = Title::makeTitleSafe( $fields['namespace'], $fields['title'] );
                if ( $title ) {
                        $job = Job::factory( $fields['type'], $title, $fields['params'] );
                        $job->metadata['sourceFields'] = $fields;
                        return $job;
                }
                return false;
        }

        /**
         * Get a connection to the server that handles all sub-queues for this queue
         *
         * @return Array (server name, Redis instance)
         * @throws MWException
         */
        protected function getConnection() {
                $conn = $this->redisPool->getConnection( $this->server );
                if ( !$conn ) {
                        throw new MWException( "Unable to connect to redis server." );
                }
                return $conn;
        }

        /**
         * @param $server string
         * @param $conn RedisConnRef
         * @param $e RedisException
         * @throws MWException
         */
        protected function throwRedisException( $server, RedisConnRef $conn, $e ) {
                $this->redisPool->handleException( $server, $conn, $e );
                throw new MWException( "Redis server error: {$e->getMessage()}\n" );
        }

        /**
         * @param $prop string
         * @return string
         */
        private function getQueueKey( $prop ) {
                list( $db, $prefix ) = wfSplitWikiID( $this->wiki );
                if ( strlen( $this->key ) ) { // namespaced queue (for testing)
                        return wfForeignMemcKey( $db, $prefix, 'jobqueue', $this->type, $this->key, $prop );
                } else {
                        return wfForeignMemcKey( $db, $prefix, 'jobqueue', $this->type, $prop );
                }
        }

        /**
         * @param string $signature Hash identifier of the root job
         * @return string
         */
        private function getRootJobKey( $signature ) {
                list( $db, $prefix ) = wfSplitWikiID( $this->wiki );
                return wfForeignMemcKey( $db, $prefix, 'jobqueue', $this->type, 'rootjob', $signature );
        }

        /**
         * @param $key string
         * @return void
         */
        public function setTestingPrefix( $key ) {
                $this->key = $key;
        }
}