Merge "Remove not used private member variable mParserWarnings from OutputPage"

[mediawiki.git] / includes / jobqueue / 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 a faster and less resource-intensive job queue than JobQueueDB.
 * All data for a queue using this class is placed into one redis server.
 * The mediawiki/services/jobrunner background service must be set up and running.
 *
 * There are eight main redis keys (per queue) used to track jobs:
 *   - l-unclaimed  : A list of job IDs used for ready unclaimed jobs
 *   - 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
 *   - z-delayed    : A sorted set of (job ID, UNIX timestamp as score) used for delayed 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
 * A job ID can be in only one of z-delayed, 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 SHA1 de-duplication value and its ID is in l-unclaimed or z-delayed, then
 * there should be no other such jobs with that SHA1. 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.
 *
 * The following keys are used to track queue states:
 *   - s-queuesWithJobs : A set of all queues with non-abandoned jobs
 *
 * The background service takes care of undelaying, recycling, and pruning jobs as well as
 * removing s-queuesWithJobs entries as queues empty.
 *
 * Additionally, "rootjob:* keys 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 persistence. Appropriate configuration should be
 * made on the servers based on what queues are using it and what tolerance they have.
 *
 * @ingroup JobQueue
 * @ingroup Redis
 * @since 1.22
 */
class JobQueueRedis extends JobQueue {
        /** @var RedisConnectionPool */
        protected $redisPool;

        /** @var string Server address */
        protected $server;
        /** @var string Compression method to use */
        protected $compression;

        const MAX_AGE_PRUNE = 604800; // integer; seconds a job can live once claimed (7 days)

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

        /**
         * @param array $params Possible keys:
         *   - redisConfig : An array of parameters to RedisConnectionPool::__construct().
         *                   Note that the serializer option is ignored as "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.
         *   - compression : The type of compression to use; one of (none,gzip).
         *   - daemonized  : Set to true if the redisJobRunnerService runs in the background.
         *                   This will disable job recycling/undelaying from the MediaWiki side
         *                   to avoid redundance and out-of-sync configuration.
         * @throws InvalidArgumentException
         */
        public function __construct( array $params ) {
                parent::__construct( $params );
                $params['redisConfig']['serializer'] = 'none'; // make it easy to use Lua
                $this->server = $params['redisServer'];
                $this->compression = isset( $params['compression'] ) ? $params['compression'] : 'none';
                $this->redisPool = RedisConnectionPool::singleton( $params['redisConfig'] );
                if ( empty( $params['daemonized'] ) ) {
                        throw new InvalidArgumentException(
                                "Non-daemonized mode is no longer supported. Please install the " .
                                "mediawiki/services/jobrunner service and update \$wgJobTypeConf as needed." );
                }
        }

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

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

        protected function supportsDelayedJobs() {
                return true;
        }

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

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

        /**
         * @see JobQueue::doGetAcquiredCount()
         * @return int
         * @throws JobQueueError
         */
        protected function doGetAcquiredCount() {
                $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( $conn, $e );
                }
        }

        /**
         * @see JobQueue::doGetDelayedCount()
         * @return int
         * @throws JobQueueError
         */
        protected function doGetDelayedCount() {
                $conn = $this->getConnection();
                try {
                        return $conn->zSize( $this->getQueueKey( 'z-delayed' ) );
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $conn, $e );
                }
        }

        /**
         * @see JobQueue::doGetAbandonedCount()
         * @return int
         * @throws JobQueueError
         */
        protected function doGetAbandonedCount() {
                $conn = $this->getConnection();
                try {
                        return $conn->zSize( $this->getQueueKey( 'z-abandoned' ) );
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $conn, $e );
                }
        }

        /**
         * @see JobQueue::doBatchPush()
         * @param array $jobs
         * @param int $flags
         * @return void
         * @throws JobQueueError
         */
        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;
                        }
                }

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

                $conn = $this->getConnection();
                try {
                        // Actually push the non-duplicate jobs into the queue...
                        if ( $flags & self::QOS_ATOMIC ) {
                                $batches = array( $items ); // all or nothing
                        } else {
                                $batches = array_chunk( $items, 100 ); // avoid tying up the server
                        }
                        $failed = 0;
                        $pushed = 0;
                        foreach ( $batches as $itemBatch ) {
                                $added = $this->pushBlobs( $conn, $itemBatch );
                                if ( is_int( $added ) ) {
                                        $pushed += $added;
                                } else {
                                        $failed += count( $itemBatch );
                                }
                        }
                        JobQueue::incrStats( 'inserts', $this->type, count( $items ) );
                        JobQueue::incrStats( 'inserts_actual', $this->type, $pushed );
                        JobQueue::incrStats( 'dupe_inserts', $this->type,
                                count( $items ) - $failed - $pushed );
                        if ( $failed > 0 ) {
                                $err = "Could not insert {$failed} {$this->type} job(s).";
                                wfDebugLog( 'JobQueueRedis', $err );
                                throw new RedisException( $err );
                        }
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $conn, $e );
                }
        }

        /**
         * @param RedisConnRef $conn
         * @param array $items List of results from JobQueueRedis::getNewJobFields()
         * @return int Number of jobs inserted (duplicates are ignored)
         * @throws RedisException
         */
        protected function pushBlobs( RedisConnRef $conn, array $items ) {
                $args = array( $this->encodeQueueName() );
                // Next args come in 4s ([id, sha1, rtime, blob [, id, sha1, rtime, blob ... ] ] )
                foreach ( $items as $item ) {
                        $args[] = (string)$item['uuid'];
                        $args[] = (string)$item['sha1'];
                        $args[] = (string)$item['rtimestamp'];
                        $args[] = (string)$this->serialize( $item );
                }
                static $script =
<<<LUA
                local kUnclaimed, kSha1ById, kIdBySha1, kDelayed, kData, kQwJobs = unpack(KEYS)
                -- First argument is the queue ID
                local queueId = ARGV[1]
                -- Next arguments all come in 4s (one per job)
                local variadicArgCount = #ARGV - 1
                if variadicArgCount % 4 ~= 0 then
                        return redis.error_reply('Unmatched arguments')
                end
                -- Insert each job into this queue as needed
                local pushed = 0
                for i = 2,#ARGV,4 do
                        local id,sha1,rtimestamp,blob = ARGV[i],ARGV[i+1],ARGV[i+2],ARGV[i+3]
                        if sha1 == '' or redis.call('hExists',kIdBySha1,sha1) == 0 then
                                if 1*rtimestamp > 0 then
                                        -- Insert into delayed queue (release time as score)
                                        redis.call('zAdd',kDelayed,rtimestamp,id)
                                else
                                        -- Insert into unclaimed queue
                                        redis.call('lPush',kUnclaimed,id)
                                end
                                if sha1 ~= '' then
                                        redis.call('hSet',kSha1ById,id,sha1)
                                        redis.call('hSet',kIdBySha1,sha1,id)
                                end
                                redis.call('hSet',kData,id,blob)
                                pushed = pushed + 1
                        end
                end
                -- Mark this queue as having jobs
                redis.call('sAdd',kQwJobs,queueId)
                return pushed
LUA;
                return $conn->luaEval( $script,
                        array_merge(
                                array(
                                        $this->getQueueKey( 'l-unclaimed' ), # KEYS[1]
                                        $this->getQueueKey( 'h-sha1ById' ), # KEYS[2]
                                        $this->getQueueKey( 'h-idBySha1' ), # KEYS[3]
                                        $this->getQueueKey( 'z-delayed' ), # KEYS[4]
                                        $this->getQueueKey( 'h-data' ), # KEYS[5]
                                        $this->getGlobalKey( 's-queuesWithJobs' ), # KEYS[6]
                                ),
                                $args
                        ),
                        6 # number of first argument(s) that are keys
                );
        }

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

                $conn = $this->getConnection();
                try {
                        do {
                                $blob = $this->popAndAcquireBlob( $conn );
                                if ( !is_string( $blob ) ) {
                                        break; // no jobs; nothing to do
                                }

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

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

                return $job;
        }

        /**
         * @param RedisConnRef $conn
         * @return array Serialized string or false
         * @throws RedisException
         */
        protected function popAndAcquireBlob( RedisConnRef $conn ) {
                static $script =
<<<LUA
                local kUnclaimed, kSha1ById, kIdBySha1, kClaimed, kAttempts, kData = unpack(KEYS)
                local rTime = unpack(ARGV)
                -- Pop an item off the queue
                local id = redis.call('rPop',kUnclaimed)
                if not id then
                        return false
                end
                -- Allow new duplicates of this job
                local sha1 = redis.call('hGet',kSha1ById,id)
                if sha1 then redis.call('hDel',kIdBySha1,sha1) end
                redis.call('hDel',kSha1ById,id)
                -- Mark the jobs as claimed and return it
                redis.call('zAdd',kClaimed,rTime,id)
                redis.call('hIncrBy',kAttempts,id,1)
                return redis.call('hGet',kData,id)
LUA;
                return $conn->luaEval( $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 UnexpectedValueException
         * @throws JobQueueError
         */
        protected function doAck( Job $job ) {
                if ( !isset( $job->metadata['uuid'] ) ) {
                        throw new UnexpectedValueException( "Job of type '{$job->getType()}' has no UUID." );
                }

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

                        if ( !$res ) {
                                wfDebugLog( 'JobQueueRedis', "Could not acknowledge {$this->type} job $uuid." );

                                return false;
                        }

                        JobQueue::incrStats( 'acks', $this->type );
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $conn, $e );
                }

                return true;
        }

        /**
         * @see JobQueue::doDeduplicateRootJob()
         * @param IJobSpecification $job
         * @return bool
         * @throws JobQueueError
         * @throws LogicException
         */
        protected function doDeduplicateRootJob( IJobSpecification $job ) {
                if ( !$job->hasRootJobParams() ) {
                        throw new LogicException( "Cannot register root job; missing parameters." );
                }
                $params = $job->getRootJobParams();

                $key = $this->getRootJobCacheKey( $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( $conn, $e );
                }
        }

        /**
         * @see JobQueue::doIsRootJobOldDuplicate()
         * @param Job $job
         * @return bool
         * @throws JobQueueError
         */
        protected function doIsRootJobOldDuplicate( Job $job ) {
                if ( !$job->hasRootJobParams() ) {
                        return false; // job has no de-deplication info
                }
                $params = $job->getRootJobParams();

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

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

        /**
         * @see JobQueue::doDelete()
         * @return bool
         * @throws JobQueueError
         */
        protected function doDelete() {
                static $props = array( 'l-unclaimed', 'z-claimed', 'z-abandoned',
                        'z-delayed', 'h-idBySha1', 'h-sha1ById', 'h-attempts', 'h-data' );

                $conn = $this->getConnection();
                try {
                        $keys = array();
                        foreach ( $props as $prop ) {
                                $keys[] = $this->getQueueKey( $prop );
                        }

                        $ok = ( $conn->delete( $keys ) !== false );
                        $conn->sRem( $this->getGlobalKey( 's-queuesWithJobs' ), $this->encodeQueueName() );

                        return $ok;
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $conn, $e );
                }
        }

        /**
         * @see JobQueue::getAllQueuedJobs()
         * @return Iterator
         * @throws JobQueueError
         */
        public function getAllQueuedJobs() {
                $conn = $this->getConnection();
                try {
                        $uids = $conn->lRange( $this->getQueueKey( 'l-unclaimed' ), 0, -1 );
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $conn, $e );
                }

                return $this->getJobIterator( $conn, $uids );
        }

        /**
         * @see JobQueue::getAllDelayedJobs()
         * @return Iterator
         * @throws JobQueueError
         */
        public function getAllDelayedJobs() {
                $conn = $this->getConnection();
                try {
                        $uids = $conn->zRange( $this->getQueueKey( 'z-delayed' ), 0, -1 );
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $conn, $e );
                }

                return $this->getJobIterator( $conn, $uids );
        }

        /**
         * @see JobQueue::getAllAcquiredJobs()
         * @return Iterator
         * @throws JobQueueError
         */
        public function getAllAcquiredJobs() {
                $conn = $this->getConnection();
                try {
                        $uids = $conn->zRange( $this->getQueueKey( 'z-claimed' ), 0, -1 );
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $conn, $e );
                }

                return $this->getJobIterator( $conn, $uids );
        }

        /**
         * @see JobQueue::getAllAbandonedJobs()
         * @return Iterator
         * @throws JobQueueError
         */
        public function getAllAbandonedJobs() {
                $conn = $this->getConnection();
                try {
                        $uids = $conn->zRange( $this->getQueueKey( 'z-abandoned' ), 0, -1 );
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $conn, $e );
                }

                return $this->getJobIterator( $conn, $uids );
        }

        /**
         * @param RedisConnRef $conn
         * @param array $uids List of job UUIDs
         * @return MappedIterator
         */
        protected function getJobIterator( RedisConnRef $conn, array $uids ) {
                $that = $this;

                return new MappedIterator(
                        $uids,
                        function ( $uid ) use ( $that, $conn ) {
                                return $that->getJobFromUidInternal( $uid, $conn );
                        },
                        array( 'accept' => function ( $job ) {
                                return is_object( $job );
                        } )
                );
        }

        public function getCoalesceLocationInternal() {
                return "RedisServer:" . $this->server;
        }

        protected function doGetSiblingQueuesWithJobs( array $types ) {
                return array_keys( array_filter( $this->doGetSiblingQueueSizes( $types ) ) );
        }

        protected function doGetSiblingQueueSizes( array $types ) {
                $sizes = array(); // (type => size)
                $types = array_values( $types ); // reindex
                $conn = $this->getConnection();
                try {
                        $conn->multi( Redis::PIPELINE );
                        foreach ( $types as $type ) {
                                $conn->lSize( $this->getQueueKey( 'l-unclaimed', $type ) );
                        }
                        $res = $conn->exec();
                        if ( is_array( $res ) ) {
                                foreach ( $res as $i => $size ) {
                                        $sizes[$types[$i]] = $size;
                                }
                        }
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $conn, $e );
                }

                return $sizes;
        }

        /**
         * This function should not be called outside JobQueueRedis
         *
         * @param string $uid
         * @param RedisConnRef $conn
         * @return Job|bool Returns false if the job does not exist
         * @throws JobQueueError
         * @throws UnexpectedValueException
         */
        public function getJobFromUidInternal( $uid, RedisConnRef $conn ) {
                try {
                        $data = $conn->hGet( $this->getQueueKey( 'h-data' ), $uid );
                        if ( $data === false ) {
                                return false; // not found
                        }
                        $item = $this->unserialize( $data );
                        if ( !is_array( $item ) ) { // this shouldn't happen
                                throw new UnexpectedValueException( "Could not find job with ID '$uid'." );
                        }
                        $title = Title::makeTitle( $item['namespace'], $item['title'] );
                        $job = Job::factory( $item['type'], $title, $item['params'] );
                        $job->metadata['uuid'] = $item['uuid'];
                        $job->metadata['timestamp'] = $item['timestamp'];
                        // Add in attempt count for debugging at showJobs.php
                        $job->metadata['attempts'] = $conn->hGet( $this->getQueueKey( 'h-attempts' ), $uid );

                        return $job;
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $conn, $e );
                }
        }

        /**
         * @return array List of (wiki,type) tuples for queues with non-abandoned jobs
         * @throws JobQueueConnectionError
         * @throws JobQueueError
         */
        public function getServerQueuesWithJobs() {
                $queues = array();

                $conn = $this->getConnection();
                try {
                        $set = $conn->sMembers( $this->getGlobalKey( 's-queuesWithJobs' ) );
                        foreach ( $set as $queue ) {
                                $queues[] = $this->decodeQueueName( $queue );
                        }
                } catch ( RedisException $e ) {
                        $this->throwRedisException( $conn, $e );
                }

                return $queues;
        }

        /**
         * @param IJobSpecification $job
         * @return array
         */
        protected function getNewJobFields( IJobSpecification $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(),
                        // Some jobs cannot run until a "release timestamp"
                        'rtimestamp' => $job->getReleaseTimestamp() ?: 0,
                        // Additional job metadata
                        'uuid' => UIDGenerator::newRawUUIDv4( UIDGenerator::QUICK_RAND ),
                        'sha1' => $job->ignoreDuplicates()
                                ? Wikimedia\base_convert( sha1( serialize( $job->getDeduplicationInfo() ) ), 16, 36, 31 )
                                : '',
                        'timestamp' => time() // UNIX timestamp
                );
        }

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

                return $job;
        }

        /**
         * @param array $fields
         * @return string Serialized and possibly compressed version of $fields
         */
        protected function serialize( array $fields ) {
                $blob = serialize( $fields );
                if ( $this->compression === 'gzip'
                        && strlen( $blob ) >= 1024
                        && function_exists( 'gzdeflate' )
                ) {
                        $object = (object)array( 'blob' => gzdeflate( $blob ), 'enc' => 'gzip' );
                        $blobz = serialize( $object );

                        return ( strlen( $blobz ) < strlen( $blob ) ) ? $blobz : $blob;
                } else {
                        return $blob;
                }
        }

        /**
         * @param string $blob
         * @return array|bool Unserialized version of $blob or false
         */
        protected function unserialize( $blob ) {
                $fields = unserialize( $blob );
                if ( is_object( $fields ) ) {
                        if ( $fields->enc === 'gzip' && function_exists( 'gzinflate' ) ) {
                                $fields = unserialize( gzinflate( $fields->blob ) );
                        } else {
                                $fields = false;
                        }
                }

                return is_array( $fields ) ? $fields : false;
        }

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

                return $conn;
        }

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

        /**
         * @return string JSON
         */
        private function encodeQueueName() {
                return json_encode( array( $this->type, $this->wiki ) );
        }

        /**
         * @param string $name JSON
         * @return array (type, wiki)
         */
        private function decodeQueueName( $name ) {
                return json_decode( $name );
        }

        /**
         * @param string $name
         * @return string
         */
        private function getGlobalKey( $name ) {
                $parts = array( 'global', 'jobqueue', $name );
                foreach ( $parts as $part ) {
                    if ( !preg_match( '/[a-zA-Z0-9_-]+/', $part ) ) {
                        throw new InvalidArgumentException( "Key part characters are out of range." );
                    }
                }

                return implode( ':', $parts );
        }

        /**
         * @param string $prop
         * @param string|null $type Override this for sibling queues
         * @return string
         */
        private function getQueueKey( $prop, $type = null ) {
                $type = is_string( $type ) ? $type : $this->type;
                list( $db, $prefix ) = wfSplitWikiID( $this->wiki );
                $keyspace = $prefix ? "$db-$prefix" : $db;

                $parts = array( $keyspace, 'jobqueue', $type, $prop );

                // Parts are typically ASCII, but encode for sanity to escape ":"
                return implode( ':', array_map( 'rawurlencode', $parts ) );
        }
}