JobQueueDB.php

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<?php
use Wikimedia\Rdbms\IDatabase;
use Wikimedia\Rdbms\DBConnRef;
use Wikimedia\Rdbms\DBConnectionError;
use Wikimedia\Rdbms\DBError;
use MediaWiki\MediaWikiServices;
use Wikimedia\ScopedCallback;
 
class JobQueueDB extends JobQueue {
    const CACHE_TTL_SHORT = 30; // integer; seconds to cache info without re-validating
    const MAX_AGE_PRUNE = 604800; // integer; seconds a job can live once claimed
    const MAX_JOB_RANDOM = 2147483647; // integer; 2^31 - 1, used for job_random
    const MAX_OFFSET = 255; // integer; maximum number of rows to skip
 
    protected $cache;
 
    protected $cluster = false;
 
    protected function __construct( array $params ) {
        parent::__construct( $params );
 
        $this->cluster = isset( $params['cluster'] ) ? $params['cluster'] : false;
        $this->cache = ObjectCache::getMainWANInstance();
    }
 
    protected function supportedOrders() {
        return [ 'random', 'timestamp', 'fifo' ];
    }
 
    protected function optimalOrder() {
        return 'random';
    }
 
    protected function doIsEmpty() {
        $dbr = $this->getReplicaDB();
        try {
            $found = $dbr->selectField( // unclaimed job
                'job', '1', [ 'job_cmd' => $this->type, 'job_token' => '' ], __METHOD__
            );
        } catch ( DBError $e ) {
            $this->throwDBException( $e );
        }
 
        return !$found;
    }
 
    protected function doGetSize() {
        $key = $this->getCacheKey( 'size' );
 
        $size = $this->cache->get( $key );
        if ( is_int( $size ) ) {
            return $size;
        }
 
        try {
            $dbr = $this->getReplicaDB();
            $size = (int)$dbr->selectField( 'job', 'COUNT(*)',
                [ 'job_cmd' => $this->type, 'job_token' => '' ],
                __METHOD__
            );
        } catch ( DBError $e ) {
            $this->throwDBException( $e );
        }
        $this->cache->set( $key, $size, self::CACHE_TTL_SHORT );
 
        return $size;
    }
 
    protected function doGetAcquiredCount() {
        if ( $this->claimTTL <= 0 ) {
            return 0; // no acknowledgements
        }
 
        $key = $this->getCacheKey( 'acquiredcount' );
 
        $count = $this->cache->get( $key );
        if ( is_int( $count ) ) {
            return $count;
        }
 
        $dbr = $this->getReplicaDB();
        try {
            $count = (int)$dbr->selectField( 'job', 'COUNT(*)',
                [ 'job_cmd' => $this->type, "job_token != {$dbr->addQuotes( '' )}" ],
                __METHOD__
            );
        } catch ( DBError $e ) {
            $this->throwDBException( $e );
        }
        $this->cache->set( $key, $count, self::CACHE_TTL_SHORT );
 
        return $count;
    }
 
    protected function doGetAbandonedCount() {
        if ( $this->claimTTL <= 0 ) {
            return 0; // no acknowledgements
        }
 
        $key = $this->getCacheKey( 'abandonedcount' );
 
        $count = $this->cache->get( $key );
        if ( is_int( $count ) ) {
            return $count;
        }
 
        $dbr = $this->getReplicaDB();
        try {
            $count = (int)$dbr->selectField( 'job', 'COUNT(*)',
                [
                    'job_cmd' => $this->type,
                    "job_token != {$dbr->addQuotes( '' )}",
                    "job_attempts >= " . $dbr->addQuotes( $this->maxTries )
                ],
                __METHOD__
            );
        } catch ( DBError $e ) {
            $this->throwDBException( $e );
        }
 
        $this->cache->set( $key, $count, self::CACHE_TTL_SHORT );
 
        return $count;
    }
 
    protected function doBatchPush( array $jobs, $flags ) {
        $dbw = $this->getMasterDB();
        // In general, there will be two cases here:
        // a) sqlite; DB connection is probably a regular round-aware handle.
        // If the connection is busy with a transaction, then defer the job writes
        // until right before the main round commit step. Any errors that bubble
        // up will rollback the main commit round.
        // b) mysql/postgres; DB connection is generally a separate CONN_TRX_AUTOCOMMIT handle.
        // No transaction is active nor will be started by writes, so enqueue the jobs
        // now so that any errors will show up immediately as the interface expects. Any
        // errors that bubble up will rollback the main commit round.
        $fname = __METHOD__;
        $dbw->onTransactionPreCommitOrIdle(
            function () use ( $dbw, $jobs, $flags, $fname ) {
                $this->doBatchPushInternal( $dbw, $jobs, $flags, $fname );
            },
            $fname
        );
    }
 
    public function doBatchPushInternal( IDatabase $dbw, array $jobs, $flags, $method ) {
        if ( !count( $jobs ) ) {
            return;
        }
 
        $rowSet = []; // (sha1 => job) map for jobs that are de-duplicated
        $rowList = []; // list of jobs for jobs that are not de-duplicated
        foreach ( $jobs as $job ) {
            $row = $this->insertFields( $job );
            if ( $job->ignoreDuplicates() ) {
                $rowSet[$row['job_sha1']] = $row;
            } else {
                $rowList[] = $row;
            }
        }
 
        if ( $flags & self::QOS_ATOMIC ) {
            $dbw->startAtomic( $method ); // wrap all the job additions in one transaction
        }
        try {
            // Strip out any duplicate jobs that are already in the queue...
            if ( count( $rowSet ) ) {
                $res = $dbw->select( 'job', 'job_sha1',
                    [
                        // No job_type condition since it's part of the job_sha1 hash
                        'job_sha1' => array_keys( $rowSet ),
                        'job_token' => '' // unclaimed
                    ],
                    $method
                );
                foreach ( $res as $row ) {
                    wfDebug( "Job with hash '{$row->job_sha1}' is a duplicate.\n" );
                    unset( $rowSet[$row->job_sha1] ); // already enqueued
                }
            }
            // Build the full list of job rows to insert
            $rows = array_merge( $rowList, array_values( $rowSet ) );
            // Insert the job rows in chunks to avoid replica DB lag...
            foreach ( array_chunk( $rows, 50 ) as $rowBatch ) {
                $dbw->insert( 'job', $rowBatch, $method );
            }
            JobQueue::incrStats( 'inserts', $this->type, count( $rows ) );
            JobQueue::incrStats( 'dupe_inserts', $this->type,
                count( $rowSet ) + count( $rowList ) - count( $rows )
            );
        } catch ( DBError $e ) {
            $this->throwDBException( $e );
        }
        if ( $flags & self::QOS_ATOMIC ) {
            $dbw->endAtomic( $method );
        }
 
        return;
    }
 
    protected function doPop() {
        $dbw = $this->getMasterDB();
        try {
            $autoTrx = $dbw->getFlag( DBO_TRX ); // get current setting
            $dbw->clearFlag( DBO_TRX ); // make each query its own transaction
            $scopedReset = new ScopedCallback( function () use ( $dbw, $autoTrx ) {
                $dbw->setFlag( $autoTrx ? DBO_TRX : 0 ); // restore old setting
            } );
 
            $uuid = wfRandomString( 32 ); // pop attempt
            $job = false; // job popped off
            do { // retry when our row is invalid or deleted as a duplicate
                // Try to reserve a row in the DB...
                if ( in_array( $this->order, [ 'fifo', 'timestamp' ] ) ) {
                    $row = $this->claimOldest( $uuid );
                } else { // random first
                    $rand = mt_rand( 0, self::MAX_JOB_RANDOM ); // encourage concurrent UPDATEs
                    $gte = (bool)mt_rand( 0, 1 ); // find rows with rand before/after $rand
                    $row = $this->claimRandom( $uuid, $rand, $gte );
                }
                // Check if we found a row to reserve...
                if ( !$row ) {
                    break; // nothing to do
                }
                JobQueue::incrStats( 'pops', $this->type );
                // Get the job object from the row...
                $title = Title::makeTitle( $row->job_namespace, $row->job_title );
                $job = Job::factory( $row->job_cmd, $title,
                    self::extractBlob( $row->job_params ), $row->job_id );
                $job->metadata['id'] = $row->job_id;
                $job->metadata['timestamp'] = $row->job_timestamp;
                break; // done
            } while ( true );
 
            if ( !$job || mt_rand( 0, 9 ) == 0 ) {
                // Handled jobs that need to be recycled/deleted;
                // any recycled jobs will be picked up next attempt
                $this->recycleAndDeleteStaleJobs();
            }
        } catch ( DBError $e ) {
            $this->throwDBException( $e );
        }
 
        return $job;
    }
 
    protected function claimRandom( $uuid, $rand, $gte ) {
        $dbw = $this->getMasterDB();
        // Check cache to see if the queue has <= OFFSET items
        $tinyQueue = $this->cache->get( $this->getCacheKey( 'small' ) );
 
        $row = false; // the row acquired
        $invertedDirection = false; // whether one job_random direction was already scanned
        // This uses a replication safe method for acquiring jobs. One could use UPDATE+LIMIT
        // instead, but that either uses ORDER BY (in which case it deadlocks in MySQL) or is
        // not replication safe. Due to https://bugs.mysql.com/bug.php?id=6980, subqueries cannot
        // be used here with MySQL.
        do {
            if ( $tinyQueue ) { // queue has <= MAX_OFFSET rows
                // For small queues, using OFFSET will overshoot and return no rows more often.
                // Instead, this uses job_random to pick a row (possibly checking both directions).
                $ineq = $gte ? '>=' : '<=';
                $dir = $gte ? 'ASC' : 'DESC';
                $row = $dbw->selectRow( 'job', self::selectFields(), // find a random job
                    [
                        'job_cmd' => $this->type,
                        'job_token' => '', // unclaimed
                        "job_random {$ineq} {$dbw->addQuotes( $rand )}" ],
                    __METHOD__,
                    [ 'ORDER BY' => "job_random {$dir}" ]
                );
                if ( !$row && !$invertedDirection ) {
                    $gte = !$gte;
                    $invertedDirection = true;
                    continue; // try the other direction
                }
            } else { // table *may* have >= MAX_OFFSET rows
                // T44614: "ORDER BY job_random" with a job_random inequality causes high CPU
                // in MySQL if there are many rows for some reason. This uses a small OFFSET
                // instead of job_random for reducing excess claim retries.
                $row = $dbw->selectRow( 'job', self::selectFields(), // find a random job
                    [
                        'job_cmd' => $this->type,
                        'job_token' => '', // unclaimed
                    ],
                    __METHOD__,
                    [ 'OFFSET' => mt_rand( 0, self::MAX_OFFSET ) ]
                );
                if ( !$row ) {
                    $tinyQueue = true; // we know the queue must have <= MAX_OFFSET rows
                    $this->cache->set( $this->getCacheKey( 'small' ), 1, 30 );
                    continue; // use job_random
                }
            }
 
            if ( $row ) { // claim the job
                $dbw->update( 'job', // update by PK
                    [
                        'job_token' => $uuid,
                        'job_token_timestamp' => $dbw->timestamp(),
                        'job_attempts = job_attempts+1' ],
                    [ 'job_cmd' => $this->type, 'job_id' => $row->job_id, 'job_token' => '' ],
                    __METHOD__
                );
                // This might get raced out by another runner when claiming the previously
                // selected row. The use of job_random should minimize this problem, however.
                if ( !$dbw->affectedRows() ) {
                    $row = false; // raced out
                }
            } else {
                break; // nothing to do
            }
        } while ( !$row );
 
        return $row;
    }
 
    protected function claimOldest( $uuid ) {
        $dbw = $this->getMasterDB();
 
        $row = false; // the row acquired
        do {
            if ( $dbw->getType() === 'mysql' ) {
                // Per https://bugs.mysql.com/bug.php?id=6980, we can't use subqueries on the
                // same table being changed in an UPDATE query in MySQL (gives Error: 1093).
                // Oracle and Postgre have no such limitation. However, MySQL offers an
                // alternative here by supporting ORDER BY + LIMIT for UPDATE queries.
                $dbw->query( "UPDATE {$dbw->tableName( 'job' )} " .
                    "SET " .
                        "job_token = {$dbw->addQuotes( $uuid ) }, " .
                        "job_token_timestamp = {$dbw->addQuotes( $dbw->timestamp() )}, " .
                        "job_attempts = job_attempts+1 " .
                    "WHERE ( " .
                        "job_cmd = {$dbw->addQuotes( $this->type )} " .
                        "AND job_token = {$dbw->addQuotes( '' )} " .
                    ") ORDER BY job_id ASC LIMIT 1",
                    __METHOD__
                );
            } else {
                // Use a subquery to find the job, within an UPDATE to claim it.
                // This uses as much of the DB wrapper functions as possible.
                $dbw->update( 'job',
                    [
                        'job_token' => $uuid,
                        'job_token_timestamp' => $dbw->timestamp(),
                        'job_attempts = job_attempts+1' ],
                    [ 'job_id = (' .
                        $dbw->selectSQLText( 'job', 'job_id',
                            [ 'job_cmd' => $this->type, 'job_token' => '' ],
                            __METHOD__,
                            [ 'ORDER BY' => 'job_id ASC', 'LIMIT' => 1 ] ) .
                        ')'
                    ],
                    __METHOD__
                );
            }
            // Fetch any row that we just reserved...
            if ( $dbw->affectedRows() ) {
                $row = $dbw->selectRow( 'job', self::selectFields(),
                    [ 'job_cmd' => $this->type, 'job_token' => $uuid ], __METHOD__
                );
                if ( !$row ) { // raced out by duplicate job removal
                    wfDebug( "Row deleted as duplicate by another process.\n" );
                }
            } else {
                break; // nothing to do
            }
        } while ( !$row );
 
        return $row;
    }
 
    protected function doAck( Job $job ) {
        if ( !isset( $job->metadata['id'] ) ) {
            throw new MWException( "Job of type '{$job->getType()}' has no ID." );
        }
 
        $dbw = $this->getMasterDB();
        try {
            $autoTrx = $dbw->getFlag( DBO_TRX ); // get current setting
            $dbw->clearFlag( DBO_TRX ); // make each query its own transaction
            $scopedReset = new ScopedCallback( function () use ( $dbw, $autoTrx ) {
                $dbw->setFlag( $autoTrx ? DBO_TRX : 0 ); // restore old setting
            } );
 
            // Delete a row with a single DELETE without holding row locks over RTTs...
            $dbw->delete( 'job',
                [ 'job_cmd' => $this->type, 'job_id' => $job->metadata['id'] ], __METHOD__ );
 
            JobQueue::incrStats( 'acks', $this->type );
        } catch ( DBError $e ) {
            $this->throwDBException( $e );
        }
    }
 
    protected function doDeduplicateRootJob( IJobSpecification $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->getRootJobCacheKey( $params['rootJobSignature'] );
        // Callers should call batchInsert() and then this function so that if the insert
        // fails, the de-duplication registration will be aborted. Since the insert is
        // deferred till "transaction idle", do the same here, so that the ordering is
        // maintained. Having only the de-duplication registration succeed would cause
        // jobs to become no-ops without any actual jobs that made them redundant.
        $dbw = $this->getMasterDB();
        $cache = $this->dupCache;
        $dbw->onTransactionIdle(
            function () use ( $cache, $params, $key, $dbw ) {
                $timestamp = $cache->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 $cache->set( $key, $params['rootJobTimestamp'], JobQueueDB::ROOTJOB_TTL );
            },
            __METHOD__
        );
 
        return true;
    }
 
    protected function doDelete() {
        $dbw = $this->getMasterDB();
        try {
            $dbw->delete( 'job', [ 'job_cmd' => $this->type ] );
        } catch ( DBError $e ) {
            $this->throwDBException( $e );
        }
 
        return true;
    }
 
    protected function doWaitForBackups() {
        $lbFactory = MediaWikiServices::getInstance()->getDBLoadBalancerFactory();
        $lbFactory->waitForReplication( [ 'wiki' => $this->wiki, 'cluster' => $this->cluster ] );
    }
 
    protected function doFlushCaches() {
        foreach ( [ 'size', 'acquiredcount' ] as $type ) {
            $this->cache->delete( $this->getCacheKey( $type ) );
        }
    }
 
    public function getAllQueuedJobs() {
        return $this->getJobIterator( [ 'job_cmd' => $this->getType(), 'job_token' => '' ] );
    }
 
    public function getAllAcquiredJobs() {
        return $this->getJobIterator( [ 'job_cmd' => $this->getType(), "job_token > ''" ] );
    }
 
    public function getAllAbandonedJobs() {
        return $this->getJobIterator( [
            'job_cmd' => $this->getType(),
            "job_token > ''",
            "job_attempts >= " . intval( $this->maxTries )
        ] );
    }
 
    protected function getJobIterator( array $conds ) {
        $dbr = $this->getReplicaDB();
        try {
            return new MappedIterator(
                $dbr->select( 'job', self::selectFields(), $conds ),
                function ( $row ) {
                    $job = Job::factory(
                        $row->job_cmd,
                        Title::makeTitle( $row->job_namespace, $row->job_title ),
                        strlen( $row->job_params ) ? unserialize( $row->job_params ) : []
                    );
                    $job->metadata['id'] = $row->job_id;
                    $job->metadata['timestamp'] = $row->job_timestamp;
 
                    return $job;
                }
            );
        } catch ( DBError $e ) {
            $this->throwDBException( $e );
        }
    }
 
    public function getCoalesceLocationInternal() {
        return $this->cluster
            ? "DBCluster:{$this->cluster}:{$this->wiki}"
            : "LBFactory:{$this->wiki}";
    }
 
    protected function doGetSiblingQueuesWithJobs( array $types ) {
        $dbr = $this->getReplicaDB();
        // @note: this does not check whether the jobs are claimed or not.
        // This is useful so JobQueueGroup::pop() also sees queues that only
        // have stale jobs. This lets recycleAndDeleteStaleJobs() re-enqueue
        // failed jobs so that they can be popped again for that edge case.
        $res = $dbr->select( 'job', 'DISTINCT job_cmd',
            [ 'job_cmd' => $types ], __METHOD__ );
 
        $types = [];
        foreach ( $res as $row ) {
            $types[] = $row->job_cmd;
        }
 
        return $types;
    }
 
    protected function doGetSiblingQueueSizes( array $types ) {
        $dbr = $this->getReplicaDB();
        $res = $dbr->select( 'job', [ 'job_cmd', 'COUNT(*) AS count' ],
            [ 'job_cmd' => $types ], __METHOD__, [ 'GROUP BY' => 'job_cmd' ] );
 
        $sizes = [];
        foreach ( $res as $row ) {
            $sizes[$row->job_cmd] = (int)$row->count;
        }
 
        return $sizes;
    }
 
    public function recycleAndDeleteStaleJobs() {
        $now = time();
        $count = 0; // affected rows
        $dbw = $this->getMasterDB();
 
        try {
            if ( !$dbw->lock( "jobqueue-recycle-{$this->type}", __METHOD__, 1 ) ) {
                return $count; // already in progress
            }
 
            // Remove claims on jobs acquired for too long if enabled...
            if ( $this->claimTTL > 0 ) {
                $claimCutoff = $dbw->timestamp( $now - $this->claimTTL );
                // Get the IDs of jobs that have be claimed but not finished after too long.
                // These jobs can be recycled into the queue by expiring the claim. Selecting
                // the IDs first means that the UPDATE can be done by primary key (less deadlocks).
                $res = $dbw->select( 'job', 'job_id',
                    [
                        'job_cmd' => $this->type,
                        "job_token != {$dbw->addQuotes( '' )}", // was acquired
                        "job_token_timestamp < {$dbw->addQuotes( $claimCutoff )}", // stale
                        "job_attempts < {$dbw->addQuotes( $this->maxTries )}" ], // retries left
                    __METHOD__
                );
                $ids = array_map(
                    function ( $o ) {
                        return $o->job_id;
                    }, iterator_to_array( $res )
                );
                if ( count( $ids ) ) {
                    // Reset job_token for these jobs so that other runners will pick them up.
                    // Set the timestamp to the current time, as it is useful to now that the job
                    // was already tried before (the timestamp becomes the "released" time).
                    $dbw->update( 'job',
                        [
                            'job_token' => '',
                            'job_token_timestamp' => $dbw->timestamp( $now ) ], // time of release
                        [
                            'job_id' => $ids ],
                        __METHOD__
                    );
                    $affected = $dbw->affectedRows();
                    $count += $affected;
                    JobQueue::incrStats( 'recycles', $this->type, $affected );
                    $this->aggr->notifyQueueNonEmpty( $this->wiki, $this->type );
                }
            }
 
            // Just destroy any stale jobs...
            $pruneCutoff = $dbw->timestamp( $now - self::MAX_AGE_PRUNE );
            $conds = [
                'job_cmd' => $this->type,
                "job_token != {$dbw->addQuotes( '' )}", // was acquired
                "job_token_timestamp < {$dbw->addQuotes( $pruneCutoff )}" // stale
            ];
            if ( $this->claimTTL > 0 ) { // only prune jobs attempted too many times...
                $conds[] = "job_attempts >= {$dbw->addQuotes( $this->maxTries )}";
            }
            // Get the IDs of jobs that are considered stale and should be removed. Selecting
            // the IDs first means that the UPDATE can be done by primary key (less deadlocks).
            $res = $dbw->select( 'job', 'job_id', $conds, __METHOD__ );
            $ids = array_map(
                function ( $o ) {
                    return $o->job_id;
                }, iterator_to_array( $res )
            );
            if ( count( $ids ) ) {
                $dbw->delete( 'job', [ 'job_id' => $ids ], __METHOD__ );
                $affected = $dbw->affectedRows();
                $count += $affected;
                JobQueue::incrStats( 'abandons', $this->type, $affected );
            }
 
            $dbw->unlock( "jobqueue-recycle-{$this->type}", __METHOD__ );
        } catch ( DBError $e ) {
            $this->throwDBException( $e );
        }
 
        return $count;
    }
 
    protected function insertFields( IJobSpecification $job ) {
        $dbw = $this->getMasterDB();
 
        return [
            // Fields that describe the nature of the job
            'job_cmd' => $job->getType(),
            'job_namespace' => $job->getTitle()->getNamespace(),
            'job_title' => $job->getTitle()->getDBkey(),
            'job_params' => self::makeBlob( $job->getParams() ),
            // Additional job metadata
            'job_timestamp' => $dbw->timestamp(),
            'job_sha1' => Wikimedia\base_convert(
                sha1( serialize( $job->getDeduplicationInfo() ) ),
                16, 36, 31
            ),
            'job_random' => mt_rand( 0, self::MAX_JOB_RANDOM )
        ];
    }
 
    protected function getReplicaDB() {
        try {
            return $this->getDB( DB_REPLICA );
        } catch ( DBConnectionError $e ) {
            throw new JobQueueConnectionError( "DBConnectionError:" . $e->getMessage() );
        }
    }
 
    protected function getMasterDB() {
        try {
            return $this->getDB( DB_MASTER );
        } catch ( DBConnectionError $e ) {
            throw new JobQueueConnectionError( "DBConnectionError:" . $e->getMessage() );
        }
    }
 
    protected function getDB( $index ) {
        $lbFactory = MediaWikiServices::getInstance()->getDBLoadBalancerFactory();
        $lb = ( $this->cluster !== false )
            ? $lbFactory->getExternalLB( $this->cluster )
            : $lbFactory->getMainLB( $this->wiki );
 
        return ( $lb->getServerType( $lb->getWriterIndex() ) !== 'sqlite' )
            // Keep a separate connection to avoid contention and deadlocks;
            // However, SQLite has the opposite behavior due to DB-level locking.
            ? $lb->getConnectionRef( $index, [], $this->wiki, $lb::CONN_TRX_AUTOCOMMIT )
            // Jobs insertion will be defered until the PRESEND stage to reduce contention.
            : $lb->getConnectionRef( $index, [], $this->wiki );
    }
 
    private function getCacheKey( $property ) {
        list( $db, $prefix ) = wfSplitWikiID( $this->wiki );
        $cluster = is_string( $this->cluster ) ? $this->cluster : 'main';
 
        return wfForeignMemcKey( $db, $prefix, 'jobqueue', $cluster, $this->type, $property );
    }
 
    protected static function makeBlob( $params ) {
        if ( $params !== false ) {
            return serialize( $params );
        } else {
            return '';
        }
    }
 
    protected static function extractBlob( $blob ) {
        if ( (string)$blob !== '' ) {
            return unserialize( $blob );
        } else {
            return false;
        }
    }
 
    protected function throwDBException( DBError $e ) {
        throw new JobQueueError( get_class( $e ) . ": " . $e->getMessage() );
    }
 
    public static function selectFields() {
        return [
            'job_id',
            'job_cmd',
            'job_namespace',
            'job_title',
            'job_timestamp',
            'job_params',
            'job_random',
            'job_attempts',
            'job_token',
            'job_token_timestamp',
            'job_sha1',
        ];
    }
}