3 * Database-backed job queue code.
5 * This program is free software; you can redistribute it and/or modify
6 * it under the terms of the GNU General Public License as published by
7 * the Free Software Foundation; either version 2 of the License, or
8 * (at your option) any later version.
10 * This program is distributed in the hope that it will be useful,
11 * but WITHOUT ANY WARRANTY; without even the implied warranty of
12 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 * GNU General Public License for more details.
15 * You should have received a copy of the GNU General Public License along
16 * with this program; if not, write to the Free Software Foundation, Inc.,
17 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
18 * http://www.gnu.org/copyleft/gpl.html
21 * @author Aaron Schulz
25 * Class to handle job queues stored in the DB
30 class JobQueueDB
extends JobQueue
{
31 const CACHE_TTL_SHORT
= 30; // integer; seconds to cache info without re-validating
32 const CACHE_TTL_LONG
= 300; // integer; seconds to cache info that is kept up to date
33 const MAX_AGE_PRUNE
= 604800; // integer; seconds a job can live once claimed
34 const MAX_JOB_RANDOM
= 2147483647; // integer; 2^31 - 1, used for job_random
35 const MAX_OFFSET
= 255; // integer; maximum number of rows to skip
40 /** @var bool|string Name of an external DB cluster. False if not set */
41 protected $cluster = false;
44 * Additional parameters include:
45 * - cluster : The name of an external cluster registered via LBFactory.
46 * If not specified, the primary DB cluster for the wiki will be used.
47 * This can be overridden with a custom cluster so that DB handles will
48 * be retrieved via LBFactory::getExternalLB() and getConnection().
49 * @param array $params
51 protected function __construct( array $params ) {
54 parent
::__construct( $params );
56 $this->cluster
= isset( $params['cluster'] ) ?
$params['cluster'] : false;
57 // Make sure that we don't use the SQL cache, which would be harmful
58 $this->cache
= ( $wgMemc instanceof SqlBagOStuff
) ?
new EmptyBagOStuff() : $wgMemc;
61 protected function supportedOrders() {
62 return array( 'random', 'timestamp', 'fifo' );
65 protected function optimalOrder() {
70 * @see JobQueue::doIsEmpty()
73 protected function doIsEmpty() {
74 $key = $this->getCacheKey( 'empty' );
76 $isEmpty = $this->cache
->get( $key );
77 if ( $isEmpty === 'true' ) {
79 } elseif ( $isEmpty === 'false' ) {
83 $dbr = $this->getSlaveDB();
85 $found = $dbr->selectField( // unclaimed job
86 'job', '1', array( 'job_cmd' => $this->type
, 'job_token' => '' ), __METHOD__
88 } catch ( DBError
$e ) {
89 $this->throwDBException( $e );
91 $this->cache
->add( $key, $found ?
'false' : 'true', self
::CACHE_TTL_LONG
);
97 * @see JobQueue::doGetSize()
100 protected function doGetSize() {
101 $key = $this->getCacheKey( 'size' );
103 $size = $this->cache
->get( $key );
104 if ( is_int( $size ) ) {
109 $dbr = $this->getSlaveDB();
110 $size = (int)$dbr->selectField( 'job', 'COUNT(*)',
111 array( 'job_cmd' => $this->type
, 'job_token' => '' ),
114 } catch ( DBError
$e ) {
115 $this->throwDBException( $e );
117 $this->cache
->set( $key, $size, self
::CACHE_TTL_SHORT
);
123 * @see JobQueue::doGetAcquiredCount()
126 protected function doGetAcquiredCount() {
127 if ( $this->claimTTL
<= 0 ) {
128 return 0; // no acknowledgements
131 $key = $this->getCacheKey( 'acquiredcount' );
133 $count = $this->cache
->get( $key );
134 if ( is_int( $count ) ) {
138 $dbr = $this->getSlaveDB();
140 $count = (int)$dbr->selectField( 'job', 'COUNT(*)',
141 array( 'job_cmd' => $this->type
, "job_token != {$dbr->addQuotes( '' )}" ),
144 } catch ( DBError
$e ) {
145 $this->throwDBException( $e );
147 $this->cache
->set( $key, $count, self
::CACHE_TTL_SHORT
);
153 * @see JobQueue::doGetAbandonedCount()
155 * @throws MWException
157 protected function doGetAbandonedCount() {
160 if ( $this->claimTTL
<= 0 ) {
161 return 0; // no acknowledgements
164 $key = $this->getCacheKey( 'abandonedcount' );
166 $count = $wgMemc->get( $key );
167 if ( is_int( $count ) ) {
171 $dbr = $this->getSlaveDB();
173 $count = (int)$dbr->selectField( 'job', 'COUNT(*)',
175 'job_cmd' => $this->type
,
176 "job_token != {$dbr->addQuotes( '' )}",
177 "job_attempts >= " . $dbr->addQuotes( $this->maxTries
)
181 } catch ( DBError
$e ) {
182 $this->throwDBException( $e );
184 $wgMemc->set( $key, $count, self
::CACHE_TTL_SHORT
);
190 * @see JobQueue::doBatchPush()
193 * @throws DBError|Exception
196 protected function doBatchPush( array $jobs, $flags ) {
197 $dbw = $this->getMasterDB();
200 $method = __METHOD__
;
201 $dbw->onTransactionIdle(
202 function () use ( $dbw, $that, $jobs, $flags, $method ) {
203 $that->doBatchPushInternal( $dbw, $jobs, $flags, $method );
211 * This function should *not* be called outside of JobQueueDB
213 * @param IDatabase $dbw
216 * @param string $method
220 public function doBatchPushInternal( IDatabase
$dbw, array $jobs, $flags, $method ) {
221 if ( !count( $jobs ) ) {
225 $rowSet = array(); // (sha1 => job) map for jobs that are de-duplicated
226 $rowList = array(); // list of jobs for jobs that are are not de-duplicated
227 foreach ( $jobs as $job ) {
228 $row = $this->insertFields( $job );
229 if ( $job->ignoreDuplicates() ) {
230 $rowSet[$row['job_sha1']] = $row;
236 if ( $flags & self
::QOS_ATOMIC
) {
237 $dbw->begin( $method ); // wrap all the job additions in one transaction
240 // Strip out any duplicate jobs that are already in the queue...
241 if ( count( $rowSet ) ) {
242 $res = $dbw->select( 'job', 'job_sha1',
244 // No job_type condition since it's part of the job_sha1 hash
245 'job_sha1' => array_keys( $rowSet ),
246 'job_token' => '' // unclaimed
250 foreach ( $res as $row ) {
251 wfDebug( "Job with hash '{$row->job_sha1}' is a duplicate.\n" );
252 unset( $rowSet[$row->job_sha1
] ); // already enqueued
255 // Build the full list of job rows to insert
256 $rows = array_merge( $rowList, array_values( $rowSet ) );
257 // Insert the job rows in chunks to avoid slave lag...
258 foreach ( array_chunk( $rows, 50 ) as $rowBatch ) {
259 $dbw->insert( 'job', $rowBatch, $method );
261 JobQueue
::incrStats( 'job-insert', $this->type
, count( $rows ), $this->wiki
);
263 'job-insert-duplicate',
265 count( $rowSet ) +
count( $rowList ) - count( $rows ),
268 } catch ( DBError
$e ) {
269 if ( $flags & self
::QOS_ATOMIC
) {
270 $dbw->rollback( $method );
274 if ( $flags & self
::QOS_ATOMIC
) {
275 $dbw->commit( $method );
278 $this->cache
->set( $this->getCacheKey( 'empty' ), 'false', JobQueueDB
::CACHE_TTL_LONG
);
284 * @see JobQueue::doPop()
287 protected function doPop() {
288 if ( $this->cache
->get( $this->getCacheKey( 'empty' ) ) === 'true' ) {
289 return false; // queue is empty
292 $dbw = $this->getMasterDB();
294 $dbw->commit( __METHOD__
, 'flush' ); // flush existing transaction
295 $autoTrx = $dbw->getFlag( DBO_TRX
); // get current setting
296 $dbw->clearFlag( DBO_TRX
); // make each query its own transaction
297 $scopedReset = new ScopedCallback( function () use ( $dbw, $autoTrx ) {
298 $dbw->setFlag( $autoTrx ? DBO_TRX
: 0 ); // restore old setting
301 $uuid = wfRandomString( 32 ); // pop attempt
302 $job = false; // job popped off
303 do { // retry when our row is invalid or deleted as a duplicate
304 // Try to reserve a row in the DB...
305 if ( in_array( $this->order
, array( 'fifo', 'timestamp' ) ) ) {
306 $row = $this->claimOldest( $uuid );
307 } else { // random first
308 $rand = mt_rand( 0, self
::MAX_JOB_RANDOM
); // encourage concurrent UPDATEs
309 $gte = (bool)mt_rand( 0, 1 ); // find rows with rand before/after $rand
310 $row = $this->claimRandom( $uuid, $rand, $gte );
312 // Check if we found a row to reserve...
314 $this->cache
->set( $this->getCacheKey( 'empty' ), 'true', self
::CACHE_TTL_LONG
);
315 break; // nothing to do
317 JobQueue
::incrStats( 'job-pop', $this->type
, 1, $this->wiki
);
318 // Get the job object from the row...
319 $title = Title
::makeTitleSafe( $row->job_namespace
, $row->job_title
);
321 $dbw->delete( 'job', array( 'job_id' => $row->job_id
), __METHOD__
);
322 wfDebug( "Row has invalid title '{$row->job_title}'.\n" );
323 continue; // try again
325 $job = Job
::factory( $row->job_cmd
, $title,
326 self
::extractBlob( $row->job_params
), $row->job_id
);
327 $job->metadata
['id'] = $row->job_id
;
330 } catch ( DBError
$e ) {
331 $this->throwDBException( $e );
338 * Reserve a row with a single UPDATE without holding row locks over RTTs...
340 * @param string $uuid 32 char hex string
341 * @param $rand integer Random unsigned integer (31 bits)
342 * @param bool $gte Search for job_random >= $random (otherwise job_random <= $random)
343 * @return stdClass|bool Row|false
345 protected function claimRandom( $uuid, $rand, $gte ) {
346 $dbw = $this->getMasterDB();
347 // Check cache to see if the queue has <= OFFSET items
348 $tinyQueue = $this->cache
->get( $this->getCacheKey( 'small' ) );
350 $row = false; // the row acquired
351 $invertedDirection = false; // whether one job_random direction was already scanned
352 // This uses a replication safe method for acquiring jobs. One could use UPDATE+LIMIT
353 // instead, but that either uses ORDER BY (in which case it deadlocks in MySQL) or is
354 // not replication safe. Due to http://bugs.mysql.com/bug.php?id=6980, subqueries cannot
355 // be used here with MySQL.
357 if ( $tinyQueue ) { // queue has <= MAX_OFFSET rows
358 // For small queues, using OFFSET will overshoot and return no rows more often.
359 // Instead, this uses job_random to pick a row (possibly checking both directions).
360 $ineq = $gte ?
'>=' : '<=';
361 $dir = $gte ?
'ASC' : 'DESC';
362 $row = $dbw->selectRow( 'job', self
::selectFields(), // find a random job
364 'job_cmd' => $this->type
,
365 'job_token' => '', // unclaimed
366 "job_random {$ineq} {$dbw->addQuotes( $rand )}" ),
368 array( 'ORDER BY' => "job_random {$dir}" )
370 if ( !$row && !$invertedDirection ) {
372 $invertedDirection = true;
373 continue; // try the other direction
375 } else { // table *may* have >= MAX_OFFSET rows
376 // Bug 42614: "ORDER BY job_random" with a job_random inequality causes high CPU
377 // in MySQL if there are many rows for some reason. This uses a small OFFSET
378 // instead of job_random for reducing excess claim retries.
379 $row = $dbw->selectRow( 'job', self
::selectFields(), // find a random job
381 'job_cmd' => $this->type
,
382 'job_token' => '', // unclaimed
385 array( 'OFFSET' => mt_rand( 0, self
::MAX_OFFSET
) )
388 $tinyQueue = true; // we know the queue must have <= MAX_OFFSET rows
389 $this->cache
->set( $this->getCacheKey( 'small' ), 1, 30 );
390 continue; // use job_random
394 if ( $row ) { // claim the job
395 $dbw->update( 'job', // update by PK
397 'job_token' => $uuid,
398 'job_token_timestamp' => $dbw->timestamp(),
399 'job_attempts = job_attempts+1' ),
400 array( 'job_cmd' => $this->type
, 'job_id' => $row->job_id
, 'job_token' => '' ),
403 // This might get raced out by another runner when claiming the previously
404 // selected row. The use of job_random should minimize this problem, however.
405 if ( !$dbw->affectedRows() ) {
406 $row = false; // raced out
409 break; // nothing to do
417 * Reserve a row with a single UPDATE without holding row locks over RTTs...
419 * @param string $uuid 32 char hex string
420 * @return stdClass|bool Row|false
422 protected function claimOldest( $uuid ) {
423 $dbw = $this->getMasterDB();
425 $row = false; // the row acquired
427 if ( $dbw->getType() === 'mysql' ) {
428 // Per http://bugs.mysql.com/bug.php?id=6980, we can't use subqueries on the
429 // same table being changed in an UPDATE query in MySQL (gives Error: 1093).
430 // Oracle and Postgre have no such limitation. However, MySQL offers an
431 // alternative here by supporting ORDER BY + LIMIT for UPDATE queries.
432 $dbw->query( "UPDATE {$dbw->tableName( 'job' )} " .
434 "job_token = {$dbw->addQuotes( $uuid ) }, " .
435 "job_token_timestamp = {$dbw->addQuotes( $dbw->timestamp() )}, " .
436 "job_attempts = job_attempts+1 " .
438 "job_cmd = {$dbw->addQuotes( $this->type )} " .
439 "AND job_token = {$dbw->addQuotes( '' )} " .
440 ") ORDER BY job_id ASC LIMIT 1",
444 // Use a subquery to find the job, within an UPDATE to claim it.
445 // This uses as much of the DB wrapper functions as possible.
448 'job_token' => $uuid,
449 'job_token_timestamp' => $dbw->timestamp(),
450 'job_attempts = job_attempts+1' ),
451 array( 'job_id = (' .
452 $dbw->selectSQLText( 'job', 'job_id',
453 array( 'job_cmd' => $this->type
, 'job_token' => '' ),
455 array( 'ORDER BY' => 'job_id ASC', 'LIMIT' => 1 ) ) .
461 // Fetch any row that we just reserved...
462 if ( $dbw->affectedRows() ) {
463 $row = $dbw->selectRow( 'job', self
::selectFields(),
464 array( 'job_cmd' => $this->type
, 'job_token' => $uuid ), __METHOD__
466 if ( !$row ) { // raced out by duplicate job removal
467 wfDebug( "Row deleted as duplicate by another process.\n" );
470 break; // nothing to do
478 * @see JobQueue::doAck()
480 * @throws MWException
483 protected function doAck( Job
$job ) {
484 if ( !isset( $job->metadata
['id'] ) ) {
485 throw new MWException( "Job of type '{$job->getType()}' has no ID." );
488 $dbw = $this->getMasterDB();
490 $dbw->commit( __METHOD__
, 'flush' ); // flush existing transaction
491 $autoTrx = $dbw->getFlag( DBO_TRX
); // get current setting
492 $dbw->clearFlag( DBO_TRX
); // make each query its own transaction
493 $scopedReset = new ScopedCallback( function () use ( $dbw, $autoTrx ) {
494 $dbw->setFlag( $autoTrx ? DBO_TRX
: 0 ); // restore old setting
497 // Delete a row with a single DELETE without holding row locks over RTTs...
499 array( 'job_cmd' => $this->type
, 'job_id' => $job->metadata
['id'] ), __METHOD__
);
500 } catch ( DBError
$e ) {
501 $this->throwDBException( $e );
508 * @see JobQueue::doDeduplicateRootJob()
510 * @throws MWException
513 protected function doDeduplicateRootJob( Job
$job ) {
514 $params = $job->getParams();
515 if ( !isset( $params['rootJobSignature'] ) ) {
516 throw new MWException( "Cannot register root job; missing 'rootJobSignature'." );
517 } elseif ( !isset( $params['rootJobTimestamp'] ) ) {
518 throw new MWException( "Cannot register root job; missing 'rootJobTimestamp'." );
520 $key = $this->getRootJobCacheKey( $params['rootJobSignature'] );
521 // Callers should call batchInsert() and then this function so that if the insert
522 // fails, the de-duplication registration will be aborted. Since the insert is
523 // deferred till "transaction idle", do the same here, so that the ordering is
524 // maintained. Having only the de-duplication registration succeed would cause
525 // jobs to become no-ops without any actual jobs that made them redundant.
526 $dbw = $this->getMasterDB();
527 $cache = $this->dupCache
;
528 $dbw->onTransactionIdle( function () use ( $cache, $params, $key, $dbw ) {
529 $timestamp = $cache->get( $key ); // current last timestamp of this job
530 if ( $timestamp && $timestamp >= $params['rootJobTimestamp'] ) {
531 return true; // a newer version of this root job was enqueued
534 // Update the timestamp of the last root job started at the location...
535 return $cache->set( $key, $params['rootJobTimestamp'], JobQueueDB
::ROOTJOB_TTL
);
542 * @see JobQueue::doDelete()
545 protected function doDelete() {
546 $dbw = $this->getMasterDB();
548 $dbw->delete( 'job', array( 'job_cmd' => $this->type
) );
549 } catch ( DBError
$e ) {
550 $this->throwDBException( $e );
557 * @see JobQueue::doWaitForBackups()
560 protected function doWaitForBackups() {
567 protected function doGetPeriodicTasks() {
569 'recycleAndDeleteStaleJobs' => array(
570 'callback' => array( $this, 'recycleAndDeleteStaleJobs' ),
571 'period' => ceil( $this->claimTTL
/ 2 )
579 protected function doFlushCaches() {
580 foreach ( array( 'empty', 'size', 'acquiredcount' ) as $type ) {
581 $this->cache
->delete( $this->getCacheKey( $type ) );
586 * @see JobQueue::getAllQueuedJobs()
589 public function getAllQueuedJobs() {
590 $dbr = $this->getSlaveDB();
592 return new MappedIterator(
593 $dbr->select( 'job', self
::selectFields(),
594 array( 'job_cmd' => $this->getType(), 'job_token' => '' ) ),
595 function ( $row ) use ( $dbr ) {
598 Title
::makeTitle( $row->job_namespace
, $row->job_title
),
599 strlen( $row->job_params
) ?
unserialize( $row->job_params
) : false
601 $job->metadata
['id'] = $row->job_id
;
605 } catch ( DBError
$e ) {
606 $this->throwDBException( $e );
610 public function getCoalesceLocationInternal() {
611 return $this->cluster
612 ?
"DBCluster:{$this->cluster}:{$this->wiki}"
613 : "LBFactory:{$this->wiki}";
616 protected function doGetSiblingQueuesWithJobs( array $types ) {
617 $dbr = $this->getSlaveDB();
618 $res = $dbr->select( 'job', 'DISTINCT job_cmd',
619 array( 'job_cmd' => $types ), __METHOD__
);
622 foreach ( $res as $row ) {
623 $types[] = $row->job_cmd
;
629 protected function doGetSiblingQueueSizes( array $types ) {
630 $dbr = $this->getSlaveDB();
631 $res = $dbr->select( 'job', array( 'job_cmd', 'COUNT(*) AS count' ),
632 array( 'job_cmd' => $types ), __METHOD__
, array( 'GROUP BY' => 'job_cmd' ) );
635 foreach ( $res as $row ) {
636 $sizes[$row->job_cmd
] = (int)$row->count
;
643 * Recycle or destroy any jobs that have been claimed for too long
645 * @return int Number of jobs recycled/deleted
647 public function recycleAndDeleteStaleJobs() {
649 $count = 0; // affected rows
650 $dbw = $this->getMasterDB();
653 if ( !$dbw->lock( "jobqueue-recycle-{$this->type}", __METHOD__
, 1 ) ) {
654 return $count; // already in progress
657 // Remove claims on jobs acquired for too long if enabled...
658 if ( $this->claimTTL
> 0 ) {
659 $claimCutoff = $dbw->timestamp( $now - $this->claimTTL
);
660 // Get the IDs of jobs that have be claimed but not finished after too long.
661 // These jobs can be recycled into the queue by expiring the claim. Selecting
662 // the IDs first means that the UPDATE can be done by primary key (less deadlocks).
663 $res = $dbw->select( 'job', 'job_id',
665 'job_cmd' => $this->type
,
666 "job_token != {$dbw->addQuotes( '' )}", // was acquired
667 "job_token_timestamp < {$dbw->addQuotes( $claimCutoff )}", // stale
668 "job_attempts < {$dbw->addQuotes( $this->maxTries )}" ), // retries left
674 }, iterator_to_array( $res )
676 if ( count( $ids ) ) {
677 // Reset job_token for these jobs so that other runners will pick them up.
678 // Set the timestamp to the current time, as it is useful to now that the job
679 // was already tried before (the timestamp becomes the "released" time).
683 'job_token_timestamp' => $dbw->timestamp( $now ) ), // time of release
688 $affected = $dbw->affectedRows();
690 JobQueue
::incrStats( 'job-recycle', $this->type
, $affected, $this->wiki
);
691 $this->cache
->set( $this->getCacheKey( 'empty' ), 'false', self
::CACHE_TTL_LONG
);
695 // Just destroy any stale jobs...
696 $pruneCutoff = $dbw->timestamp( $now - self
::MAX_AGE_PRUNE
);
698 'job_cmd' => $this->type
,
699 "job_token != {$dbw->addQuotes( '' )}", // was acquired
700 "job_token_timestamp < {$dbw->addQuotes( $pruneCutoff )}" // stale
702 if ( $this->claimTTL
> 0 ) { // only prune jobs attempted too many times...
703 $conds[] = "job_attempts >= {$dbw->addQuotes( $this->maxTries )}";
705 // Get the IDs of jobs that are considered stale and should be removed. Selecting
706 // the IDs first means that the UPDATE can be done by primary key (less deadlocks).
707 $res = $dbw->select( 'job', 'job_id', $conds, __METHOD__
);
711 }, iterator_to_array( $res )
713 if ( count( $ids ) ) {
714 $dbw->delete( 'job', array( 'job_id' => $ids ), __METHOD__
);
715 $affected = $dbw->affectedRows();
717 JobQueue
::incrStats( 'job-abandon', $this->type
, $affected, $this->wiki
);
720 $dbw->unlock( "jobqueue-recycle-{$this->type}", __METHOD__
);
721 } catch ( DBError
$e ) {
722 $this->throwDBException( $e );
729 * @param IJobSpecification $job
732 protected function insertFields( IJobSpecification
$job ) {
733 $dbw = $this->getMasterDB();
736 // Fields that describe the nature of the job
737 'job_cmd' => $job->getType(),
738 'job_namespace' => $job->getTitle()->getNamespace(),
739 'job_title' => $job->getTitle()->getDBkey(),
740 'job_params' => self
::makeBlob( $job->getParams() ),
741 // Additional job metadata
742 'job_id' => $dbw->nextSequenceValue( 'job_job_id_seq' ),
743 'job_timestamp' => $dbw->timestamp(),
744 'job_sha1' => wfBaseConvert(
745 sha1( serialize( $job->getDeduplicationInfo() ) ),
748 'job_random' => mt_rand( 0, self
::MAX_JOB_RANDOM
)
753 * @throws JobQueueConnectionError
756 protected function getSlaveDB() {
758 return $this->getDB( DB_SLAVE
);
759 } catch ( DBConnectionError
$e ) {
760 throw new JobQueueConnectionError( "DBConnectionError:" . $e->getMessage() );
765 * @throws JobQueueConnectionError
768 protected function getMasterDB() {
770 return $this->getDB( DB_MASTER
);
771 } catch ( DBConnectionError
$e ) {
772 throw new JobQueueConnectionError( "DBConnectionError:" . $e->getMessage() );
777 * @param $index integer (DB_SLAVE/DB_MASTER)
780 protected function getDB( $index ) {
781 $lb = ( $this->cluster
!== false )
782 ?
wfGetLBFactory()->getExternalLB( $this->cluster
, $this->wiki
)
783 : wfGetLB( $this->wiki
);
785 return $lb->getConnectionRef( $index, array(), $this->wiki
);
792 private function getCacheKey( $property ) {
793 list( $db, $prefix ) = wfSplitWikiID( $this->wiki
);
794 $cluster = is_string( $this->cluster
) ?
$this->cluster
: 'main';
796 return wfForeignMemcKey( $db, $prefix, 'jobqueue', $cluster, $this->type
, $property );
803 protected static function makeBlob( $params ) {
804 if ( $params !== false ) {
805 return serialize( $params );
815 protected static function extractBlob( $blob ) {
816 if ( (string)$blob !== '' ) {
817 return unserialize( $blob );
825 * @throws JobQueueError
827 protected function throwDBException( DBError
$e ) {
828 throw new JobQueueError( get_class( $e ) . ": " . $e->getMessage() );
832 * Return the list of job fields that should be selected.
836 public static function selectFields() {
847 'job_token_timestamp',