summaryrefslogtreecommitdiff
path: root/includes/job/JobQueue.php
blob: 6556ee85c7e2de3f5e99af340a295e8836cbf24e (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
<?php
/**
 * Job queue base 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
 * @defgroup JobQueue JobQueue
 * @author Aaron Schulz
 */

/**
 * Class to handle enqueueing and running of background jobs
 *
 * @ingroup JobQueue
 * @since 1.21
 */
abstract class JobQueue {
	protected $wiki; // string; wiki ID
	protected $type; // string; job type
	protected $order; // string; job priority for pop()
	protected $claimTTL; // integer; seconds
	protected $maxTries; // integer; maximum number of times to try a job
	protected $checkDelay; // boolean; allow delayed jobs

	/** @var BagOStuff */
	protected $dupCache;

	const QOS_ATOMIC = 1; // integer; "all-or-nothing" job insertions

	const ROOTJOB_TTL = 2419200; // integer; seconds to remember root jobs (28 days)

	/**
	 * @param $params array
	 */
	protected function __construct( array $params ) {
		$this->wiki = $params['wiki'];
		$this->type = $params['type'];
		$this->claimTTL = isset( $params['claimTTL'] ) ? $params['claimTTL'] : 0;
		$this->maxTries = isset( $params['maxTries'] ) ? $params['maxTries'] : 3;
		if ( isset( $params['order'] ) && $params['order'] !== 'any' ) {
			$this->order = $params['order'];
		} else {
			$this->order = $this->optimalOrder();
		}
		if ( !in_array( $this->order, $this->supportedOrders() ) ) {
			throw new MWException( __CLASS__ . " does not support '{$this->order}' order." );
		}
		$this->checkDelay = !empty( $params['checkDelay'] );
		if ( $this->checkDelay && !$this->supportsDelayedJobs() ) {
			throw new MWException( __CLASS__ . " does not support delayed jobs." );
		}
		$this->dupCache = wfGetCache( CACHE_ANYTHING );
	}

	/**
	 * Get a job queue object of the specified type.
	 * $params includes:
	 *   - class      : What job class to use (determines job type)
	 *   - wiki       : wiki ID of the wiki the jobs are for (defaults to current wiki)
	 *   - type       : The name of the job types this queue handles
	 *   - order      : Order that pop() selects jobs, one of "fifo", "timestamp" or "random".
	 *                  If "fifo" is used, the queue will effectively be FIFO. Note that job
	 *                  completion will not appear to be exactly FIFO if there are multiple
	 *                  job runners since jobs can take different times to finish once popped.
	 *                  If "timestamp" is used, the queue will at least be loosely ordered
	 *                  by timestamp, allowing for some jobs to be popped off out of order.
	 *                  If "random" is used, pop() will pick jobs in random order.
	 *                  Note that it may only be weakly random (e.g. a lottery of the oldest X).
	 *                  If "any" is choosen, the queue will use whatever order is the fastest.
	 *                  This might be useful for improving concurrency for job acquisition.
	 *   - claimTTL   : If supported, the queue will recycle jobs that have been popped
	 *                  but not acknowledged as completed after this many seconds. Recycling
	 *                  of jobs simple means re-inserting them into the queue. Jobs can be
	 *                  attempted up to three times before being discarded.
	 *   - checkDelay : If supported, respect Job::getReleaseTimestamp() in the push functions.
	 *                  This lets delayed jobs wait in a staging area until a given timestamp is
	 *                  reached, at which point they will enter the queue. If this is not enabled
	 *                  or not supported, an exception will be thrown on delayed job insertion.
	 *
	 * Queue classes should throw an exception if they do not support the options given.
	 *
	 * @param $params array
	 * @return JobQueue
	 * @throws MWException
	 */
	final public static function factory( array $params ) {
		$class = $params['class'];
		if ( !class_exists( $class ) ) {
			throw new MWException( "Invalid job queue class '$class'." );
		}
		$obj = new $class( $params );
		if ( !( $obj instanceof self ) ) {
			throw new MWException( "Class '$class' is not a " . __CLASS__ . " class." );
		}
		return $obj;
	}

	/**
	 * @return string Wiki ID
	 */
	final public function getWiki() {
		return $this->wiki;
	}

	/**
	 * @return string Job type that this queue handles
	 */
	final public function getType() {
		return $this->type;
	}

	/**
	 * @return string One of (random, timestamp, fifo, undefined)
	 */
	final public function getOrder() {
		return $this->order;
	}

	/**
	 * @return bool Whether delayed jobs are enabled
	 * @since 1.22
	 */
	final public function delayedJobsEnabled() {
		return $this->checkDelay;
	}

	/**
	 * Get the allowed queue orders for configuration validation
	 *
	 * @return Array Subset of (random, timestamp, fifo, undefined)
	 */
	abstract protected function supportedOrders();

	/**
	 * Get the default queue order to use if configuration does not specify one
	 *
	 * @return string One of (random, timestamp, fifo, undefined)
	 */
	abstract protected function optimalOrder();

	/**
	 * Find out if delayed jobs are supported for configuration validation
	 *
	 * @return boolean Whether delayed jobs are supported
	 */
	protected function supportsDelayedJobs() {
		return false; // not implemented
	}

	/**
	 * Quickly check if the queue has no available (unacquired, non-delayed) jobs.
	 * Queue classes should use caching if they are any slower without memcached.
	 *
	 * If caching is used, this might return false when there are actually no jobs.
	 * If pop() is called and returns false then it should correct the cache. Also,
	 * calling flushCaches() first prevents this. However, this affect is typically
	 * not distinguishable from the race condition between isEmpty() and pop().
	 *
	 * @return bool
	 * @throws JobQueueError
	 */
	final public function isEmpty() {
		wfProfileIn( __METHOD__ );
		$res = $this->doIsEmpty();
		wfProfileOut( __METHOD__ );
		return $res;
	}

	/**
	 * @see JobQueue::isEmpty()
	 * @return bool
	 */
	abstract protected function doIsEmpty();

	/**
	 * Get the number of available (unacquired, non-delayed) jobs in the queue.
	 * Queue classes should use caching if they are any slower without memcached.
	 *
	 * If caching is used, this number might be out of date for a minute.
	 *
	 * @return integer
	 * @throws JobQueueError
	 */
	final public function getSize() {
		wfProfileIn( __METHOD__ );
		$res = $this->doGetSize();
		wfProfileOut( __METHOD__ );
		return $res;
	}

	/**
	 * @see JobQueue::getSize()
	 * @return integer
	 */
	abstract protected function doGetSize();

	/**
	 * Get the number of acquired jobs (these are temporarily out of the queue).
	 * Queue classes should use caching if they are any slower without memcached.
	 *
	 * If caching is used, this number might be out of date for a minute.
	 *
	 * @return integer
	 * @throws JobQueueError
	 */
	final public function getAcquiredCount() {
		wfProfileIn( __METHOD__ );
		$res = $this->doGetAcquiredCount();
		wfProfileOut( __METHOD__ );
		return $res;
	}

	/**
	 * @see JobQueue::getAcquiredCount()
	 * @return integer
	 */
	abstract protected function doGetAcquiredCount();

	/**
	 * Get the number of delayed jobs (these are temporarily out of the queue).
	 * Queue classes should use caching if they are any slower without memcached.
	 *
	 * If caching is used, this number might be out of date for a minute.
	 *
	 * @return integer
	 * @throws JobQueueError
	 * @since 1.22
	 */
	final public function getDelayedCount() {
		wfProfileIn( __METHOD__ );
		$res = $this->doGetDelayedCount();
		wfProfileOut( __METHOD__ );
		return $res;
	}

	/**
	 * @see JobQueue::getDelayedCount()
	 * @return integer
	 */
	protected function doGetDelayedCount() {
		return 0; // not implemented
	}

	/**
	 * Get the number of acquired jobs that can no longer be attempted.
	 * Queue classes should use caching if they are any slower without memcached.
	 *
	 * If caching is used, this number might be out of date for a minute.
	 *
	 * @return integer
	 * @throws JobQueueError
	 */
	final public function getAbandonedCount() {
		wfProfileIn( __METHOD__ );
		$res = $this->doGetAbandonedCount();
		wfProfileOut( __METHOD__ );
		return $res;
	}

	/**
	 * @see JobQueue::getAbandonedCount()
	 * @return integer
	 */
	protected function doGetAbandonedCount() {
		return 0; // not implemented
	}

	/**
	 * Push a single jobs into the queue.
	 * This does not require $wgJobClasses to be set for the given job type.
	 * Outside callers should use JobQueueGroup::push() instead of this function.
	 *
	 * @param $jobs Job|Array
	 * @param $flags integer Bitfield (supports JobQueue::QOS_ATOMIC)
	 * @return bool Returns false on failure
	 * @throws JobQueueError
	 */
	final public function push( $jobs, $flags = 0 ) {
		return $this->batchPush( is_array( $jobs ) ? $jobs : array( $jobs ), $flags );
	}

	/**
	 * Push a batch of jobs into the queue.
	 * This does not require $wgJobClasses to be set for the given job type.
	 * Outside callers should use JobQueueGroup::push() instead of this function.
	 *
	 * @param array $jobs List of Jobs
	 * @param $flags integer Bitfield (supports JobQueue::QOS_ATOMIC)
	 * @return bool Returns false on failure
	 * @throws JobQueueError
	 */
	final public function batchPush( array $jobs, $flags = 0 ) {
		if ( !count( $jobs ) ) {
			return true; // nothing to do
		}

		foreach ( $jobs as $job ) {
			if ( $job->getType() !== $this->type ) {
				throw new MWException(
					"Got '{$job->getType()}' job; expected a '{$this->type}' job." );
			} elseif ( $job->getReleaseTimestamp() && !$this->checkDelay ) {
				throw new MWException(
					"Got delayed '{$job->getType()}' job; delays are not supported." );
			}
		}

		wfProfileIn( __METHOD__ );
		$ok = $this->doBatchPush( $jobs, $flags );
		wfProfileOut( __METHOD__ );
		return $ok;
	}

	/**
	 * @see JobQueue::batchPush()
	 * @return bool
	 */
	abstract protected function doBatchPush( array $jobs, $flags );

	/**
	 * Pop a job off of the queue.
	 * This requires $wgJobClasses to be set for the given job type.
	 * Outside callers should use JobQueueGroup::pop() instead of this function.
	 *
	 * @return Job|bool Returns false if there are no jobs
	 * @throws JobQueueError
	 */
	final public function pop() {
		global $wgJobClasses;

		if ( $this->wiki !== wfWikiID() ) {
			throw new MWException( "Cannot pop '{$this->type}' job off foreign wiki queue." );
		} elseif ( !isset( $wgJobClasses[$this->type] ) ) {
			// Do not pop jobs if there is no class for the queue type
			throw new MWException( "Unrecognized job type '{$this->type}'." );
		}

		wfProfileIn( __METHOD__ );
		$job = $this->doPop();
		wfProfileOut( __METHOD__ );

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

		return $job;
	}

	/**
	 * @see JobQueue::pop()
	 * @return Job
	 */
	abstract protected function doPop();

	/**
	 * Acknowledge that a job was completed.
	 *
	 * This does nothing for certain queue classes or if "claimTTL" is not set.
	 * Outside callers should use JobQueueGroup::ack() instead of this function.
	 *
	 * @param $job Job
	 * @return bool
	 * @throws JobQueueError
	 */
	final public function ack( Job $job ) {
		if ( $job->getType() !== $this->type ) {
			throw new MWException( "Got '{$job->getType()}' job; expected '{$this->type}'." );
		}
		wfProfileIn( __METHOD__ );
		$ok = $this->doAck( $job );
		wfProfileOut( __METHOD__ );
		return $ok;
	}

	/**
	 * @see JobQueue::ack()
	 * @return bool
	 */
	abstract protected function doAck( Job $job );

	/**
	 * Register the "root job" of a given job into the queue for de-duplication.
	 * This should only be called right *after* all the new jobs have been inserted.
	 * This is used to turn older, duplicate, job entries into no-ops. The root job
	 * information will remain in the registry until it simply falls out of cache.
	 *
	 * This requires that $job has two special fields in the "params" array:
	 *   - rootJobSignature : hash (e.g. SHA1) that identifies the task
	 *   - rootJobTimestamp : TS_MW timestamp of this instance of the task
	 *
	 * A "root job" is a conceptual job that consist of potentially many smaller jobs
	 * that are actually inserted into the queue. For example, "refreshLinks" jobs are
	 * spawned when a template is edited. One can think of the task as "update links
	 * of pages that use template X" and an instance of that task as a "root job".
	 * However, what actually goes into the queue are potentially many refreshLinks2 jobs.
	 * Since these jobs include things like page ID ranges and DB master positions, and morph
	 * into smaller refreshLinks2 jobs recursively, simple duplicate detection (like job_sha1)
	 * for individual jobs being identical is not useful.
	 *
	 * In the case of "refreshLinks", if these jobs are still in the queue when the template
	 * is edited again, we want all of these old refreshLinks jobs for that template to become
	 * no-ops. This can greatly reduce server load, since refreshLinks jobs involves parsing.
	 * Essentially, the new batch of jobs belong to a new "root job" and the older ones to a
	 * previous "root job" for the same task of "update links of pages that use template X".
	 *
	 * This does nothing for certain queue classes.
	 *
	 * @param $job Job
	 * @return bool
	 * @throws JobQueueError
	 */
	final public function deduplicateRootJob( Job $job ) {
		if ( $job->getType() !== $this->type ) {
			throw new MWException( "Got '{$job->getType()}' job; expected '{$this->type}'." );
		}
		wfProfileIn( __METHOD__ );
		$ok = $this->doDeduplicateRootJob( $job );
		wfProfileOut( __METHOD__ );
		return $ok;
	}

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

		$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.
		$timestamp = $this->dupCache->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 $this->dupCache->set( $key, $params['rootJobTimestamp'], JobQueueDB::ROOTJOB_TTL );
	}

	/**
	 * Check if the "root" job of a given job has been superseded by a newer one
	 *
	 * @param $job Job
	 * @return bool
	 * @throws JobQueueError
	 */
	final protected function isRootJobOldDuplicate( Job $job ) {
		if ( $job->getType() !== $this->type ) {
			throw new MWException( "Got '{$job->getType()}' job; expected '{$this->type}'." );
		}
		wfProfileIn( __METHOD__ );
		$isDuplicate = $this->doIsRootJobOldDuplicate( $job );
		wfProfileOut( __METHOD__ );
		return $isDuplicate;
	}

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

		$key = $this->getRootJobCacheKey( $params['rootJobSignature'] );
		// Get the last time this root job was enqueued
		$timestamp = $this->dupCache->get( $key );

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

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

	/**
	 * Deleted all unclaimed and delayed jobs from the queue
	 *
	 * @return bool Success
	 * @throws JobQueueError
	 * @since 1.22
	 */
	final public function delete() {
		wfProfileIn( __METHOD__ );
		$res = $this->doDelete();
		wfProfileOut( __METHOD__ );
		return $res;
	}

	/**
	 * @see JobQueue::delete()
	 * @return bool Success
	 */
	protected function doDelete() {
		throw new MWException( "This method is not implemented." );
	}

	/**
	 * Wait for any slaves or backup servers to catch up.
	 *
	 * This does nothing for certain queue classes.
	 *
	 * @return void
	 * @throws JobQueueError
	 */
	final public function waitForBackups() {
		wfProfileIn( __METHOD__ );
		$this->doWaitForBackups();
		wfProfileOut( __METHOD__ );
	}

	/**
	 * @see JobQueue::waitForBackups()
	 * @return void
	 */
	protected function doWaitForBackups() {}

	/**
	 * Return a map of task names to task definition maps.
	 * A "task" is a fast periodic queue maintenance action.
	 * Mutually exclusive tasks must implement their own locking in the callback.
	 *
	 * Each task value is an associative array with:
	 *   - name     : the name of the task
	 *   - callback : a PHP callable that performs the task
	 *   - period   : the period in seconds corresponding to the task frequency
	 *
	 * @return Array
	 */
	final public function getPeriodicTasks() {
		$tasks = $this->doGetPeriodicTasks();
		foreach ( $tasks as $name => &$def ) {
			$def['name'] = $name;
		}
		return $tasks;
	}

	/**
	 * @see JobQueue::getPeriodicTasks()
	 * @return Array
	 */
	protected function doGetPeriodicTasks() {
		return array();
	}

	/**
	 * Clear any process and persistent caches
	 *
	 * @return void
	 */
	final public function flushCaches() {
		wfProfileIn( __METHOD__ );
		$this->doFlushCaches();
		wfProfileOut( __METHOD__ );
	}

	/**
	 * @see JobQueue::flushCaches()
	 * @return void
	 */
	protected function doFlushCaches() {}

	/**
	 * Get an iterator to traverse over all available jobs in this queue.
	 * This does not include jobs that are currently acquired or delayed.
	 * Note: results may be stale if the queue is concurrently modified.
	 *
	 * @return Iterator
	 * @throws JobQueueError
	 */
	abstract public function getAllQueuedJobs();

	/**
	 * Get an iterator to traverse over all delayed jobs in this queue.
	 * Note: results may be stale if the queue is concurrently modified.
	 *
	 * @return Iterator
	 * @throws JobQueueError
	 * @since 1.22
	 */
	public function getAllDelayedJobs() {
		return new ArrayIterator( array() ); // not implemented
	}

	/**
	 * Do not use this function outside of JobQueue/JobQueueGroup
	 *
	 * @return string
	 * @since 1.22
	 */
	public function getCoalesceLocationInternal() {
		return null;
	}

	/**
	 * Check whether each of the given queues are empty.
	 * This is used for batching checks for queues stored at the same place.
	 *
	 * @param array $types List of queues types
	 * @return array|null (list of non-empty queue types) or null if unsupported
	 * @throws MWException
	 * @since 1.22
	 */
	final public function getSiblingQueuesWithJobs( array $types ) {
		$section = new ProfileSection( __METHOD__ );
		return $this->doGetSiblingQueuesWithJobs( $types );
	}

	/**
	 * @see JobQueue::getSiblingQueuesWithJobs()
	 * @param array $types List of queues types
	 * @return array|null (list of queue types) or null if unsupported
	 */
	protected function doGetSiblingQueuesWithJobs( array $types ) {
		return null; // not supported
	}

	/**
	 * Check the size of each of the given queues.
	 * For queues not served by the same store as this one, 0 is returned.
	 * This is used for batching checks for queues stored at the same place.
	 *
	 * @param array $types List of queues types
	 * @return array|null (job type => whether queue is empty) or null if unsupported
	 * @throws MWException
	 * @since 1.22
	 */
	final public function getSiblingQueueSizes( array $types ) {
		$section = new ProfileSection( __METHOD__ );
		return $this->doGetSiblingQueueSizes( $types );
	}

	/**
	 * @see JobQueue::getSiblingQueuesSize()
	 * @param array $types List of queues types
	 * @return array|null (list of queue types) or null if unsupported
	 */
	protected function doGetSiblingQueueSizes( array $types ) {
		return null; // not supported
	}

	/**
	 * Call wfIncrStats() for the queue overall and for the queue type
	 *
	 * @param string $key Event type
	 * @param string $type Job type
	 * @param integer $delta
	 * @since 1.22
	 */
	public static function incrStats( $key, $type, $delta = 1 ) {
		wfIncrStats( $key, $delta );
		wfIncrStats( "{$key}-{$type}", $delta );
	}

	/**
	 * Namespace the queue with a key to isolate it for testing
	 *
	 * @param $key string
	 * @return void
	 * @throws MWException
	 */
	public function setTestingPrefix( $key ) {
		throw new MWException( "Queue namespacing not supported for this queue type." );
	}
}

/**
 * @ingroup JobQueue
 * @since 1.22
 */
class JobQueueError extends MWException {}
class JobQueueConnectionError extends JobQueueError {}