includes/job/JobQueueDB.php

   1 <?php
   2 /**
   3  * Database-backed job queue code.
   4  *
   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.
   9  *
  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.
  14  *
  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
  19  *
  20  * @file
  21  * @author Aaron Schulz
  22  */
  23
  24 /**
  25  * Class to handle job queues stored in the DB
  26  *
  27  * @ingroup JobQueue
  28  * @since 1.21
  29  */
  30 class JobQueueDB extends JobQueue {
  31         const ROOTJOB_TTL = 1209600; // integer; seconds to remember root jobs (14 days)
  32         const CACHE_TTL_SHORT = 30; // integer; seconds to cache info without re-validating
  33         const CACHE_TTL_LONG = 300; // integer; seconds to cache info that is kept up to date
  34         const MAX_AGE_PRUNE = 604800; // integer; seconds a job can live once claimed
  35         const MAX_JOB_RANDOM = 2147483647; // integer; 2^31 - 1, used for job_random
  36         const MAX_OFFSET = 255; // integer; maximum number of rows to skip
  37
  38         /** @var BagOStuff */
  39         protected $cache;
  40
  41         protected $cluster = false; // string; name of an external DB cluster
  42
  43         /**
  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 $params array
  50          */
  51         protected function __construct( array $params ) {
  52                 parent::__construct( $params );
  53                 $this->cluster = isset( $params['cluster'] ) ? $params['cluster'] : false;
  54                 $this->cache = wfGetMainCache();
  55         }
  56
  57         protected function supportedOrders() {
  58                 return array( 'random', 'timestamp', 'fifo' );
  59         }
  60
  61         protected function optimalOrder() {
  62                 return 'random';
  63         }
  64
  65         /**
  66          * @see JobQueue::doIsEmpty()
  67          * @return bool
  68          */
  69         protected function doIsEmpty() {
  70                 $key = $this->getCacheKey( 'empty' );
  71
  72                 $isEmpty = $this->cache->get( $key );
  73                 if ( $isEmpty === 'true' ) {
  74                         return true;
  75                 } elseif ( $isEmpty === 'false' ) {
  76                         return false;
  77                 }
  78
  79                 list( $dbr, $scope ) = $this->getSlaveDB();
  80                 $found = $dbr->selectField( // unclaimed job
  81                         'job', '1', array( 'job_cmd' => $this->type, 'job_token' => '' ), __METHOD__
  82                 );
  83                 $this->cache->add( $key, $found ? 'false' : 'true', self::CACHE_TTL_LONG );
  84
  85                 return !$found;
  86         }
  87
  88         /**
  89          * @see JobQueue::doGetSize()
  90          * @return integer
  91          */
  92         protected function doGetSize() {
  93                 $key = $this->getCacheKey( 'size' );
  94
  95                 $size = $this->cache->get( $key );
  96                 if ( is_int( $size ) ) {
  97                         return $size;
  98                 }
  99
 100                 list( $dbr, $scope ) = $this->getSlaveDB();
 101                 $size = (int)$dbr->selectField( 'job', 'COUNT(*)',
 102                         array( 'job_cmd' => $this->type, 'job_token' => '' ),
 103                         __METHOD__
 104                 );
 105                 $this->cache->set( $key, $size, self::CACHE_TTL_SHORT );
 106
 107                 return $size;
 108         }
 109
 110         /**
 111          * @see JobQueue::doGetAcquiredCount()
 112          * @return integer
 113          */
 114         protected function doGetAcquiredCount() {
 115                 if ( $this->claimTTL <= 0 ) {
 116                         return 0; // no acknowledgements
 117                 }
 118
 119                 $key = $this->getCacheKey( 'acquiredcount' );
 120
 121                 $count = $this->cache->get( $key );
 122                 if ( is_int( $count ) ) {
 123                         return $count;
 124                 }
 125
 126                 list( $dbr, $scope ) = $this->getSlaveDB();
 127                 $count = (int)$dbr->selectField( 'job', 'COUNT(*)',
 128                         array( 'job_cmd' => $this->type, "job_token != {$dbr->addQuotes( '' )}" ),
 129                         __METHOD__
 130                 );
 131                 $this->cache->set( $key, $count, self::CACHE_TTL_SHORT );
 132
 133                 return $count;
 134         }
 135
 136         /**
 137          * @see JobQueue::doBatchPush()
 138          * @param array $jobs
 139          * @param $flags
 140          * @throws DBError|Exception
 141          * @return bool
 142          */
 143         protected function doBatchPush( array $jobs, $flags ) {
 144                 if ( count( $jobs ) ) {
 145                         list( $dbw, $scope ) = $this->getMasterDB();
 146
 147                         $rowSet = array(); // (sha1 => job) map for jobs that are de-duplicated
 148                         $rowList = array(); // list of jobs for jobs that are are not de-duplicated
 149
 150                         foreach ( $jobs as $job ) {
 151                                 $row = $this->insertFields( $job );
 152                                 if ( $job->ignoreDuplicates() ) {
 153                                         $rowSet[$row['job_sha1']] = $row;
 154                                 } else {
 155                                         $rowList[] = $row;
 156                                 }
 157                         }
 158
 159                         $key = $this->getCacheKey( 'empty' );
 160                         $atomic = ( $flags & self::QoS_Atomic );
 161                         $cache = $this->cache;
 162
 163                         $dbw->onTransactionIdle(
 164                                 function() use ( $dbw, $cache, $rowSet, $rowList, $atomic, $key, $scope
 165                         ) {
 166                                 if ( $atomic ) {
 167                                         $dbw->begin( __METHOD__ ); // wrap all the job additions in one transaction
 168                                 }
 169                                 try {
 170                                         // Strip out any duplicate jobs that are already in the queue...
 171                                         if ( count( $rowSet ) ) {
 172                                                 $res = $dbw->select( 'job', 'job_sha1',
 173                                                         array(
 174                                                                 // No job_type condition since it's part of the job_sha1 hash
 175                                                                 'job_sha1'  => array_keys( $rowSet ),
 176                                                                 'job_token' => '' // unclaimed
 177                                                         ),
 178                                                         __METHOD__
 179                                                 );
 180                                                 foreach ( $res as $row ) {
 181                                                         wfDebug( "Job with hash '{$row->job_sha1}' is a duplicate." );
 182                                                         unset( $rowSet[$row->job_sha1] ); // already enqueued
 183                                                 }
 184                                         }
 185                                         // Build the full list of job rows to insert
 186                                         $rows = array_merge( $rowList, array_values( $rowSet ) );
 187                                         // Insert the job rows in chunks to avoid slave lag...
 188                                         foreach ( array_chunk( $rows, 50 ) as $rowBatch ) {
 189                                                 $dbw->insert( 'job', $rowBatch, __METHOD__ );
 190                                         }
 191                                         wfIncrStats( 'job-insert', count( $rows ) );
 192                                         wfIncrStats( 'job-insert-duplicate',
 193                                                 count( $rowSet ) + count( $rowList ) - count( $rows ) );
 194                                 } catch ( DBError $e ) {
 195                                         if ( $atomic ) {
 196                                                 $dbw->rollback( __METHOD__ );
 197                                         }
 198                                         throw $e;
 199                                 }
 200                                 if ( $atomic ) {
 201                                         $dbw->commit( __METHOD__ );
 202                                 }
 203
 204                                 $cache->set( $key, 'false', JobQueueDB::CACHE_TTL_LONG );
 205                         } );
 206                 }
 207
 208                 return true;
 209         }
 210
 211         /**
 212          * @see JobQueue::doPop()
 213          * @return Job|bool
 214          */
 215         protected function doPop() {
 216                 if ( $this->cache->get( $this->getCacheKey( 'empty' ) ) === 'true' ) {
 217                         return false; // queue is empty
 218                 }
 219
 220                 list( $dbw, $scope ) = $this->getMasterDB();
 221                 $dbw->commit( __METHOD__, 'flush' ); // flush existing transaction
 222
 223                 $uuid = wfRandomString( 32 ); // pop attempt
 224                 $job = false; // job popped off
 225                 do { // retry when our row is invalid or deleted as a duplicate
 226                         // Try to reserve a row in the DB...
 227                         if ( in_array( $this->order, array( 'fifo', 'timestamp' ) ) ) {
 228                                 $row = $this->claimOldest( $uuid );
 229                         } else { // random first
 230                                 $rand = mt_rand( 0, self::MAX_JOB_RANDOM ); // encourage concurrent UPDATEs
 231                                 $gte = (bool)mt_rand( 0, 1 ); // find rows with rand before/after $rand
 232                                 $row = $this->claimRandom( $uuid, $rand, $gte );
 233                         }
 234                         // Check if we found a row to reserve...
 235                         if ( !$row ) {
 236                                 $this->cache->set( $this->getCacheKey( 'empty' ), 'true', self::CACHE_TTL_LONG );
 237                                 break; // nothing to do
 238                         }
 239                         wfIncrStats( 'job-pop' );
 240                         // Get the job object from the row...
 241                         $title = Title::makeTitleSafe( $row->job_namespace, $row->job_title );
 242                         if ( !$title ) {
 243                                 $dbw->delete( 'job', array( 'job_id' => $row->job_id ), __METHOD__ );
 244                                 wfDebugLog( 'JobQueueDB', "Row has invalid title '{$row->job_title}'." );
 245                                 continue; // try again
 246                         }
 247                         $job = Job::factory( $row->job_cmd, $title,
 248                                 self::extractBlob( $row->job_params ), $row->job_id );
 249                         $job->id = $row->job_id; // XXX: work around broken subclasses
 250                         // Flag this job as an old duplicate based on its "root" job...
 251                         if ( $this->isRootJobOldDuplicate( $job ) ) {
 252                                 wfIncrStats( 'job-pop-duplicate' );
 253                                 $job = DuplicateJob::newFromJob( $job ); // convert to a no-op
 254                         }
 255                         break; // done
 256                 } while( true );
 257
 258                 return $job;
 259         }
 260
 261         /**
 262          * Reserve a row with a single UPDATE without holding row locks over RTTs...
 263          *
 264          * @param string $uuid 32 char hex string
 265          * @param $rand integer Random unsigned integer (31 bits)
 266          * @param bool $gte Search for job_random >= $random (otherwise job_random <= $random)
 267          * @return Row|false
 268          */
 269         protected function claimRandom( $uuid, $rand, $gte ) {
 270                 list( $dbw, $scope ) = $this->getMasterDB();
 271                 // Check cache to see if the queue has <= OFFSET items
 272                 $tinyQueue = $this->cache->get( $this->getCacheKey( 'small' ) );
 273
 274                 $row = false; // the row acquired
 275                 $invertedDirection = false; // whether one job_random direction was already scanned
 276                 // This uses a replication safe method for acquiring jobs. One could use UPDATE+LIMIT
 277                 // instead, but that either uses ORDER BY (in which case it deadlocks in MySQL) or is
 278                 // not replication safe. Due to http://bugs.mysql.com/bug.php?id=6980, subqueries cannot
 279                 // be used here with MySQL.
 280                 do {
 281                         if ( $tinyQueue ) { // queue has <= MAX_OFFSET rows
 282                                 // For small queues, using OFFSET will overshoot and return no rows more often.
 283                                 // Instead, this uses job_random to pick a row (possibly checking both directions).
 284                                 $ineq = $gte ? '>=' : '<=';
 285                                 $dir = $gte ? 'ASC' : 'DESC';
 286                                 $row = $dbw->selectRow( 'job', '*', // find a random job
 287                                         array(
 288                                                 'job_cmd'   => $this->type,
 289                                                 'job_token' => '', // unclaimed
 290                                                 "job_random {$ineq} {$dbw->addQuotes( $rand )}" ),
 291                                         __METHOD__,
 292                                         array( 'ORDER BY' => "job_random {$dir}" )
 293                                 );
 294                                 if ( !$row && !$invertedDirection ) {
 295                                         $gte = !$gte;
 296                                         $invertedDirection = true;
 297                                         continue; // try the other direction
 298                                 }
 299                         } else { // table *may* have >= MAX_OFFSET rows
 300                                 // Bug 42614: "ORDER BY job_random" with a job_random inequality causes high CPU
 301                                 // in MySQL if there are many rows for some reason. This uses a small OFFSET
 302                                 // instead of job_random for reducing excess claim retries.
 303                                 $row = $dbw->selectRow( 'job', '*', // find a random job
 304                                         array(
 305                                                 'job_cmd'   => $this->type,
 306                                                 'job_token' => '', // unclaimed
 307                                         ),
 308                                         __METHOD__,
 309                                         array( 'OFFSET' => mt_rand( 0, self::MAX_OFFSET ) )
 310                                 );
 311                                 if ( !$row ) {
 312                                         $tinyQueue = true; // we know the queue must have <= MAX_OFFSET rows
 313                                         $this->cache->set( $this->getCacheKey( 'small' ), 1, 30 );
 314                                         continue; // use job_random
 315                                 }
 316                         }
 317                         if ( $row ) { // claim the job
 318                                 $dbw->update( 'job', // update by PK
 319                                         array(
 320                                                 'job_token'           => $uuid,
 321                                                 'job_token_timestamp' => $dbw->timestamp(),
 322                                                 'job_attempts = job_attempts+1' ),
 323                                         array( 'job_cmd' => $this->type, 'job_id' => $row->job_id, 'job_token' => '' ),
 324                                         __METHOD__
 325                                 );
 326                                 // This might get raced out by another runner when claiming the previously
 327                                 // selected row. The use of job_random should minimize this problem, however.
 328                                 if ( !$dbw->affectedRows() ) {
 329                                         $row = false; // raced out
 330                                 }
 331                         } else {
 332                                 break; // nothing to do
 333                         }
 334                 } while ( !$row );
 335
 336                 return $row;
 337         }
 338
 339         /**
 340          * Reserve a row with a single UPDATE without holding row locks over RTTs...
 341          *
 342          * @param string $uuid 32 char hex string
 343          * @return Row|false
 344          */
 345         protected function claimOldest( $uuid ) {
 346                 list( $dbw, $scope ) = $this->getMasterDB();
 347
 348                 $row = false; // the row acquired
 349                 do {
 350                         if ( $dbw->getType() === 'mysql' ) {
 351                                 // Per http://bugs.mysql.com/bug.php?id=6980, we can't use subqueries on the
 352                                 // same table being changed in an UPDATE query in MySQL (gives Error: 1093).
 353                                 // Oracle and Postgre have no such limitation. However, MySQL offers an
 354                                 // alternative here by supporting ORDER BY + LIMIT for UPDATE queries.
 355                                 $dbw->query( "UPDATE {$dbw->tableName( 'job' )} " .
 356                                         "SET " .
 357                                                 "job_token = {$dbw->addQuotes( $uuid ) }, " .
 358                                                 "job_token_timestamp = {$dbw->addQuotes( $dbw->timestamp() )}, " .
 359                                                 "job_attempts = job_attempts+1 " .
 360                                         "WHERE ( " .
 361                                                 "job_cmd = {$dbw->addQuotes( $this->type )} " .
 362                                                 "AND job_token = {$dbw->addQuotes( '' )} " .
 363                                         ") ORDER BY job_id ASC LIMIT 1",
 364                                         __METHOD__
 365                                 );
 366                         } else {
 367                                 // Use a subquery to find the job, within an UPDATE to claim it.
 368                                 // This uses as much of the DB wrapper functions as possible.
 369                                 $dbw->update( 'job',
 370                                         array(
 371                                                 'job_token'           => $uuid,
 372                                                 'job_token_timestamp' => $dbw->timestamp(),
 373                                                 'job_attempts = job_attempts+1' ),
 374                                         array( 'job_id = (' .
 375                                                 $dbw->selectSQLText( 'job', 'job_id',
 376                                                         array( 'job_cmd' => $this->type, 'job_token' => '' ),
 377                                                         __METHOD__,
 378                                                         array( 'ORDER BY' => 'job_id ASC', 'LIMIT' => 1 ) ) .
 379                                                 ')'
 380                                         ),
 381                                         __METHOD__
 382                                 );
 383                         }
 384                         // Fetch any row that we just reserved...
 385                         if ( $dbw->affectedRows() ) {
 386                                 $row = $dbw->selectRow( 'job', '*',
 387                                         array( 'job_cmd' => $this->type, 'job_token' => $uuid ), __METHOD__
 388                                 );
 389                                 if ( !$row ) { // raced out by duplicate job removal
 390                                         wfDebugLog( 'JobQueueDB', "Row deleted as duplicate by another process." );
 391                                 }
 392                         } else {
 393                                 break; // nothing to do
 394                         }
 395                 } while ( !$row );
 396
 397                 return $row;
 398         }
 399
 400         /**
 401          * Recycle or destroy any jobs that have been claimed for too long
 402          *
 403          * @return integer Number of jobs recycled/deleted
 404          */
 405         public function recycleAndDeleteStaleJobs() {
 406                 $now = time();
 407                 list( $dbw, $scope ) = $this->getMasterDB();
 408                 $count = 0; // affected rows
 409
 410                 if ( !$dbw->lock( "jobqueue-recycle-{$this->type}", __METHOD__, 1 ) ) {
 411                         return $count; // already in progress
 412                 }
 413
 414                 // Remove claims on jobs acquired for too long if enabled...
 415                 if ( $this->claimTTL > 0 ) {
 416                         $claimCutoff = $dbw->timestamp( $now - $this->claimTTL );
 417                         // Get the IDs of jobs that have be claimed but not finished after too long.
 418                         // These jobs can be recycled into the queue by expiring the claim. Selecting
 419                         // the IDs first means that the UPDATE can be done by primary key (less deadlocks).
 420                         $res = $dbw->select( 'job', 'job_id',
 421                                 array(
 422                                         'job_cmd' => $this->type,
 423                                         "job_token != {$dbw->addQuotes( '' )}", // was acquired
 424                                         "job_token_timestamp < {$dbw->addQuotes( $claimCutoff )}", // stale
 425                                         "job_attempts < {$dbw->addQuotes( $this->maxTries )}" ), // retries left
 426                                 __METHOD__
 427                         );
 428                         $ids = array_map( function( $o ) { return $o->job_id; }, iterator_to_array( $res ) );
 429                         if ( count( $ids ) ) {
 430                                 // Reset job_token for these jobs so that other runners will pick them up.
 431                                 // Set the timestamp to the current time, as it is useful to now that the job
 432                                 // was already tried before (the timestamp becomes the "released" time).
 433                                 $dbw->update( 'job',
 434                                         array(
 435                                                 'job_token' => '',
 436                                                 'job_token_timestamp' => $dbw->timestamp( $now ) ), // time of release
 437                                         array(
 438                                                 'job_id' => $ids ),
 439                                         __METHOD__
 440                                 );
 441                                 $count += $dbw->affectedRows();
 442                                 wfIncrStats( 'job-recycle', $dbw->affectedRows() );
 443                                 $this->cache->set( $this->getCacheKey( 'empty' ), 'false', self::CACHE_TTL_LONG );
 444                         }
 445                 }
 446
 447                 // Just destroy any stale jobs...
 448                 $pruneCutoff = $dbw->timestamp( $now - self::MAX_AGE_PRUNE );
 449                 $conds = array(
 450                         'job_cmd' => $this->type,
 451                         "job_token != {$dbw->addQuotes( '' )}", // was acquired
 452                         "job_token_timestamp < {$dbw->addQuotes( $pruneCutoff )}" // stale
 453                 );
 454                 if ( $this->claimTTL > 0 ) { // only prune jobs attempted too many times...
 455                         $conds[] = "job_attempts >= {$dbw->addQuotes( $this->maxTries )}";
 456                 }
 457                 // Get the IDs of jobs that are considered stale and should be removed. Selecting
 458                 // the IDs first means that the UPDATE can be done by primary key (less deadlocks).
 459                 $res = $dbw->select( 'job', 'job_id', $conds, __METHOD__ );
 460                 $ids = array_map( function( $o ) { return $o->job_id; }, iterator_to_array( $res ) );
 461                 if ( count( $ids ) ) {
 462                         $dbw->delete( 'job', array( 'job_id' => $ids ), __METHOD__ );
 463                         $count += $dbw->affectedRows();
 464                 }
 465
 466                 $dbw->unlock( "jobqueue-recycle-{$this->type}", __METHOD__ );
 467
 468                 return $count;
 469         }
 470
 471         /**
 472          * @see JobQueue::doAck()
 473          * @param Job $job
 474          * @throws MWException
 475          * @return Job|bool
 476          */
 477         protected function doAck( Job $job ) {
 478                 if ( !$job->getId() ) {
 479                         throw new MWException( "Job of type '{$job->getType()}' has no ID." );
 480                 }
 481
 482                 list( $dbw, $scope ) = $this->getMasterDB();
 483                 $dbw->commit( __METHOD__, 'flush' ); // flush existing transaction
 484
 485                 // Delete a row with a single DELETE without holding row locks over RTTs...
 486                 $dbw->delete( 'job',
 487                         array( 'job_cmd' => $this->type, 'job_id' => $job->getId() ), __METHOD__ );
 488
 489                 return true;
 490         }
 491
 492         /**
 493          * @see JobQueue::doDeduplicateRootJob()
 494          * @param Job $job
 495          * @throws MWException
 496          * @return bool
 497          */
 498         protected function doDeduplicateRootJob( Job $job ) {
 499                 $params = $job->getParams();
 500                 if ( !isset( $params['rootJobSignature'] ) ) {
 501                         throw new MWException( "Cannot register root job; missing 'rootJobSignature'." );
 502                 } elseif ( !isset( $params['rootJobTimestamp'] ) ) {
 503                         throw new MWException( "Cannot register root job; missing 'rootJobTimestamp'." );
 504                 }
 505                 $key = $this->getRootJobCacheKey( $params['rootJobSignature'] );
 506                 // Callers should call batchInsert() and then this function so that if the insert
 507                 // fails, the de-duplication registration will be aborted. Since the insert is
 508                 // deferred till "transaction idle", do the same here, so that the ordering is
 509                 // maintained. Having only the de-duplication registration succeed would cause
 510                 // jobs to become no-ops without any actual jobs that made them redundant.
 511                 list( $dbw, $scope ) = $this->getMasterDB();
 512                 $cache = $this->cache;
 513                 $dbw->onTransactionIdle( function() use ( $cache, $params, $key, $scope ) {
 514                         $timestamp = $cache->get( $key ); // current last timestamp of this job
 515                         if ( $timestamp && $timestamp >= $params['rootJobTimestamp'] ) {
 516                                 return true; // a newer version of this root job was enqueued
 517                         }
 518
 519                         // Update the timestamp of the last root job started at the location...
 520                         return $cache->set( $key, $params['rootJobTimestamp'], JobQueueDB::ROOTJOB_TTL );
 521                 } );
 522
 523                 return true;
 524         }
 525
 526         /**
 527          * Check if the "root" job of a given job has been superseded by a newer one
 528          *
 529          * @param $job Job
 530          * @return bool
 531          */
 532         protected function isRootJobOldDuplicate( Job $job ) {
 533                 $params = $job->getParams();
 534                 if ( !isset( $params['rootJobSignature'] ) ) {
 535                         return false; // job has no de-deplication info
 536                 } elseif ( !isset( $params['rootJobTimestamp'] ) ) {
 537                         trigger_error( "Cannot check root job; missing 'rootJobTimestamp'." );
 538                         return false;
 539                 }
 540
 541                 // Get the last time this root job was enqueued
 542                 $timestamp = $this->cache->get( $this->getRootJobCacheKey( $params['rootJobSignature'] ) );
 543
 544                 // Check if a new root job was started at the location after this one's...
 545                 return ( $timestamp && $timestamp > $params['rootJobTimestamp'] );
 546         }
 547
 548         /**
 549          * @see JobQueue::doWaitForBackups()
 550          * @return void
 551          */
 552         protected function doWaitForBackups() {
 553                 wfWaitForSlaves();
 554         }
 555
 556         /**
 557          * @return Array
 558          */
 559         protected function doGetPeriodicTasks() {
 560                 return array(
 561                         'recycleAndDeleteStaleJobs' => array(
 562                                 'callback' => array( $this, 'recycleAndDeleteStaleJobs' ),
 563                                 'period'   => ceil( $this->claimTTL / 2 )
 564                         )
 565                 );
 566         }
 567
 568         /**
 569          * @return void
 570          */
 571         protected function doFlushCaches() {
 572                 foreach ( array( 'empty', 'size', 'acquiredcount' ) as $type ) {
 573                         $this->cache->delete( $this->getCacheKey( $type ) );
 574                 }
 575         }
 576
 577         /**
 578          * @see JobQueue::getAllQueuedJobs()
 579          * @return Iterator
 580          */
 581         public function getAllQueuedJobs() {
 582                 list( $dbr, $scope ) = $this->getSlaveDB();
 583                 return new MappedIterator(
 584                         $dbr->select( 'job', '*', array( 'job_cmd' => $this->getType(), 'job_token' => '' ) ),
 585                         function( $row ) use ( $scope ) {
 586                                 $job = Job::factory(
 587                                         $row->job_cmd,
 588                                         Title::makeTitle( $row->job_namespace, $row->job_title ),
 589                                         strlen( $row->job_params ) ? unserialize( $row->job_params ) : false,
 590                                         $row->job_id
 591                                 );
 592                                 $job->id = $row->job_id; // XXX: work around broken subclasses
 593                                 return $job;
 594                         }
 595                 );
 596         }
 597
 598         /**
 599          * @return Array (DatabaseBase, ScopedCallback)
 600          */
 601         protected function getSlaveDB() {
 602                 return $this->getDB( DB_SLAVE );
 603         }
 604
 605         /**
 606          * @return Array (DatabaseBase, ScopedCallback)
 607          */
 608         protected function getMasterDB() {
 609                 return $this->getDB( DB_MASTER );
 610         }
 611
 612         /**
 613          * @param $index integer (DB_SLAVE/DB_MASTER)
 614          * @return Array (DatabaseBase, ScopedCallback)
 615          */
 616         protected function getDB( $index ) {
 617                 $lb = ( $this->cluster !== false )
 618                         ? wfGetLBFactory()->getExternalLB( $this->cluster, $this->wiki )
 619                         : wfGetLB( $this->wiki );
 620                 $conn = $lb->getConnection( $index, array(), $this->wiki );
 621                 return array(
 622                         $conn,
 623                         new ScopedCallback( function() use ( $lb, $conn ) {
 624                                 $lb->reuseConnection( $conn );
 625                         } )
 626                 );
 627         }
 628
 629         /**
 630          * @param $job Job
 631          * @return array
 632          */
 633         protected function insertFields( Job $job ) {
 634                 list( $dbw, $scope ) = $this->getMasterDB();
 635                 return array(
 636                         // Fields that describe the nature of the job
 637                         'job_cmd'       => $job->getType(),
 638                         'job_namespace' => $job->getTitle()->getNamespace(),
 639                         'job_title'     => $job->getTitle()->getDBkey(),
 640                         'job_params'    => self::makeBlob( $job->getParams() ),
 641                         // Additional job metadata
 642                         'job_id'        => $dbw->nextSequenceValue( 'job_job_id_seq' ),
 643                         'job_timestamp' => $dbw->timestamp(),
 644                         'job_sha1'      => wfBaseConvert(
 645                                 sha1( serialize( $job->getDeduplicationInfo() ) ),
 646                                 16, 36, 31
 647                         ),
 648                         'job_random'    => mt_rand( 0, self::MAX_JOB_RANDOM )
 649                 );
 650         }
 651
 652         /**
 653          * @return string
 654          */
 655         private function getCacheKey( $property ) {
 656                 list( $db, $prefix ) = wfSplitWikiID( $this->wiki );
 657                 return wfForeignMemcKey( $db, $prefix, 'jobqueue', $this->type, $property );
 658         }
 659
 660         /**
 661          * @param string $signature Hash identifier of the root job
 662          * @return string
 663          */
 664         private function getRootJobCacheKey( $signature ) {
 665                 list( $db, $prefix ) = wfSplitWikiID( $this->wiki );
 666                 return wfForeignMemcKey( $db, $prefix, 'jobqueue', $this->type, 'rootjob', $signature );
 667         }
 668
 669         /**
 670          * @param $params
 671          * @return string
 672          */
 673         protected static function makeBlob( $params ) {
 674                 if ( $params !== false ) {
 675                         return serialize( $params );
 676                 } else {
 677                         return '';
 678                 }
 679         }
 680
 681         /**
 682          * @param $blob
 683          * @return bool|mixed
 684          */
 685         protected static function extractBlob( $blob ) {
 686                 if ( (string)$blob !== '' ) {
 687                         return unserialize( $blob );
 688                 } else {
 689                         return false;
 690                 }
 691         }
 692 }