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 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
  36
  37         /** @var BagOStuff */
  38         protected $cache;
  39
  40         protected $cluster = false; // string; name of an external DB cluster
  41
  42         /**
  43          * Additional parameters include:
  44          *   - cluster : The name of an external cluster registered via LBFactory.
  45          *               If not specified, the primary DB cluster for the wiki will be used.
  46          *               This can be overridden with a custom cluster so that DB handles will
  47          *               be retrieved via LBFactory::getExternalLB() and getConnection().
  48          * @param $params array
  49          */
  50         protected function __construct( array $params ) {
  51                 global $wgMemc;
  52
  53                 parent::__construct( $params );
  54
  55                 $this->cluster = isset( $params['cluster'] ) ? $params['cluster'] : false;
  56                 // Make sure that we don't use the SQL cache, which would be harmful
  57                 $this->cache = ( $wgMemc instanceof SqlBagOStuff ) ? new EmptyBagOStuff() : $wgMemc;
  58         }
  59
  60         protected function supportedOrders() {
  61                 return array( 'random', 'timestamp', 'fifo' );
  62         }
  63
  64         protected function optimalOrder() {
  65                 return 'random';
  66         }
  67
  68         /**
  69          * @see JobQueue::doIsEmpty()
  70          * @return bool
  71          */
  72         protected function doIsEmpty() {
  73                 $key = $this->getCacheKey( 'empty' );
  74
  75                 $isEmpty = $this->cache->get( $key );
  76                 if ( $isEmpty === 'true' ) {
  77                         return true;
  78                 } elseif ( $isEmpty === 'false' ) {
  79                         return false;
  80                 }
  81
  82                 list( $dbr, $scope ) = $this->getSlaveDB();
  83                 $found = $dbr->selectField( // unclaimed job
  84                         'job', '1', array( 'job_cmd' => $this->type, 'job_token' => '' ), __METHOD__
  85                 );
  86                 $this->cache->add( $key, $found ? 'false' : 'true', self::CACHE_TTL_LONG );
  87
  88                 return !$found;
  89         }
  90
  91         /**
  92          * @see JobQueue::doGetSize()
  93          * @return integer
  94          */
  95         protected function doGetSize() {
  96                 $key = $this->getCacheKey( 'size' );
  97
  98                 $size = $this->cache->get( $key );
  99                 if ( is_int( $size ) ) {
 100                         return $size;
 101                 }
 102
 103                 list( $dbr, $scope ) = $this->getSlaveDB();
 104                 $size = (int)$dbr->selectField( 'job', 'COUNT(*)',
 105                         array( 'job_cmd' => $this->type, 'job_token' => '' ),
 106                         __METHOD__
 107                 );
 108                 $this->cache->set( $key, $size, self::CACHE_TTL_SHORT );
 109
 110                 return $size;
 111         }
 112
 113         /**
 114          * @see JobQueue::doGetAcquiredCount()
 115          * @return integer
 116          */
 117         protected function doGetAcquiredCount() {
 118                 if ( $this->claimTTL <= 0 ) {
 119                         return 0; // no acknowledgements
 120                 }
 121
 122                 $key = $this->getCacheKey( 'acquiredcount' );
 123
 124                 $count = $this->cache->get( $key );
 125                 if ( is_int( $count ) ) {
 126                         return $count;
 127                 }
 128
 129                 list( $dbr, $scope ) = $this->getSlaveDB();
 130                 $count = (int)$dbr->selectField( 'job', 'COUNT(*)',
 131                         array( 'job_cmd' => $this->type, "job_token != {$dbr->addQuotes( '' )}" ),
 132                         __METHOD__
 133                 );
 134                 $this->cache->set( $key, $count, self::CACHE_TTL_SHORT );
 135
 136                 return $count;
 137         }
 138
 139         /**
 140          * @see JobQueue::doGetAbandonedCount()
 141          * @return integer
 142          * @throws MWException
 143          */
 144         protected function doGetAbandonedCount() {
 145                 global $wgMemc;
 146
 147                 if ( $this->claimTTL <= 0 ) {
 148                         return 0; // no acknowledgements
 149                 }
 150
 151                 $key = $this->getCacheKey( 'abandonedcount' );
 152
 153                 $count = $wgMemc->get( $key );
 154                 if ( is_int( $count ) ) {
 155                         return $count;
 156                 }
 157
 158                 list( $dbr, $scope ) = $this->getSlaveDB();
 159                 $count = (int)$dbr->selectField( 'job', 'COUNT(*)',
 160                         array(
 161                                 'job_cmd' => $this->type,
 162                                 "job_token != {$dbr->addQuotes( '' )}",
 163                                 "job_attempts >= " . $dbr->addQuotes( $this->maxTries )
 164                         ),
 165                         __METHOD__
 166                 );
 167                 $wgMemc->set( $key, $count, self::CACHE_TTL_SHORT );
 168
 169                 return $count;
 170         }
 171
 172         /**
 173          * @see JobQueue::doBatchPush()
 174          * @param array $jobs
 175          * @param $flags
 176          * @throws DBError|Exception
 177          * @return bool
 178          */
 179         protected function doBatchPush( array $jobs, $flags ) {
 180                 if ( count( $jobs ) ) {
 181                         list( $dbw, $scope ) = $this->getMasterDB();
 182
 183                         $rowSet = array(); // (sha1 => job) map for jobs that are de-duplicated
 184                         $rowList = array(); // list of jobs for jobs that are are not de-duplicated
 185
 186                         foreach ( $jobs as $job ) {
 187                                 $row = $this->insertFields( $job );
 188                                 if ( $job->ignoreDuplicates() ) {
 189                                         $rowSet[$row['job_sha1']] = $row;
 190                                 } else {
 191                                         $rowList[] = $row;
 192                                 }
 193                         }
 194
 195                         $key = $this->getCacheKey( 'empty' );
 196                         $atomic = ( $flags & self::QOS_ATOMIC );
 197                         $cache = $this->cache;
 198
 199                         $dbw->onTransactionIdle(
 200                                 function() use ( $dbw, $cache, $rowSet, $rowList, $atomic, $key, $scope
 201                         ) {
 202                                 if ( $atomic ) {
 203                                         $dbw->begin( __METHOD__ ); // wrap all the job additions in one transaction
 204                                 }
 205                                 try {
 206                                         // Strip out any duplicate jobs that are already in the queue...
 207                                         if ( count( $rowSet ) ) {
 208                                                 $res = $dbw->select( 'job', 'job_sha1',
 209                                                         array(
 210                                                                 // No job_type condition since it's part of the job_sha1 hash
 211                                                                 'job_sha1'  => array_keys( $rowSet ),
 212                                                                 'job_token' => '' // unclaimed
 213                                                         ),
 214                                                         __METHOD__
 215                                                 );
 216                                                 foreach ( $res as $row ) {
 217                                                         wfDebug( "Job with hash '{$row->job_sha1}' is a duplicate." );
 218                                                         unset( $rowSet[$row->job_sha1] ); // already enqueued
 219                                                 }
 220                                         }
 221                                         // Build the full list of job rows to insert
 222                                         $rows = array_merge( $rowList, array_values( $rowSet ) );
 223                                         // Insert the job rows in chunks to avoid slave lag...
 224                                         foreach ( array_chunk( $rows, 50 ) as $rowBatch ) {
 225                                                 $dbw->insert( 'job', $rowBatch, __METHOD__ );
 226                                         }
 227                                         wfIncrStats( 'job-insert', count( $rows ) );
 228                                         wfIncrStats( 'job-insert-duplicate',
 229                                                 count( $rowSet ) + count( $rowList ) - count( $rows ) );
 230                                 } catch ( DBError $e ) {
 231                                         if ( $atomic ) {
 232                                                 $dbw->rollback( __METHOD__ );
 233                                         }
 234                                         throw $e;
 235                                 }
 236                                 if ( $atomic ) {
 237                                         $dbw->commit( __METHOD__ );
 238                                 }
 239
 240                                 $cache->set( $key, 'false', JobQueueDB::CACHE_TTL_LONG );
 241                         } );
 242                 }
 243
 244                 return true;
 245         }
 246
 247         /**
 248          * @see JobQueue::doPop()
 249          * @return Job|bool
 250          */
 251         protected function doPop() {
 252                 if ( $this->cache->get( $this->getCacheKey( 'empty' ) ) === 'true' ) {
 253                         return false; // queue is empty
 254                 }
 255
 256                 list( $dbw, $scope ) = $this->getMasterDB();
 257                 $dbw->commit( __METHOD__, 'flush' ); // flush existing transaction
 258
 259                 $uuid = wfRandomString( 32 ); // pop attempt
 260                 $job = false; // job popped off
 261                 do { // retry when our row is invalid or deleted as a duplicate
 262                         // Try to reserve a row in the DB...
 263                         if ( in_array( $this->order, array( 'fifo', 'timestamp' ) ) ) {
 264                                 $row = $this->claimOldest( $uuid );
 265                         } else { // random first
 266                                 $rand = mt_rand( 0, self::MAX_JOB_RANDOM ); // encourage concurrent UPDATEs
 267                                 $gte = (bool)mt_rand( 0, 1 ); // find rows with rand before/after $rand
 268                                 $row = $this->claimRandom( $uuid, $rand, $gte );
 269                         }
 270                         // Check if we found a row to reserve...
 271                         if ( !$row ) {
 272                                 $this->cache->set( $this->getCacheKey( 'empty' ), 'true', self::CACHE_TTL_LONG );
 273                                 break; // nothing to do
 274                         }
 275                         wfIncrStats( 'job-pop' );
 276                         // Get the job object from the row...
 277                         $title = Title::makeTitleSafe( $row->job_namespace, $row->job_title );
 278                         if ( !$title ) {
 279                                 $dbw->delete( 'job', array( 'job_id' => $row->job_id ), __METHOD__ );
 280                                 wfDebugLog( 'JobQueueDB', "Row has invalid title '{$row->job_title}'." );
 281                                 continue; // try again
 282                         }
 283                         $job = Job::factory( $row->job_cmd, $title,
 284                                 self::extractBlob( $row->job_params ), $row->job_id );
 285                         $job->id = $row->job_id; // XXX: work around broken subclasses
 286                         break; // done
 287                 } while( true );
 288
 289                 return $job;
 290         }
 291
 292         /**
 293          * Reserve a row with a single UPDATE without holding row locks over RTTs...
 294          *
 295          * @param string $uuid 32 char hex string
 296          * @param $rand integer Random unsigned integer (31 bits)
 297          * @param bool $gte Search for job_random >= $random (otherwise job_random <= $random)
 298          * @return Row|false
 299          */
 300         protected function claimRandom( $uuid, $rand, $gte ) {
 301                 list( $dbw, $scope ) = $this->getMasterDB();
 302                 // Check cache to see if the queue has <= OFFSET items
 303                 $tinyQueue = $this->cache->get( $this->getCacheKey( 'small' ) );
 304
 305                 $row = false; // the row acquired
 306                 $invertedDirection = false; // whether one job_random direction was already scanned
 307                 // This uses a replication safe method for acquiring jobs. One could use UPDATE+LIMIT
 308                 // instead, but that either uses ORDER BY (in which case it deadlocks in MySQL) or is
 309                 // not replication safe. Due to http://bugs.mysql.com/bug.php?id=6980, subqueries cannot
 310                 // be used here with MySQL.
 311                 do {
 312                         if ( $tinyQueue ) { // queue has <= MAX_OFFSET rows
 313                                 // For small queues, using OFFSET will overshoot and return no rows more often.
 314                                 // Instead, this uses job_random to pick a row (possibly checking both directions).
 315                                 $ineq = $gte ? '>=' : '<=';
 316                                 $dir = $gte ? 'ASC' : 'DESC';
 317                                 $row = $dbw->selectRow( 'job', '*', // find a random job
 318                                         array(
 319                                                 'job_cmd'   => $this->type,
 320                                                 'job_token' => '', // unclaimed
 321                                                 "job_random {$ineq} {$dbw->addQuotes( $rand )}" ),
 322                                         __METHOD__,
 323                                         array( 'ORDER BY' => "job_random {$dir}" )
 324                                 );
 325                                 if ( !$row && !$invertedDirection ) {
 326                                         $gte = !$gte;
 327                                         $invertedDirection = true;
 328                                         continue; // try the other direction
 329                                 }
 330                         } else { // table *may* have >= MAX_OFFSET rows
 331                                 // Bug 42614: "ORDER BY job_random" with a job_random inequality causes high CPU
 332                                 // in MySQL if there are many rows for some reason. This uses a small OFFSET
 333                                 // instead of job_random for reducing excess claim retries.
 334                                 $row = $dbw->selectRow( 'job', '*', // find a random job
 335                                         array(
 336                                                 'job_cmd'   => $this->type,
 337                                                 'job_token' => '', // unclaimed
 338                                         ),
 339                                         __METHOD__,
 340                                         array( 'OFFSET' => mt_rand( 0, self::MAX_OFFSET ) )
 341                                 );
 342                                 if ( !$row ) {
 343                                         $tinyQueue = true; // we know the queue must have <= MAX_OFFSET rows
 344                                         $this->cache->set( $this->getCacheKey( 'small' ), 1, 30 );
 345                                         continue; // use job_random
 346                                 }
 347                         }
 348                         if ( $row ) { // claim the job
 349                                 $dbw->update( 'job', // update by PK
 350                                         array(
 351                                                 'job_token'           => $uuid,
 352                                                 'job_token_timestamp' => $dbw->timestamp(),
 353                                                 'job_attempts = job_attempts+1' ),
 354                                         array( 'job_cmd' => $this->type, 'job_id' => $row->job_id, 'job_token' => '' ),
 355                                         __METHOD__
 356                                 );
 357                                 // This might get raced out by another runner when claiming the previously
 358                                 // selected row. The use of job_random should minimize this problem, however.
 359                                 if ( !$dbw->affectedRows() ) {
 360                                         $row = false; // raced out
 361                                 }
 362                         } else {
 363                                 break; // nothing to do
 364                         }
 365                 } while ( !$row );
 366
 367                 return $row;
 368         }
 369
 370         /**
 371          * Reserve a row with a single UPDATE without holding row locks over RTTs...
 372          *
 373          * @param string $uuid 32 char hex string
 374          * @return Row|false
 375          */
 376         protected function claimOldest( $uuid ) {
 377                 list( $dbw, $scope ) = $this->getMasterDB();
 378
 379                 $row = false; // the row acquired
 380                 do {
 381                         if ( $dbw->getType() === 'mysql' ) {
 382                                 // Per http://bugs.mysql.com/bug.php?id=6980, we can't use subqueries on the
 383                                 // same table being changed in an UPDATE query in MySQL (gives Error: 1093).
 384                                 // Oracle and Postgre have no such limitation. However, MySQL offers an
 385                                 // alternative here by supporting ORDER BY + LIMIT for UPDATE queries.
 386                                 $dbw->query( "UPDATE {$dbw->tableName( 'job' )} " .
 387                                         "SET " .
 388                                                 "job_token = {$dbw->addQuotes( $uuid ) }, " .
 389                                                 "job_token_timestamp = {$dbw->addQuotes( $dbw->timestamp() )}, " .
 390                                                 "job_attempts = job_attempts+1 " .
 391                                         "WHERE ( " .
 392                                                 "job_cmd = {$dbw->addQuotes( $this->type )} " .
 393                                                 "AND job_token = {$dbw->addQuotes( '' )} " .
 394                                         ") ORDER BY job_id ASC LIMIT 1",
 395                                         __METHOD__
 396                                 );
 397                         } else {
 398                                 // Use a subquery to find the job, within an UPDATE to claim it.
 399                                 // This uses as much of the DB wrapper functions as possible.
 400                                 $dbw->update( 'job',
 401                                         array(
 402                                                 'job_token'           => $uuid,
 403                                                 'job_token_timestamp' => $dbw->timestamp(),
 404                                                 'job_attempts = job_attempts+1' ),
 405                                         array( 'job_id = (' .
 406                                                 $dbw->selectSQLText( 'job', 'job_id',
 407                                                         array( 'job_cmd' => $this->type, 'job_token' => '' ),
 408                                                         __METHOD__,
 409                                                         array( 'ORDER BY' => 'job_id ASC', 'LIMIT' => 1 ) ) .
 410                                                 ')'
 411                                         ),
 412                                         __METHOD__
 413                                 );
 414                         }
 415                         // Fetch any row that we just reserved...
 416                         if ( $dbw->affectedRows() ) {
 417                                 $row = $dbw->selectRow( 'job', '*',
 418                                         array( 'job_cmd' => $this->type, 'job_token' => $uuid ), __METHOD__
 419                                 );
 420                                 if ( !$row ) { // raced out by duplicate job removal
 421                                         wfDebugLog( 'JobQueueDB', "Row deleted as duplicate by another process." );
 422                                 }
 423                         } else {
 424                                 break; // nothing to do
 425                         }
 426                 } while ( !$row );
 427
 428                 return $row;
 429         }
 430
 431         /**
 432          * Recycle or destroy any jobs that have been claimed for too long
 433          *
 434          * @return integer Number of jobs recycled/deleted
 435          */
 436         public function recycleAndDeleteStaleJobs() {
 437                 $now = time();
 438                 list( $dbw, $scope ) = $this->getMasterDB();
 439                 $count = 0; // affected rows
 440
 441                 if ( !$dbw->lock( "jobqueue-recycle-{$this->type}", __METHOD__, 1 ) ) {
 442                         return $count; // already in progress
 443                 }
 444
 445                 // Remove claims on jobs acquired for too long if enabled...
 446                 if ( $this->claimTTL > 0 ) {
 447                         $claimCutoff = $dbw->timestamp( $now - $this->claimTTL );
 448                         // Get the IDs of jobs that have be claimed but not finished after too long.
 449                         // These jobs can be recycled into the queue by expiring the claim. Selecting
 450                         // the IDs first means that the UPDATE can be done by primary key (less deadlocks).
 451                         $res = $dbw->select( 'job', 'job_id',
 452                                 array(
 453                                         'job_cmd' => $this->type,
 454                                         "job_token != {$dbw->addQuotes( '' )}", // was acquired
 455                                         "job_token_timestamp < {$dbw->addQuotes( $claimCutoff )}", // stale
 456                                         "job_attempts < {$dbw->addQuotes( $this->maxTries )}" ), // retries left
 457                                 __METHOD__
 458                         );
 459                         $ids = array_map( function( $o ) { return $o->job_id; }, iterator_to_array( $res ) );
 460                         if ( count( $ids ) ) {
 461                                 // Reset job_token for these jobs so that other runners will pick them up.
 462                                 // Set the timestamp to the current time, as it is useful to now that the job
 463                                 // was already tried before (the timestamp becomes the "released" time).
 464                                 $dbw->update( 'job',
 465                                         array(
 466                                                 'job_token' => '',
 467                                                 'job_token_timestamp' => $dbw->timestamp( $now ) ), // time of release
 468                                         array(
 469                                                 'job_id' => $ids ),
 470                                         __METHOD__
 471                                 );
 472                                 $count += $dbw->affectedRows();
 473                                 wfIncrStats( 'job-recycle', $dbw->affectedRows() );
 474                                 $this->cache->set( $this->getCacheKey( 'empty' ), 'false', self::CACHE_TTL_LONG );
 475                         }
 476                 }
 477
 478                 // Just destroy any stale jobs...
 479                 $pruneCutoff = $dbw->timestamp( $now - self::MAX_AGE_PRUNE );
 480                 $conds = array(
 481                         'job_cmd' => $this->type,
 482                         "job_token != {$dbw->addQuotes( '' )}", // was acquired
 483                         "job_token_timestamp < {$dbw->addQuotes( $pruneCutoff )}" // stale
 484                 );
 485                 if ( $this->claimTTL > 0 ) { // only prune jobs attempted too many times...
 486                         $conds[] = "job_attempts >= {$dbw->addQuotes( $this->maxTries )}";
 487                 }
 488                 // Get the IDs of jobs that are considered stale and should be removed. Selecting
 489                 // the IDs first means that the UPDATE can be done by primary key (less deadlocks).
 490                 $res = $dbw->select( 'job', 'job_id', $conds, __METHOD__ );
 491                 $ids = array_map( function( $o ) { return $o->job_id; }, iterator_to_array( $res ) );
 492                 if ( count( $ids ) ) {
 493                         $dbw->delete( 'job', array( 'job_id' => $ids ), __METHOD__ );
 494                         $count += $dbw->affectedRows();
 495                 }
 496
 497                 $dbw->unlock( "jobqueue-recycle-{$this->type}", __METHOD__ );
 498
 499                 return $count;
 500         }
 501
 502         /**
 503          * @see JobQueue::doAck()
 504          * @param Job $job
 505          * @throws MWException
 506          * @return Job|bool
 507          */
 508         protected function doAck( Job $job ) {
 509                 if ( !$job->getId() ) {
 510                         throw new MWException( "Job of type '{$job->getType()}' has no ID." );
 511                 }
 512
 513                 list( $dbw, $scope ) = $this->getMasterDB();
 514                 $dbw->commit( __METHOD__, 'flush' ); // flush existing transaction
 515
 516                 // Delete a row with a single DELETE without holding row locks over RTTs...
 517                 $dbw->delete( 'job',
 518                         array( 'job_cmd' => $this->type, 'job_id' => $job->getId() ), __METHOD__ );
 519
 520                 return true;
 521         }
 522
 523         /**
 524          * @see JobQueue::doDeduplicateRootJob()
 525          * @param Job $job
 526          * @throws MWException
 527          * @return bool
 528          */
 529         protected function doDeduplicateRootJob( Job $job ) {
 530                 $params = $job->getParams();
 531                 if ( !isset( $params['rootJobSignature'] ) ) {
 532                         throw new MWException( "Cannot register root job; missing 'rootJobSignature'." );
 533                 } elseif ( !isset( $params['rootJobTimestamp'] ) ) {
 534                         throw new MWException( "Cannot register root job; missing 'rootJobTimestamp'." );
 535                 }
 536                 $key = $this->getRootJobCacheKey( $params['rootJobSignature'] );
 537                 // Callers should call batchInsert() and then this function so that if the insert
 538                 // fails, the de-duplication registration will be aborted. Since the insert is
 539                 // deferred till "transaction idle", do the same here, so that the ordering is
 540                 // maintained. Having only the de-duplication registration succeed would cause
 541                 // jobs to become no-ops without any actual jobs that made them redundant.
 542                 list( $dbw, $scope ) = $this->getMasterDB();
 543                 $cache = $this->cache;
 544                 $dbw->onTransactionIdle( function() use ( $cache, $params, $key, $scope ) {
 545                         $timestamp = $cache->get( $key ); // current last timestamp of this job
 546                         if ( $timestamp && $timestamp >= $params['rootJobTimestamp'] ) {
 547                                 return true; // a newer version of this root job was enqueued
 548                         }
 549
 550                         // Update the timestamp of the last root job started at the location...
 551                         return $cache->set( $key, $params['rootJobTimestamp'], JobQueueDB::ROOTJOB_TTL );
 552                 } );
 553
 554                 return true;
 555         }
 556
 557         /**
 558          * @see JobQueue::doWaitForBackups()
 559          * @return void
 560          */
 561         protected function doWaitForBackups() {
 562                 wfWaitForSlaves();
 563         }
 564
 565         /**
 566          * @return Array
 567          */
 568         protected function doGetPeriodicTasks() {
 569                 return array(
 570                         'recycleAndDeleteStaleJobs' => array(
 571                                 'callback' => array( $this, 'recycleAndDeleteStaleJobs' ),
 572                                 'period'   => ceil( $this->claimTTL / 2 )
 573                         )
 574                 );
 575         }
 576
 577         /**
 578          * @return void
 579          */
 580         protected function doFlushCaches() {
 581                 foreach ( array( 'empty', 'size', 'acquiredcount' ) as $type ) {
 582                         $this->cache->delete( $this->getCacheKey( $type ) );
 583                 }
 584         }
 585
 586         /**
 587          * @see JobQueue::getAllQueuedJobs()
 588          * @return Iterator
 589          */
 590         public function getAllQueuedJobs() {
 591                 list( $dbr, $scope ) = $this->getSlaveDB();
 592                 return new MappedIterator(
 593                         $dbr->select( 'job', '*', array( 'job_cmd' => $this->getType(), 'job_token' => '' ) ),
 594                         function( $row ) use ( $scope ) {
 595                                 $job = Job::factory(
 596                                         $row->job_cmd,
 597                                         Title::makeTitle( $row->job_namespace, $row->job_title ),
 598                                         strlen( $row->job_params ) ? unserialize( $row->job_params ) : false,
 599                                         $row->job_id
 600                                 );
 601                                 $job->id = $row->job_id; // XXX: work around broken subclasses
 602                                 return $job;
 603                         }
 604                 );
 605         }
 606
 607         /**
 608          * @return Array (DatabaseBase, ScopedCallback)
 609          */
 610         protected function getSlaveDB() {
 611                 return $this->getDB( DB_SLAVE );
 612         }
 613
 614         /**
 615          * @return Array (DatabaseBase, ScopedCallback)
 616          */
 617         protected function getMasterDB() {
 618                 return $this->getDB( DB_MASTER );
 619         }
 620
 621         /**
 622          * @param $index integer (DB_SLAVE/DB_MASTER)
 623          * @return Array (DatabaseBase, ScopedCallback)
 624          */
 625         protected function getDB( $index ) {
 626                 $lb = ( $this->cluster !== false )
 627                         ? wfGetLBFactory()->getExternalLB( $this->cluster, $this->wiki )
 628                         : wfGetLB( $this->wiki );
 629                 $conn = $lb->getConnection( $index, array(), $this->wiki );
 630                 return array(
 631                         $conn,
 632                         new ScopedCallback( function() use ( $lb, $conn ) {
 633                                 $lb->reuseConnection( $conn );
 634                         } )
 635                 );
 636         }
 637
 638         /**
 639          * @param $job Job
 640          * @return array
 641          */
 642         protected function insertFields( Job $job ) {
 643                 list( $dbw, $scope ) = $this->getMasterDB();
 644                 return array(
 645                         // Fields that describe the nature of the job
 646                         'job_cmd'       => $job->getType(),
 647                         'job_namespace' => $job->getTitle()->getNamespace(),
 648                         'job_title'     => $job->getTitle()->getDBkey(),
 649                         'job_params'    => self::makeBlob( $job->getParams() ),
 650                         // Additional job metadata
 651                         'job_id'        => $dbw->nextSequenceValue( 'job_job_id_seq' ),
 652                         'job_timestamp' => $dbw->timestamp(),
 653                         'job_sha1'      => wfBaseConvert(
 654                                 sha1( serialize( $job->getDeduplicationInfo() ) ),
 655                                 16, 36, 31
 656                         ),
 657                         'job_random'    => mt_rand( 0, self::MAX_JOB_RANDOM )
 658                 );
 659         }
 660
 661         /**
 662          * @return string
 663          */
 664         private function getCacheKey( $property ) {
 665                 list( $db, $prefix ) = wfSplitWikiID( $this->wiki );
 666                 return wfForeignMemcKey( $db, $prefix, 'jobqueue', $this->type, $property );
 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 }