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         protected $cluster = false; // string; name of an external DB cluster
  38
  39         /**
  40          * Additional parameters include:
  41          *   - cluster : The name of an external cluster registered via LBFactory.
  42          *               If not specified, the primary DB cluster for the wiki will be used.
  43          *               This can be overridden with a custom cluster so that DB handles will
  44          *               be retrieved via LBFactory::getExternalLB() and getConnection().
  45          * @param $params array
  46          */
  47         protected function __construct( array $params ) {
  48                 parent::__construct( $params );
  49                 $this->cluster = isset( $params['cluster'] ) ? $params['cluster'] : false;
  50         }
  51
  52         /**
  53          * @see JobQueue::doIsEmpty()
  54          * @return bool
  55          */
  56         protected function doIsEmpty() {
  57                 global $wgMemc;
  58
  59                 $key = $this->getCacheKey( 'empty' );
  60
  61                 $isEmpty = $wgMemc->get( $key );
  62                 if ( $isEmpty === 'true' ) {
  63                         return true;
  64                 } elseif ( $isEmpty === 'false' ) {
  65                         return false;
  66                 }
  67
  68                 list( $dbr, $scope ) = $this->getSlaveDB();
  69                 $found = $dbr->selectField( // unclaimed job
  70                         'job', '1', array( 'job_cmd' => $this->type, 'job_token' => '' ), __METHOD__
  71                 );
  72                 $wgMemc->add( $key, $found ? 'false' : 'true', self::CACHE_TTL_LONG );
  73
  74                 return !$found;
  75         }
  76
  77         /**
  78          * @see JobQueue::doGetSize()
  79          * @return integer
  80          */
  81         protected function doGetSize() {
  82                 global $wgMemc;
  83
  84                 $key = $this->getCacheKey( 'size' );
  85
  86                 $size = $wgMemc->get( $key );
  87                 if ( is_int( $size ) ) {
  88                         return $size;
  89                 }
  90
  91                 list( $dbr, $scope ) = $this->getSlaveDB();
  92                 $size = (int)$dbr->selectField( 'job', 'COUNT(*)',
  93                         array( 'job_cmd' => $this->type, 'job_token' => '' ),
  94                         __METHOD__
  95                 );
  96                 $wgMemc->set( $key, $size, self::CACHE_TTL_SHORT );
  97
  98                 return $size;
  99         }
 100
 101         /**
 102          * @see JobQueue::doGetAcquiredCount()
 103          * @return integer
 104          */
 105         protected function doGetAcquiredCount() {
 106                 global $wgMemc;
 107
 108                 $key = $this->getCacheKey( 'acquiredcount' );
 109
 110                 $count = $wgMemc->get( $key );
 111                 if ( is_int( $count ) ) {
 112                         return $count;
 113                 }
 114
 115                 list( $dbr, $scope ) = $this->getSlaveDB();
 116                 $count = (int)$dbr->selectField( 'job', 'COUNT(*)',
 117                         array( 'job_cmd' => $this->type, "job_token != {$dbr->addQuotes( '' )}" ),
 118                         __METHOD__
 119                 );
 120                 $wgMemc->set( $key, $count, self::CACHE_TTL_SHORT );
 121
 122                 return $count;
 123         }
 124
 125         /**
 126          * @see JobQueue::doBatchPush()
 127          * @param array $jobs
 128          * @param $flags
 129          * @throws DBError|Exception
 130          * @return bool
 131          */
 132         protected function doBatchPush( array $jobs, $flags ) {
 133                 if ( count( $jobs ) ) {
 134                         list( $dbw, $scope ) = $this->getMasterDB();
 135
 136                         $rowSet = array(); // (sha1 => job) map for jobs that are de-duplicated
 137                         $rowList = array(); // list of jobs for jobs that are are not de-duplicated
 138
 139                         foreach ( $jobs as $job ) {
 140                                 $row = $this->insertFields( $job );
 141                                 if ( $job->ignoreDuplicates() ) {
 142                                         $rowSet[$row['job_sha1']] = $row;
 143                                 } else {
 144                                         $rowList[] = $row;
 145                                 }
 146                         }
 147
 148                         $atomic = ( $flags & self::QoS_Atomic );
 149                         $key    = $this->getCacheKey( 'empty' );
 150                         $ttl    = self::CACHE_TTL_LONG;
 151
 152                         $dbw->onTransactionIdle(
 153                                 function() use ( $dbw, $rowSet, $rowList, $atomic, $key, $ttl, $scope
 154                         ) {
 155                                 global $wgMemc;
 156
 157                                 if ( $atomic ) {
 158                                         $dbw->begin( __METHOD__ ); // wrap all the job additions in one transaction
 159                                 }
 160                                 try {
 161                                         // Strip out any duplicate jobs that are already in the queue...
 162                                         if ( count( $rowSet ) ) {
 163                                                 $res = $dbw->select( 'job', 'job_sha1',
 164                                                         array(
 165                                                                 // No job_type condition since it's part of the job_sha1 hash
 166                                                                 'job_sha1'  => array_keys( $rowSet ),
 167                                                                 'job_token' => '' // unclaimed
 168                                                         ),
 169                                                         __METHOD__
 170                                                 );
 171                                                 foreach ( $res as $row ) {
 172                                                         wfDebug( "Job with hash '{$row->job_sha1}' is a duplicate." );
 173                                                         unset( $rowSet[$row->job_sha1] ); // already enqueued
 174                                                 }
 175                                         }
 176                                         // Build the full list of job rows to insert
 177                                         $rows = array_merge( $rowList, array_values( $rowSet ) );
 178                                         // Insert the job rows in chunks to avoid slave lag...
 179                                         foreach ( array_chunk( $rows, 50 ) as $rowBatch ) {
 180                                                 $dbw->insert( 'job', $rowBatch, __METHOD__ );
 181                                         }
 182                                         wfIncrStats( 'job-insert', count( $rows ) );
 183                                         wfIncrStats( 'job-insert-duplicate',
 184                                                 count( $rowSet ) + count( $rowList ) - count( $rows ) );
 185                                 } catch ( DBError $e ) {
 186                                         if ( $atomic ) {
 187                                                 $dbw->rollback( __METHOD__ );
 188                                         }
 189                                         throw $e;
 190                                 }
 191                                 if ( $atomic ) {
 192                                         $dbw->commit( __METHOD__ );
 193                                 }
 194
 195                                 $wgMemc->set( $key, 'false', $ttl ); // queue is not empty
 196                         } );
 197                 }
 198
 199                 return true;
 200         }
 201
 202         /**
 203          * @see JobQueue::doPop()
 204          * @return Job|bool
 205          */
 206         protected function doPop() {
 207                 global $wgMemc;
 208
 209                 if ( $wgMemc->get( $this->getCacheKey( 'empty' ) ) === 'true' ) {
 210                         return false; // queue is empty
 211                 }
 212
 213                 list( $dbw, $scope ) = $this->getMasterDB();
 214                 $dbw->commit( __METHOD__, 'flush' ); // flush existing transaction
 215
 216                 $uuid = wfRandomString( 32 ); // pop attempt
 217                 $job = false; // job popped off
 218                 // Occasionally recycle jobs back into the queue that have been claimed too long
 219                 if ( mt_rand( 0, 99 ) == 0 ) {
 220                         $this->recycleStaleJobs();
 221                 }
 222                 do { // retry when our row is invalid or deleted as a duplicate
 223                         // Try to reserve a row in the DB...
 224                         if ( in_array( $this->order, array( 'fifo', 'timestamp' ) ) ) {
 225                                 $row = $this->claimOldest( $uuid );
 226                         } else { // random first
 227                                 $rand = mt_rand( 0, self::MAX_JOB_RANDOM ); // encourage concurrent UPDATEs
 228                                 $gte  = (bool)mt_rand( 0, 1 ); // find rows with rand before/after $rand
 229                                 $row  = $this->claimRandom( $uuid, $rand, $gte );
 230                         }
 231                         // Check if we found a row to reserve...
 232                         if ( !$row ) {
 233                                 $wgMemc->set( $this->getCacheKey( 'empty' ), 'true', self::CACHE_TTL_LONG );
 234                                 break; // nothing to do
 235                         }
 236                         wfIncrStats( 'job-pop' );
 237                         // Get the job object from the row...
 238                         $title = Title::makeTitleSafe( $row->job_namespace, $row->job_title );
 239                         if ( !$title ) {
 240                                 $dbw->delete( 'job', array( 'job_id' => $row->job_id ), __METHOD__ );
 241                                 wfDebugLog( 'JobQueueDB', "Row has invalid title '{$row->job_title}'." );
 242                                 continue; // try again
 243                         }
 244                         $job = Job::factory( $row->job_cmd, $title,
 245                                 self::extractBlob( $row->job_params ), $row->job_id );
 246                         $job->id = $row->job_id; // XXX: work around broken subclasses
 247                         // Flag this job as an old duplicate based on its "root" job...
 248                         if ( $this->isRootJobOldDuplicate( $job ) ) {
 249                                 wfIncrStats( 'job-pop-duplicate' );
 250                                 $job = DuplicateJob::newFromJob( $job ); // convert to a no-op
 251                         }
 252                         break; // done
 253                 } while( true );
 254
 255                 return $job;
 256         }
 257
 258         /**
 259          * Reserve a row with a single UPDATE without holding row locks over RTTs...
 260          *
 261          * @param $uuid string 32 char hex string
 262          * @param $rand integer Random unsigned integer (31 bits)
 263          * @param $gte bool Search for job_random >= $random (otherwise job_random <= $random)
 264          * @return Row|false
 265          */
 266         protected function claimRandom( $uuid, $rand, $gte ) {
 267                 global $wgMemc;
 268
 269                 list( $dbw, $scope ) = $this->getMasterDB();
 270                 // Check cache to see if the queue has <= OFFSET items
 271                 $tinyQueue = $wgMemc->get( $this->getCacheKey( 'small' ) );
 272
 273                 $row = false; // the row acquired
 274                 $invertedDirection = false; // whether one job_random direction was already scanned
 275                 // This uses a replication safe method for acquiring jobs. One could use UPDATE+LIMIT
 276                 // instead, but that either uses ORDER BY (in which case it deadlocks in MySQL) or is
 277                 // not replication safe. Due to http://bugs.mysql.com/bug.php?id=6980, subqueries cannot
 278                 // be used here with MySQL.
 279                 do {
 280                         if ( $tinyQueue ) { // queue has <= MAX_OFFSET rows
 281                                 // For small queues, using OFFSET will overshoot and return no rows more often.
 282                                 // Instead, this uses job_random to pick a row (possibly checking both directions).
 283                                 $ineq = $gte ? '>=' : '<=';
 284                                 $dir  = $gte ? 'ASC' : 'DESC';
 285                                 $row  = $dbw->selectRow( 'job', '*', // find a random job
 286                                         array(
 287                                                 'job_cmd'   => $this->type,
 288                                                 'job_token' => '', // unclaimed
 289                                                 "job_random {$ineq} {$dbw->addQuotes( $rand )}" ),
 290                                         __METHOD__,
 291                                         array( 'ORDER BY' => "job_random {$dir}" )
 292                                 );
 293                                 if ( !$row && !$invertedDirection ) {
 294                                         $gte = !$gte;
 295                                         $invertedDirection = true;
 296                                         continue; // try the other direction
 297                                 }
 298                         } else { // table *may* have >= MAX_OFFSET rows
 299                                 // Bug 42614: "ORDER BY job_random" with a job_random inequality causes high CPU
 300                                 // in MySQL if there are many rows for some reason. This uses a small OFFSET
 301                                 // instead of job_random for reducing excess claim retries.
 302                                 $row = $dbw->selectRow( 'job', '*', // find a random job
 303                                         array(
 304                                                 'job_cmd'   => $this->type,
 305                                                 'job_token' => '', // unclaimed
 306                                         ),
 307                                         __METHOD__,
 308                                         array( 'OFFSET' => mt_rand( 0, self::MAX_OFFSET ) )
 309                                 );
 310                                 if ( !$row ) {
 311                                         $tinyQueue = true; // we know the queue must have <= MAX_OFFSET rows
 312                                         $wgMemc->set( $this->getCacheKey( 'small' ), 1, 30 );
 313                                         continue; // use job_random
 314                                 }
 315                         }
 316                         if ( $row ) { // claim the job
 317                                 $dbw->update( 'job', // update by PK
 318                                         array(
 319                                                 'job_token'           => $uuid,
 320                                                 'job_token_timestamp' => $dbw->timestamp(),
 321                                                 'job_attempts = job_attempts+1' ),
 322                                         array( 'job_cmd' => $this->type, 'job_id' => $row->job_id, 'job_token' => '' ),
 323                                         __METHOD__
 324                                 );
 325                                 // This might get raced out by another runner when claiming the previously
 326                                 // selected row. The use of job_random should minimize this problem, however.
 327                                 if ( !$dbw->affectedRows() ) {
 328                                         $row = false; // raced out
 329                                 }
 330                         } else {
 331                                 break; // nothing to do
 332                         }
 333                 } while ( !$row );
 334
 335                 return $row;
 336         }
 337
 338         /**
 339          * Reserve a row with a single UPDATE without holding row locks over RTTs...
 340          *
 341          * @param $uuid string 32 char hex string
 342          * @return Row|false
 343          */
 344         protected function claimOldest( $uuid ) {
 345                 list( $dbw, $scope ) = $this->getMasterDB();
 346
 347                 $row = false; // the row acquired
 348                 do {
 349                         if ( $dbw->getType() === 'mysql' ) {
 350                                 // Per http://bugs.mysql.com/bug.php?id=6980, we can't use subqueries on the
 351                                 // same table being changed in an UPDATE query in MySQL (gives Error: 1093).
 352                                 // Oracle and Postgre have no such limitation. However, MySQL offers an
 353                                 // alternative here by supporting ORDER BY + LIMIT for UPDATE queries.
 354                                 $dbw->query( "UPDATE {$dbw->tableName( 'job' )} " .
 355                                         "SET " .
 356                                                 "job_token = {$dbw->addQuotes( $uuid ) }, " .
 357                                                 "job_token_timestamp = {$dbw->addQuotes( $dbw->timestamp() )}, " .
 358                                                 "job_attempts = job_attempts+1 " .
 359                                         "WHERE ( " .
 360                                                 "job_cmd = {$dbw->addQuotes( $this->type )} " .
 361                                                 "AND job_token = {$dbw->addQuotes( '' )} " .
 362                                         ") ORDER BY job_id ASC LIMIT 1",
 363                                         __METHOD__
 364                                 );
 365                         } else {
 366                                 // Use a subquery to find the job, within an UPDATE to claim it.
 367                                 // This uses as much of the DB wrapper functions as possible.
 368                                 $dbw->update( 'job',
 369                                         array(
 370                                                 'job_token'           => $uuid,
 371                                                 'job_token_timestamp' => $dbw->timestamp(),
 372                                                 'job_attempts = job_attempts+1' ),
 373                                         array( 'job_id = (' .
 374                                                 $dbw->selectSQLText( 'job', 'job_id',
 375                                                         array( 'job_cmd' => $this->type, 'job_token' => '' ),
 376                                                         __METHOD__,
 377                                                         array( 'ORDER BY' => 'job_id ASC', 'LIMIT' => 1 ) ) .
 378                                                 ')'
 379                                         ),
 380                                         __METHOD__
 381                                 );
 382                         }
 383                         // Fetch any row that we just reserved...
 384                         if ( $dbw->affectedRows() ) {
 385                                 $row = $dbw->selectRow( 'job', '*',
 386                                         array( 'job_cmd' => $this->type, 'job_token' => $uuid ), __METHOD__
 387                                 );
 388                                 if ( !$row ) { // raced out by duplicate job removal
 389                                         wfDebugLog( 'JobQueueDB', "Row deleted as duplicate by another process." );
 390                                 }
 391                         } else {
 392                                 break; // nothing to do
 393                         }
 394                 } while ( !$row );
 395
 396                 return $row;
 397         }
 398
 399         /**
 400          * Recycle or destroy any jobs that have been claimed for too long
 401          *
 402          * @return integer Number of jobs recycled/deleted
 403          */
 404         protected function recycleStaleJobs() {
 405                 $now   = time();
 406                 list( $dbw, $scope ) = $this->getMasterDB();
 407                 $count = 0; // affected rows
 408
 409                 if ( !$dbw->lock( "jobqueue-recycle-{$this->type}", __METHOD__, 1 ) ) {
 410                         return $count; // already in progress
 411                 }
 412
 413                 // Remove claims on jobs acquired for too long if enabled...
 414                 if ( $this->claimTTL > 0 ) {
 415                         $claimCutoff = $dbw->timestamp( $now - $this->claimTTL );
 416                         // Get the IDs of jobs that have be claimed but not finished after too long.
 417                         // These jobs can be recycled into the queue by expiring the claim. Selecting
 418                         // the IDs first means that the UPDATE can be done by primary key (less deadlocks).
 419                         $res = $dbw->select( 'job', 'job_id',
 420                                 array(
 421                                         'job_cmd' => $this->type,
 422                                         "job_token != {$dbw->addQuotes( '' )}", // was acquired
 423                                         "job_token_timestamp < {$dbw->addQuotes( $claimCutoff )}", // stale
 424                                         "job_attempts < {$dbw->addQuotes( self::MAX_ATTEMPTS )}" ), // retries left
 425                                 __METHOD__
 426                         );
 427                         $ids = array_map( function( $o ) { return $o->job_id; }, iterator_to_array( $res ) );
 428                         if ( count( $ids ) ) {
 429                                 // Reset job_token for these jobs so that other runners will pick them up.
 430                                 // Set the timestamp to the current time, as it is useful to now that the job
 431                                 // was already tried before (the timestamp becomes the "released" time).
 432                                 $dbw->update( 'job',
 433                                         array(
 434                                                 'job_token' => '',
 435                                                 'job_token_timestamp' => $dbw->timestamp( $now ) ), // time of release
 436                                         array(
 437                                                 'job_id' => $ids ),
 438                                         __METHOD__
 439                                 );
 440                                 $count += $dbw->affectedRows();
 441                                 wfIncrStats( 'job-recycle', $dbw->affectedRows() );
 442                         }
 443                 }
 444
 445                 // Just destroy any stale jobs...
 446                 $pruneCutoff = $dbw->timestamp( $now - self::MAX_AGE_PRUNE );
 447                 $conds = array(
 448                         'job_cmd' => $this->type,
 449                         "job_token != {$dbw->addQuotes( '' )}", // was acquired
 450                         "job_token_timestamp < {$dbw->addQuotes( $pruneCutoff )}" // stale
 451                 );
 452                 if ( $this->claimTTL > 0 ) { // only prune jobs attempted too many times...
 453                         $conds[] = "job_attempts >= {$dbw->addQuotes( self::MAX_ATTEMPTS )}";
 454                 }
 455                 // Get the IDs of jobs that are considered stale and should be removed. Selecting
 456                 // the IDs first means that the UPDATE can be done by primary key (less deadlocks).
 457                 $res = $dbw->select( 'job', 'job_id', $conds, __METHOD__ );
 458                 $ids = array_map( function( $o ) { return $o->job_id; }, iterator_to_array( $res ) );
 459                 if ( count( $ids ) ) {
 460                         $dbw->delete( 'job', array( 'job_id' => $ids ), __METHOD__ );
 461                         $count += $dbw->affectedRows();
 462                 }
 463
 464                 $dbw->unlock( "jobqueue-recycle-{$this->type}", __METHOD__ );
 465
 466                 return $count;
 467         }
 468
 469         /**
 470          * @see JobQueue::doAck()
 471          * @param Job $job
 472          * @throws MWException
 473          * @return Job|bool
 474          */
 475         protected function doAck( Job $job ) {
 476                 if ( !$job->getId() ) {
 477                         throw new MWException( "Job of type '{$job->getType()}' has no ID." );
 478                 }
 479
 480                 list( $dbw, $scope ) = $this->getMasterDB();
 481                 $dbw->commit( __METHOD__, 'flush' ); // flush existing transaction
 482
 483                 // Delete a row with a single DELETE without holding row locks over RTTs...
 484                 $dbw->delete( 'job',
 485                         array( 'job_cmd' => $this->type, 'job_id' => $job->getId() ), __METHOD__ );
 486
 487                 return true;
 488         }
 489
 490         /**
 491          * @see JobQueue::doDeduplicateRootJob()
 492          * @param Job $job
 493          * @throws MWException
 494          * @return bool
 495          */
 496         protected function doDeduplicateRootJob( Job $job ) {
 497                 $params = $job->getParams();
 498                 if ( !isset( $params['rootJobSignature'] ) ) {
 499                         throw new MWException( "Cannot register root job; missing 'rootJobSignature'." );
 500                 } elseif ( !isset( $params['rootJobTimestamp'] ) ) {
 501                         throw new MWException( "Cannot register root job; missing 'rootJobTimestamp'." );
 502                 }
 503                 $key = $this->getRootJobCacheKey( $params['rootJobSignature'] );
 504                 // Callers should call batchInsert() and then this function so that if the insert
 505                 // fails, the de-duplication registration will be aborted. Since the insert is
 506                 // deferred till "transaction idle", do the same here, so that the ordering is
 507                 // maintained. Having only the de-duplication registration succeed would cause
 508                 // jobs to become no-ops without any actual jobs that made them redundant.
 509                 list( $dbw, $scope ) = $this->getMasterDB();
 510                 $dbw->onTransactionIdle( function() use ( $params, $key, $scope ) {
 511                         global $wgMemc;
 512
 513                         $timestamp = $wgMemc->get( $key ); // current last timestamp of this job
 514                         if ( $timestamp && $timestamp >= $params['rootJobTimestamp'] ) {
 515                                 return true; // a newer version of this root job was enqueued
 516                         }
 517
 518                         // Update the timestamp of the last root job started at the location...
 519                         return $wgMemc->set( $key, $params['rootJobTimestamp'], 14*86400 ); // 2 weeks
 520                 } );
 521
 522                 return true;
 523         }
 524
 525         /**
 526          * Check if the "root" job of a given job has been superseded by a newer one
 527          *
 528          * @param $job Job
 529          * @return bool
 530          */
 531         protected function isRootJobOldDuplicate( Job $job ) {
 532                 global $wgMemc;
 533
 534                 $params = $job->getParams();
 535                 if ( !isset( $params['rootJobSignature'] ) ) {
 536                         return false; // job has no de-deplication info
 537                 } elseif ( !isset( $params['rootJobTimestamp'] ) ) {
 538                         trigger_error( "Cannot check root job; missing 'rootJobTimestamp'." );
 539                         return false;
 540                 }
 541
 542                 // Get the last time this root job was enqueued
 543                 $timestamp = $wgMemc->get( $this->getRootJobCacheKey( $params['rootJobSignature'] ) );
 544
 545                 // Check if a new root job was started at the location after this one's...
 546                 return ( $timestamp && $timestamp > $params['rootJobTimestamp'] );
 547         }
 548
 549         /**
 550          * @see JobQueue::doWaitForBackups()
 551          * @return void
 552          */
 553         protected function doWaitForBackups() {
 554                 wfWaitForSlaves();
 555         }
 556
 557         /**
 558          * @return Array (DatabaseBase, ScopedCallback)
 559          */
 560         protected function getSlaveDB() {
 561                 return $this->getDB( DB_SLAVE );
 562         }
 563
 564         /**
 565          * @return Array (DatabaseBase, ScopedCallback)
 566          */
 567         protected function getMasterDB() {
 568                 return $this->getDB( DB_MASTER );
 569         }
 570
 571         /**
 572          * @param $index integer (DB_SLAVE/DB_MASTER)
 573          * @return Array (DatabaseBase, ScopedCallback)
 574          */
 575         protected function getDB( $index ) {
 576                 $lb = ( $this->cluster !== false )
 577                         ? wfGetLBFactory()->getExternalLB( $this->cluster, $this->wiki )
 578                         : wfGetLB( $this->wiki );
 579                 $conn = $lb->getConnection( $index, array(), $this->wiki );
 580                 return array(
 581                         $conn,
 582                         new ScopedCallback( function() use ( $lb, $conn ) {
 583                                 $lb->reuseConnection( $conn );
 584                         } )
 585                 );
 586         }
 587
 588         /**
 589          * @param $job Job
 590          * @return array
 591          */
 592         protected function insertFields( Job $job ) {
 593                 list( $dbw, $scope ) = $this->getMasterDB();
 594                 return array(
 595                         // Fields that describe the nature of the job
 596                         'job_cmd'       => $job->getType(),
 597                         'job_namespace' => $job->getTitle()->getNamespace(),
 598                         'job_title'     => $job->getTitle()->getDBkey(),
 599                         'job_params'    => self::makeBlob( $job->getParams() ),
 600                         // Additional job metadata
 601                         'job_id'        => $dbw->nextSequenceValue( 'job_job_id_seq' ),
 602                         'job_timestamp' => $dbw->timestamp(),
 603                         'job_sha1'      => wfBaseConvert(
 604                                 sha1( serialize( $job->getDeduplicationInfo() ) ),
 605                                 16, 36, 31
 606                         ),
 607                         'job_random'    => mt_rand( 0, self::MAX_JOB_RANDOM )
 608                 );
 609         }
 610
 611         /**
 612          * @return string
 613          */
 614         private function getCacheKey( $property ) {
 615                 list( $db, $prefix ) = wfSplitWikiID( $this->wiki );
 616                 return wfForeignMemcKey( $db, $prefix, 'jobqueue', $this->type, $property );
 617         }
 618
 619         /**
 620          * @param string $signature Hash identifier of the root job
 621          * @return string
 622          */
 623         private function getRootJobCacheKey( $signature ) {
 624                 list( $db, $prefix ) = wfSplitWikiID( $this->wiki );
 625                 return wfForeignMemcKey( $db, $prefix, 'jobqueue', $this->type, 'rootjob', $signature );
 626         }
 627
 628         /**
 629          * @param $params
 630          * @return string
 631          */
 632         protected static function makeBlob( $params ) {
 633                 if ( $params !== false ) {
 634                         return serialize( $params );
 635                 } else {
 636                         return '';
 637                 }
 638         }
 639
 640         /**
 641          * @param $blob
 642          * @return bool|mixed
 643          */
 644         protected static function extractBlob( $blob ) {
 645                 if ( (string)$blob !== '' ) {
 646                         return unserialize( $blob );
 647                 } else {
 648                         return false;
 649                 }
 650         }
 651 }