JobQueueDB.php

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<?php
class JobQueueDB extends JobQueue {
        const ROOTJOB_TTL = 1209600; // integer; seconds to remember root jobs (14 days)
        const CACHE_TTL_SHORT = 30; // integer; seconds to cache info without re-validating
        const CACHE_TTL_LONG = 300; // integer; seconds to cache info that is kept up to date
        const MAX_AGE_PRUNE = 604800; // integer; seconds a job can live once claimed
        const MAX_JOB_RANDOM = 2147483647; // integer; 2^31 - 1, used for job_random
        const MAX_OFFSET = 255; // integer; maximum number of rows to skip

        protected $cluster = false; // string; name of an external DB cluster

        protected function __construct( array $params ) {
                parent::__construct( $params );
                $this->cluster = isset( $params['cluster'] ) ? $params['cluster'] : false;
        }

        protected function supportedOrders() {
                return array( 'random', 'timestamp', 'fifo' );
        }

        protected function optimalOrder() {
                return 'random';
        }

        protected function doIsEmpty() {
                global $wgMemc;

                $key = $this->getCacheKey( 'empty' );

                $isEmpty = $wgMemc->get( $key );
                if ( $isEmpty === 'true' ) {
                        return true;
                } elseif ( $isEmpty === 'false' ) {
                        return false;
                }

                list( $dbr, $scope ) = $this->getSlaveDB();
                $found = $dbr->selectField( // unclaimed job
                        'job', '1', array( 'job_cmd' => $this->type, 'job_token' => '' ), __METHOD__
                );
                $wgMemc->add( $key, $found ? 'false' : 'true', self::CACHE_TTL_LONG );

                return !$found;
        }

        protected function doGetSize() {
                global $wgMemc;

                $key = $this->getCacheKey( 'size' );

                $size = $wgMemc->get( $key );
                if ( is_int( $size ) ) {
                        return $size;
                }

                list( $dbr, $scope ) = $this->getSlaveDB();
                $size = (int)$dbr->selectField( 'job', 'COUNT(*)',
                        array( 'job_cmd' => $this->type, 'job_token' => '' ),
                        __METHOD__
                );
                $wgMemc->set( $key, $size, self::CACHE_TTL_SHORT );

                return $size;
        }

        protected function doGetAcquiredCount() {
                global $wgMemc;

                if ( $this->claimTTL <= 0 ) {
                        return 0; // no acknowledgements
                }

                $key = $this->getCacheKey( 'acquiredcount' );

                $count = $wgMemc->get( $key );
                if ( is_int( $count ) ) {
                        return $count;
                }

                list( $dbr, $scope ) = $this->getSlaveDB();
                $count = (int)$dbr->selectField( 'job', 'COUNT(*)',
                        array( 'job_cmd' => $this->type, "job_token != {$dbr->addQuotes( '' )}" ),
                        __METHOD__
                );
                $wgMemc->set( $key, $count, self::CACHE_TTL_SHORT );

                return $count;
        }

        protected function doBatchPush( array $jobs, $flags ) {
                if ( count( $jobs ) ) {
                        list( $dbw, $scope ) = $this->getMasterDB();

                        $rowSet = array(); // (sha1 => job) map for jobs that are de-duplicated
                        $rowList = array(); // list of jobs for jobs that are are not de-duplicated

                        foreach ( $jobs as $job ) {
                                $row = $this->insertFields( $job );
                                if ( $job->ignoreDuplicates() ) {
                                        $rowSet[$row['job_sha1']] = $row;
                                } else {
                                        $rowList[] = $row;
                                }
                        }

                        $key = $this->getCacheKey( 'empty' );
                        $atomic = ( $flags & self::QoS_Atomic );

                        $dbw->onTransactionIdle(
                                function() use ( $dbw, $rowSet, $rowList, $atomic, $key, $scope
                        ) {
                                global $wgMemc;

                                if ( $atomic ) {
                                        $dbw->begin( __METHOD__ ); // wrap all the job additions in one transaction
                                }
                                try {
                                        // Strip out any duplicate jobs that are already in the queue...
                                        if ( count( $rowSet ) ) {
                                                $res = $dbw->select( 'job', 'job_sha1',
                                                        array(
                                                                // No job_type condition since it's part of the job_sha1 hash
                                                                'job_sha1'  => array_keys( $rowSet ),
                                                                'job_token' => '' // unclaimed
                                                        ),
                                                        __METHOD__
                                                );
                                                foreach ( $res as $row ) {
                                                        wfDebug( "Job with hash '{$row->job_sha1}' is a duplicate." );
                                                        unset( $rowSet[$row->job_sha1] ); // already enqueued
                                                }
                                        }
                                        // Build the full list of job rows to insert
                                        $rows = array_merge( $rowList, array_values( $rowSet ) );
                                        // Insert the job rows in chunks to avoid slave lag...
                                        foreach ( array_chunk( $rows, 50 ) as $rowBatch ) {
                                                $dbw->insert( 'job', $rowBatch, __METHOD__ );
                                        }
                                        wfIncrStats( 'job-insert', count( $rows ) );
                                        wfIncrStats( 'job-insert-duplicate',
                                                count( $rowSet ) + count( $rowList ) - count( $rows ) );
                                } catch ( DBError $e ) {
                                        if ( $atomic ) {
                                                $dbw->rollback( __METHOD__ );
                                        }
                                        throw $e;
                                }
                                if ( $atomic ) {
                                        $dbw->commit( __METHOD__ );
                                }

                                $wgMemc->set( $key, 'false', JobQueueDB::CACHE_TTL_LONG );
                        } );
                }

                return true;
        }

        protected function doPop() {
                global $wgMemc;

                if ( $wgMemc->get( $this->getCacheKey( 'empty' ) ) === 'true' ) {
                        return false; // queue is empty
                }

                list( $dbw, $scope ) = $this->getMasterDB();
                $dbw->commit( __METHOD__, 'flush' ); // flush existing transaction
                $autoTrx = $dbw->getFlag( DBO_TRX ); // get current setting
                $dbw->clearFlag( DBO_TRX ); // make each query its own transaction
                $scopedReset = new ScopedCallback( function() use ( $dbw, $autoTrx ) {
                        $dbw->setFlag( $autoTrx ? DBO_TRX : 0 ); // restore old setting
                } );

                $uuid = wfRandomString( 32 ); // pop attempt
                $job = false; // job popped off
                do { // retry when our row is invalid or deleted as a duplicate
                        // Try to reserve a row in the DB...
                        if ( in_array( $this->order, array( 'fifo', 'timestamp' ) ) ) {
                                $row = $this->claimOldest( $uuid );
                        } else { // random first
                                $rand = mt_rand( 0, self::MAX_JOB_RANDOM ); // encourage concurrent UPDATEs
                                $gte = (bool)mt_rand( 0, 1 ); // find rows with rand before/after $rand
                                $row = $this->claimRandom( $uuid, $rand, $gte );
                        }
                        // Check if we found a row to reserve...
                        if ( !$row ) {
                                $wgMemc->set( $this->getCacheKey( 'empty' ), 'true', self::CACHE_TTL_LONG );
                                break; // nothing to do
                        }
                        wfIncrStats( 'job-pop' );
                        // Get the job object from the row...
                        $title = Title::makeTitleSafe( $row->job_namespace, $row->job_title );
                        if ( !$title ) {
                                $dbw->delete( 'job', array( 'job_id' => $row->job_id ), __METHOD__ );
                                wfDebugLog( 'JobQueueDB', "Row has invalid title '{$row->job_title}'." );
                                continue; // try again
                        }
                        $job = Job::factory( $row->job_cmd, $title,
                                self::extractBlob( $row->job_params ), $row->job_id );
                        $job->id = $row->job_id; // XXX: work around broken subclasses
                        // Flag this job as an old duplicate based on its "root" job...
                        if ( $this->isRootJobOldDuplicate( $job ) ) {
                                wfIncrStats( 'job-pop-duplicate' );
                                $job = DuplicateJob::newFromJob( $job ); // convert to a no-op
                        }
                        break; // done
                } while( true );

                return $job;
        }

        protected function claimRandom( $uuid, $rand, $gte ) {
                global $wgMemc;

                list( $dbw, $scope ) = $this->getMasterDB();
                // Check cache to see if the queue has <= OFFSET items
                $tinyQueue = $wgMemc->get( $this->getCacheKey( 'small' ) );

                $row = false; // the row acquired
                $invertedDirection = false; // whether one job_random direction was already scanned
                // This uses a replication safe method for acquiring jobs. One could use UPDATE+LIMIT
                // instead, but that either uses ORDER BY (in which case it deadlocks in MySQL) or is
                // not replication safe. Due to http://bugs.mysql.com/bug.php?id=6980, subqueries cannot
                // be used here with MySQL.
                do {
                        if ( $tinyQueue ) { // queue has <= MAX_OFFSET rows
                                // For small queues, using OFFSET will overshoot and return no rows more often.
                                // Instead, this uses job_random to pick a row (possibly checking both directions).
                                $ineq = $gte ? '>=' : '<=';
                                $dir = $gte ? 'ASC' : 'DESC';
                                $row = $dbw->selectRow( 'job', '*', // find a random job
                                        array(
                                                'job_cmd'   => $this->type,
                                                'job_token' => '', // unclaimed
                                                "job_random {$ineq} {$dbw->addQuotes( $rand )}" ),
                                        __METHOD__,
                                        array( 'ORDER BY' => "job_random {$dir}" )
                                );
                                if ( !$row && !$invertedDirection ) {
                                        $gte = !$gte;
                                        $invertedDirection = true;
                                        continue; // try the other direction
                                }
                        } else { // table *may* have >= MAX_OFFSET rows
                                // Bug 42614: "ORDER BY job_random" with a job_random inequality causes high CPU
                                // in MySQL if there are many rows for some reason. This uses a small OFFSET
                                // instead of job_random for reducing excess claim retries.
                                $row = $dbw->selectRow( 'job', '*', // find a random job
                                        array(
                                                'job_cmd'   => $this->type,
                                                'job_token' => '', // unclaimed
                                        ),
                                        __METHOD__,
                                        array( 'OFFSET' => mt_rand( 0, self::MAX_OFFSET ) )
                                );
                                if ( !$row ) {
                                        $tinyQueue = true; // we know the queue must have <= MAX_OFFSET rows
                                        $wgMemc->set( $this->getCacheKey( 'small' ), 1, 30 );
                                        continue; // use job_random
                                }
                        }
                        if ( $row ) { // claim the job
                                $dbw->update( 'job', // update by PK
                                        array(
                                                'job_token'           => $uuid,
                                                'job_token_timestamp' => $dbw->timestamp(),
                                                'job_attempts = job_attempts+1' ),
                                        array( 'job_cmd' => $this->type, 'job_id' => $row->job_id, 'job_token' => '' ),
                                        __METHOD__
                                );
                                // This might get raced out by another runner when claiming the previously
                                // selected row. The use of job_random should minimize this problem, however.
                                if ( !$dbw->affectedRows() ) {
                                        $row = false; // raced out
                                }
                        } else {
                                break; // nothing to do
                        }
                } while ( !$row );

                return $row;
        }

        protected function claimOldest( $uuid ) {
                list( $dbw, $scope ) = $this->getMasterDB();

                $row = false; // the row acquired
                do {
                        if ( $dbw->getType() === 'mysql' ) {
                                // Per http://bugs.mysql.com/bug.php?id=6980, we can't use subqueries on the
                                // same table being changed in an UPDATE query in MySQL (gives Error: 1093).
                                // Oracle and Postgre have no such limitation. However, MySQL offers an
                                // alternative here by supporting ORDER BY + LIMIT for UPDATE queries.
                                $dbw->query( "UPDATE {$dbw->tableName( 'job' )} " .
                                        "SET " .
                                                "job_token = {$dbw->addQuotes( $uuid ) }, " .
                                                "job_token_timestamp = {$dbw->addQuotes( $dbw->timestamp() )}, " .
                                                "job_attempts = job_attempts+1 " .
                                        "WHERE ( " .
                                                "job_cmd = {$dbw->addQuotes( $this->type )} " .
                                                "AND job_token = {$dbw->addQuotes( '' )} " .
                                        ") ORDER BY job_id ASC LIMIT 1",
                                        __METHOD__
                                );
                        } else {
                                // Use a subquery to find the job, within an UPDATE to claim it.
                                // This uses as much of the DB wrapper functions as possible.
                                $dbw->update( 'job',
                                        array(
                                                'job_token'           => $uuid,
                                                'job_token_timestamp' => $dbw->timestamp(),
                                                'job_attempts = job_attempts+1' ),
                                        array( 'job_id = (' .
                                                $dbw->selectSQLText( 'job', 'job_id',
                                                        array( 'job_cmd' => $this->type, 'job_token' => '' ),
                                                        __METHOD__,
                                                        array( 'ORDER BY' => 'job_id ASC', 'LIMIT' => 1 ) ) .
                                                ')'
                                        ),
                                        __METHOD__
                                );
                        }
                        // Fetch any row that we just reserved...
                        if ( $dbw->affectedRows() ) {
                                $row = $dbw->selectRow( 'job', '*',
                                        array( 'job_cmd' => $this->type, 'job_token' => $uuid ), __METHOD__
                                );
                                if ( !$row ) { // raced out by duplicate job removal
                                        wfDebugLog( 'JobQueueDB', "Row deleted as duplicate by another process." );
                                }
                        } else {
                                break; // nothing to do
                        }
                } while ( !$row );

                return $row;
        }

        public function recycleAndDeleteStaleJobs() {
                global $wgMemc;

                $now = time();
                list( $dbw, $scope ) = $this->getMasterDB();
                $count = 0; // affected rows

                if ( !$dbw->lock( "jobqueue-recycle-{$this->type}", __METHOD__, 1 ) ) {
                        return $count; // already in progress
                }

                // Remove claims on jobs acquired for too long if enabled...
                if ( $this->claimTTL > 0 ) {
                        $claimCutoff = $dbw->timestamp( $now - $this->claimTTL );
                        // Get the IDs of jobs that have be claimed but not finished after too long.
                        // These jobs can be recycled into the queue by expiring the claim. Selecting
                        // the IDs first means that the UPDATE can be done by primary key (less deadlocks).
                        $res = $dbw->select( 'job', 'job_id',
                                array(
                                        'job_cmd' => $this->type,
                                        "job_token != {$dbw->addQuotes( '' )}", // was acquired
                                        "job_token_timestamp < {$dbw->addQuotes( $claimCutoff )}", // stale
                                        "job_attempts < {$dbw->addQuotes( $this->maxTries )}" ), // retries left
                                __METHOD__
                        );
                        $ids = array_map( function( $o ) { return $o->job_id; }, iterator_to_array( $res ) );
                        if ( count( $ids ) ) {
                                // Reset job_token for these jobs so that other runners will pick them up.
                                // Set the timestamp to the current time, as it is useful to now that the job
                                // was already tried before (the timestamp becomes the "released" time).
                                $dbw->update( 'job',
                                        array(
                                                'job_token' => '',
                                                'job_token_timestamp' => $dbw->timestamp( $now ) ), // time of release
                                        array(
                                                'job_id' => $ids ),
                                        __METHOD__
                                );
                                $count += $dbw->affectedRows();
                                wfIncrStats( 'job-recycle', $dbw->affectedRows() );
                                $wgMemc->set( $this->getCacheKey( 'empty' ), 'false', self::CACHE_TTL_LONG );
                        }
                }

                // Just destroy any stale jobs...
                $pruneCutoff = $dbw->timestamp( $now - self::MAX_AGE_PRUNE );
                $conds = array(
                        'job_cmd' => $this->type,
                        "job_token != {$dbw->addQuotes( '' )}", // was acquired
                        "job_token_timestamp < {$dbw->addQuotes( $pruneCutoff )}" // stale
                );
                if ( $this->claimTTL > 0 ) { // only prune jobs attempted too many times...
                        $conds[] = "job_attempts >= {$dbw->addQuotes( $this->maxTries )}";
                }
                // Get the IDs of jobs that are considered stale and should be removed. Selecting
                // the IDs first means that the UPDATE can be done by primary key (less deadlocks).
                $res = $dbw->select( 'job', 'job_id', $conds, __METHOD__ );
                $ids = array_map( function( $o ) { return $o->job_id; }, iterator_to_array( $res ) );
                if ( count( $ids ) ) {
                        $dbw->delete( 'job', array( 'job_id' => $ids ), __METHOD__ );
                        $count += $dbw->affectedRows();
                }

                $dbw->unlock( "jobqueue-recycle-{$this->type}", __METHOD__ );

                return $count;
        }

        protected function doAck( Job $job ) {
                if ( !$job->getId() ) {
                        throw new MWException( "Job of type '{$job->getType()}' has no ID." );
                }

                list( $dbw, $scope ) = $this->getMasterDB();
                $dbw->commit( __METHOD__, 'flush' ); // flush existing transaction
                $autoTrx = $dbw->getFlag( DBO_TRX ); // get current setting
                $dbw->clearFlag( DBO_TRX ); // make each query its own transaction
                $scopedReset = new ScopedCallback( function() use ( $dbw, $autoTrx ) {
                        $dbw->setFlag( $autoTrx ? DBO_TRX : 0 ); // restore old setting
                } );

                // Delete a row with a single DELETE without holding row locks over RTTs...
                $dbw->delete( 'job',
                        array( 'job_cmd' => $this->type, 'job_id' => $job->getId() ), __METHOD__ );

                return true;
        }

        protected function doDeduplicateRootJob( Job $job ) {
                $params = $job->getParams();
                if ( !isset( $params['rootJobSignature'] ) ) {
                        throw new MWException( "Cannot register root job; missing 'rootJobSignature'." );
                } elseif ( !isset( $params['rootJobTimestamp'] ) ) {
                        throw new MWException( "Cannot register root job; missing 'rootJobTimestamp'." );
                }
                $key = $this->getRootJobCacheKey( $params['rootJobSignature'] );
                // Callers should call batchInsert() and then this function so that if the insert
                // fails, the de-duplication registration will be aborted. Since the insert is
                // deferred till "transaction idle", do the same here, so that the ordering is
                // maintained. Having only the de-duplication registration succeed would cause
                // jobs to become no-ops without any actual jobs that made them redundant.
                list( $dbw, $scope ) = $this->getMasterDB();
                $dbw->onTransactionIdle( function() use ( $params, $key, $scope ) {
                        global $wgMemc;

                        $timestamp = $wgMemc->get( $key ); // current last timestamp of this job
                        if ( $timestamp && $timestamp >= $params['rootJobTimestamp'] ) {
                                return true; // a newer version of this root job was enqueued
                        }

                        // Update the timestamp of the last root job started at the location...
                        return $wgMemc->set( $key, $params['rootJobTimestamp'], JobQueueDB::ROOTJOB_TTL );
                } );

                return true;
        }

        protected function isRootJobOldDuplicate( Job $job ) {
                global $wgMemc;

                $params = $job->getParams();
                if ( !isset( $params['rootJobSignature'] ) ) {
                        return false; // job has no de-deplication info
                } elseif ( !isset( $params['rootJobTimestamp'] ) ) {
                        trigger_error( "Cannot check root job; missing 'rootJobTimestamp'." );
                        return false;
                }

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

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

        protected function doWaitForBackups() {
                wfWaitForSlaves();
        }

        protected function doGetPeriodicTasks() {
                return array(
                        'recycleAndDeleteStaleJobs' => array(
                                'callback' => array( $this, 'recycleAndDeleteStaleJobs' ),
                                'period'   => ceil( $this->claimTTL / 2 )
                        )
                );
        }

        protected function doFlushCaches() {
                global $wgMemc;

                foreach ( array( 'empty', 'size', 'acquiredcount' ) as $type ) {
                        $wgMemc->delete( $this->getCacheKey( $type ) );
                }
        }

        public function getAllQueuedJobs() {
                list( $dbr, $scope ) = $this->getSlaveDB();
                return new MappedIterator(
                        $dbr->select( 'job', '*', array( 'job_cmd' => $this->getType(), 'job_token' => '' ) ),
                        function( $row ) use ( $scope ) {
                                $job = Job::factory(
                                        $row->job_cmd,
                                        Title::makeTitle( $row->job_namespace, $row->job_title ),
                                        strlen( $row->job_params ) ? unserialize( $row->job_params ) : false,
                                        $row->job_id
                                );
                                $job->id = $row->job_id; // XXX: work around broken subclasses
                                return $job;
                        }
                );
        }

        protected function getSlaveDB() {
                return $this->getDB( DB_SLAVE );
        }

        protected function getMasterDB() {
                return $this->getDB( DB_MASTER );
        }

        protected function getDB( $index ) {
                $lb = ( $this->cluster !== false )
                        ? wfGetLBFactory()->getExternalLB( $this->cluster, $this->wiki )
                        : wfGetLB( $this->wiki );
                $conn = $lb->getConnection( $index, array(), $this->wiki );
                return array(
                        $conn,
                        new ScopedCallback( function() use ( $lb, $conn ) {
                                $lb->reuseConnection( $conn );
                        } )
                );
        }

        protected function insertFields( Job $job ) {
                list( $dbw, $scope ) = $this->getMasterDB();
                return array(
                        // Fields that describe the nature of the job
                        'job_cmd'       => $job->getType(),
                        'job_namespace' => $job->getTitle()->getNamespace(),
                        'job_title'     => $job->getTitle()->getDBkey(),
                        'job_params'    => self::makeBlob( $job->getParams() ),
                        // Additional job metadata
                        'job_id'        => $dbw->nextSequenceValue( 'job_job_id_seq' ),
                        'job_timestamp' => $dbw->timestamp(),
                        'job_sha1'      => wfBaseConvert(
                                sha1( serialize( $job->getDeduplicationInfo() ) ),
                                16, 36, 31
                        ),
                        'job_random'    => mt_rand( 0, self::MAX_JOB_RANDOM )
                );
        }

        private function getCacheKey( $property ) {
                list( $db, $prefix ) = wfSplitWikiID( $this->wiki );
                return wfForeignMemcKey( $db, $prefix, 'jobqueue', $this->type, $property );
        }

        private function getRootJobCacheKey( $signature ) {
                list( $db, $prefix ) = wfSplitWikiID( $this->wiki );
                return wfForeignMemcKey( $db, $prefix, 'jobqueue', $this->type, 'rootjob', $signature );
        }

        protected static function makeBlob( $params ) {
                if ( $params !== false ) {
                        return serialize( $params );
                } else {
                        return '';
                }
        }

        protected static function extractBlob( $blob ) {
                if ( (string)$blob !== '' ) {
                        return unserialize( $blob );
                } else {
                        return false;
                }
        }
}