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+<?php
+
+/**
+ * @file
+ * Queue functionality.
+ */
+
+/**
+ * @defgroup queue Queue operations
+ * @{
+ * Queue items to allow later processing.
+ *
+ * The queue system allows placing items in a queue and processing them later.
+ * The system tries to ensure that only one consumer can process an item.
+ *
+ * Before a queue can be used it needs to be created by
+ * DrupalQueueInterface::createQueue().
+ *
+ * Items can be added to the queue by passing an arbitrary data object to
+ * DrupalQueueInterface::createItem().
+ *
+ * To process an item, call DrupalQueueInterface::claimItem() and specify how
+ * long you want to have a lease for working on that item. When finished
+ * processing, the item needs to be deleted by calling
+ * DrupalQueueInterface::deleteItem(). If the consumer dies, the item will be
+ * made available again by the DrupalQueueInterface implementation once the
+ * lease expires. Another consumer will then be able to receive it when calling
+ * DrupalQueueInterface::claimItem(). Due to this, the processing code should
+ * be aware that an item might be handed over for processing more than once.
+ *
+ * The $item object used by the DrupalQueueInterface can contain arbitrary
+ * metadata depending on the implementation. Systems using the interface should
+ * only rely on the data property which will contain the information passed to
+ * DrupalQueueInterface::createItem(). The full queue item returned by
+ * DrupalQueueInterface::claimItem() needs to be passed to
+ * DrupalQueueInterface::deleteItem() once processing is completed.
+ *
+ * There are two kinds of queue backends available: reliable, which preserves
+ * the order of messages and guarantees that every item will be executed at
+ * least once. The non-reliable kind only does a best effort to preserve order
+ * in messages and to execute them at least once but there is a small chance
+ * that some items get lost. For example, some distributed back-ends like
+ * Amazon SQS will be managing jobs for a large set of producers and consumers
+ * where a strict FIFO ordering will likely not be preserved. Another example
+ * would be an in-memory queue backend which might lose items if it crashes.
+ * However, such a backend would be able to deal with significantly more writes
+ * than a reliable queue and for many tasks this is more important. See
+ * aggregator_cron() for an example of how to effectively utilize a
+ * non-reliable queue. Another example is doing Twitter statistics -- the small
+ * possibility of losing a few items is insignificant next to power of the
+ * queue being able to keep up with writes. As described in the processing
+ * section, regardless of the queue being reliable or not, the processing code
+ * should be aware that an item might be handed over for processing more than
+ * once (because the processing code might time out before it finishes).
+ */
+
+/**
+ * Factory class for interacting with queues.
+ */
+class DrupalQueue {
+ /**
+ * Returns the queue object for a given name.
+ *
+ * The following variables can be set by variable_set or $conf overrides:
+ * - queue_class_$name: the class to be used for the queue $name.
+ * - queue_default_class: the class to use when queue_class_$name is not
+ * defined. Defaults to SystemQueue, a reliable backend using SQL.
+ * - queue_default_reliable_class: the class to use when queue_class_$name is
+ * not defined and the queue_default_class is not reliable. Defaults to
+ * SystemQueue.
+ *
+ * @param $name
+ * Arbitrary string. The name of the queue to work with.
+ * @param $reliable
+ * TRUE if the ordering of items and guaranteeing every item executes at
+ * least once is important, FALSE if scalability is the main concern.
+ *
+ * @return
+ * The queue object for a given name.
+ */
+ public static function get($name, $reliable = FALSE) {
+ static $queues;
+ if (!isset($queues[$name])) {
+ $class = variable_get('queue_class_' . $name, NULL);
+ if (!$class) {
+ $class = variable_get('queue_default_class', 'SystemQueue');
+ }
+ $object = new $class($name);
+ if ($reliable && !$object instanceof DrupalReliableQueueInterface) {
+ $class = variable_get('queue_default_reliable_class', 'SystemQueue');
+ $object = new $class($name);
+ }
+ $queues[$name] = $object;
+ }
+ return $queues[$name];
+ }
+}
+
+interface DrupalQueueInterface {
+
+ /**
+ * Add a queue item and store it directly to the queue.
+ *
+ * @param $data
+ * Arbitrary data to be associated with the new task in the queue.
+ * @return
+ * TRUE if the item was successfully created and was (best effort) added
+ * to the queue, otherwise FALSE. We don't guarantee the item was
+ * committed to disk etc, but as far as we know, the item is now in the
+ * queue.
+ */
+ public function createItem($data);
+
+ /**
+ * Retrieve the number of items in the queue.
+ *
+ * This is intended to provide a "best guess" count of the number of items in
+ * the queue. Depending on the implementation and the setup, the accuracy of
+ * the results of this function may vary.
+ *
+ * e.g. On a busy system with a large number of consumers and items, the
+ * result might only be valid for a fraction of a second and not provide an
+ * accurate representation.
+ *
+ * @return
+ * An integer estimate of the number of items in the queue.
+ */
+ public function numberOfItems();
+
+ /**
+ * Claim an item in the queue for processing.
+ *
+ * @param $lease_time
+ * How long the processing is expected to take in seconds, defaults to an
+ * hour. After this lease expires, the item will be reset and another
+ * consumer can claim the item. For idempotent tasks (which can be run
+ * multiple times without side effects), shorter lease times would result
+ * in lower latency in case a consumer fails. For tasks that should not be
+ * run more than once (non-idempotent), a larger lease time will make it
+ * more rare for a given task to run multiple times in cases of failure,
+ * at the cost of higher latency.
+ * @return
+ * On success we return an item object. If the queue is unable to claim an
+ * item it returns false. This implies a best effort to retrieve an item
+ * and either the queue is empty or there is some other non-recoverable
+ * problem.
+ */
+ public function claimItem($lease_time = 3600);
+
+ /**
+ * Delete a finished item from the queue.
+ *
+ * @param $item
+ * The item returned by DrupalQueueInterface::claimItem().
+ */
+ public function deleteItem($item);
+
+ /**
+ * Release an item that the worker could not process, so another
+ * worker can come in and process it before the timeout expires.
+ *
+ * @param $item
+ * @return boolean
+ */
+ public function releaseItem($item);
+
+ /**
+ * Create a queue.
+ *
+ * Called during installation and should be used to perform any necessary
+ * initialization operations. This should not be confused with the
+ * constructor for these objects, which is called every time an object is
+ * instantiated to operate on a queue. This operation is only needed the
+ * first time a given queue is going to be initialized (for example, to make
+ * a new database table or directory to hold tasks for the queue -- it
+ * depends on the queue implementation if this is necessary at all).
+ */
+ public function createQueue();
+
+ /**
+ * Delete a queue and every item in the queue.
+ */
+ public function deleteQueue();
+}
+
+/**
+ * Reliable queue interface.
+ *
+ * Classes implementing this interface preserve the order of messages and
+ * guarantee that every item will be executed at least once.
+ */
+interface DrupalReliableQueueInterface extends DrupalQueueInterface {
+}
+
+/**
+ * Default queue implementation.
+ */
+class SystemQueue implements DrupalReliableQueueInterface {
+ /**
+ * The name of the queue this instance is working with.
+ *
+ * @var string
+ */
+ protected $name;
+
+ public function __construct($name) {
+ $this->name = $name;
+ }
+
+ public function createItem($data) {
+ // During a Drupal 6.x to 7.x update, drupal_get_schema() does not contain
+ // the queue table yet, so we cannot rely on drupal_write_record().
+ $query = db_insert('queue')
+ ->fields(array(
+ 'name' => $this->name,
+ 'data' => serialize($data),
+ // We cannot rely on REQUEST_TIME because many items might be created
+ // by a single request which takes longer than 1 second.
+ 'created' => time(),
+ ));
+ return (bool) $query->execute();
+ }
+
+ public function numberOfItems() {
+ return db_query('SELECT COUNT(item_id) FROM {queue} WHERE name = :name', array(':name' => $this->name))->fetchField();
+ }
+
+ public function claimItem($lease_time = 30) {
+ // Claim an item by updating its expire fields. If claim is not successful
+ // another thread may have claimed the item in the meantime. Therefore loop
+ // until an item is successfully claimed or we are reasonably sure there
+ // are no unclaimed items left.
+ while (TRUE) {
+ $item = db_query_range('SELECT data, item_id FROM {queue} q WHERE expire = 0 AND name = :name ORDER BY created ASC', 0, 1, array(':name' => $this->name))->fetchObject();
+ if ($item) {
+ // Try to update the item. Only one thread can succeed in UPDATEing the
+ // same row. We cannot rely on REQUEST_TIME because items might be
+ // claimed by a single consumer which runs longer than 1 second. If we
+ // continue to use REQUEST_TIME instead of the current time(), we steal
+ // time from the lease, and will tend to reset items before the lease
+ // should really expire.
+ $update = db_update('queue')
+ ->fields(array(
+ 'expire' => time() + $lease_time,
+ ))
+ ->condition('item_id', $item->item_id)
+ ->condition('expire', 0);
+ // If there are affected rows, this update succeeded.
+ if ($update->execute()) {
+ $item->data = unserialize($item->data);
+ return $item;
+ }
+ }
+ else {
+ // No items currently available to claim.
+ return FALSE;
+ }
+ }
+ }
+
+ public function releaseItem($item) {
+ $update = db_update('queue')
+ ->fields(array(
+ 'expire' => 0,
+ ))
+ ->condition('item_id', $item->item_id);
+ return $update->execute();
+ }
+
+ public function deleteItem($item) {
+ db_delete('queue')
+ ->condition('item_id', $item->item_id)
+ ->execute();
+ }
+
+ public function createQueue() {
+ // All tasks are stored in a single database table (which is created when
+ // Drupal is first installed) so there is nothing we need to do to create
+ // a new queue.
+ }
+
+ public function deleteQueue() {
+ db_delete('queue')
+ ->condition('name', $this->name)
+ ->execute();
+ }
+}
+
+/**
+ * Static queue implementation.
+ *
+ * This allows "undelayed" variants of processes relying on the Queue
+ * interface. The queue data resides in memory. It should only be used for
+ * items that will be queued and dequeued within a given page request.
+ */
+class MemoryQueue implements DrupalQueueInterface {
+ /**
+ * The queue data.
+ *
+ * @var array
+ */
+ protected $queue;
+
+ /**
+ * Counter for item ids.
+ *
+ * @var int
+ */
+ protected $id_sequence;
+
+ /**
+ * Start working with a queue.
+ *
+ * @param $name
+ * Arbitrary string. The name of the queue to work with.
+ */
+ public function __construct($name) {
+ $this->queue = array();
+ $this->id_sequence = 0;
+ }
+
+ public function createItem($data) {
+ $item = new stdClass();
+ $item->item_id = $this->id_sequence++;
+ $item->data = $data;
+ $item->created = time();
+ $item->expire = 0;
+ $this->queue[$item->item_id] = $item;
+ }
+
+ public function numberOfItems() {
+ return count($this->queue);
+ }
+
+ public function claimItem($lease_time = 30) {
+ foreach ($this->queue as $key => $item) {
+ if ($item->expire == 0) {
+ $item->expire = time() + $lease_time;
+ $this->queue[$key] = $item;
+ return $item;
+ }
+ }
+ return FALSE;
+ }
+
+ public function deleteItem($item) {
+ unset($this->queue[$item->item_id]);
+ }
+
+ public function releaseItem($item) {
+ if (isset($this->queue[$item->item_id]) && $this->queue[$item->item_id]->expire != 0) {
+ $this->queue[$item->item_id]->expire = 0;
+ return TRUE;
+ }
+ return FALSE;
+ }
+
+ public function createQueue() {
+ // Nothing needed here.
+ }
+
+ public function deleteQueue() {
+ $this->queue = array();
+ $this->id_sequence = 0;
+ }
+}
+
+/**
+ * @} End of "defgroup queue".
+ */