<?php
// $Id$

/**
 * @file
 * Queue functionality.
 */

/**
 * @defgroup queue Queue operations
 * @{
 * 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 DrapalQueueInterface implementation once the
 * lease expires. Another consumer will then be able to receive it when calling
 * DrupalQueueInterface::claimItem().
 *
 * 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.
 *
 * While the queue system makes a best effort to preserve order in messages,
 * due to the pluggable nature of the queue, there is no guarantee that items
 * will be delivered on claim in the order they were sent. For example, some
 * implementations like beanstalkd or others with 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.
 *
 * The system also makes no guarantees about a task only being executed once:
 * callers that have non-idempotent tasks either need to live with the
 * possiblity of the task being invoked multiple times in cases where a claim
 * lease expires, or need to implement their own transactions to make their
 * tasks idempotent.
 */

/**
 * Factory class for interacting with queues.
 */
class DrupalQueue {
  /**
   * Get a queue object for a given name.
   *
   * @param $name
   *   Arbitrary string. The name of the queue to work with.
   * @return
   *   The queue object for a given name.
   */
  public static function get($name) {
    static $queues;
    if (!isset($queues[$name])) {
      $class = variable_get('queue_module_' . $name, 'System') . 'Queue';
      $queues[$name] = new $class($name);
    }
    return $queues[$name];
  }
}

interface DrupalQueueInterface {
  /**
   * Start working with a queue.
   *
   * @param $name
   *   Arbitrary string. The name of the queue to work with.
   */
  public function __construct($name);

  /**
   * Add a queue item and store it directly to the queue.
   *
   * @param $data
   *   Arbitrary data to be associated with the new task in the queue.
   * @return
   *   TRUE if the item was successfully created and was (best effort) added
   *   to the queue, otherwise FALSE. We don't guarantee the item was
   *   committed to disk, that your disk wasn't hit by a meteor, etc, but as
   *   far as we know, the item is now in the queue.
   */
  public function createItem($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);

  /**
   * 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();
}

/**
 * Default queue implementation.
 */
class SystemQueue implements DrupalQueueInterface {
  /**
   * Our internal consumer ID for this queue instance.
   *
   * This is created lazily when we start consuming items with claimItem().
   *
   * @var integer
   */
  protected $consumerId;

  /**
   * 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) {
    $record = new stdClass();
    $record->name = $this->name;
    $record->data = $data;
    // We cannot rely on REQUEST_TIME because many items might be created by a
    // single request which takes longer than 1 second.
    $record->created = time();
    return drupal_write_record('queue', $record) !== FALSE;
  }

  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 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 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();
  }
}

/**
 * @} End of "defgroup queue".
 */