interface QueueInterface

interface QueueInterface {
    
    /**
     * Adds a queue item and store it directly to the queue.
     *
     * @param $data
     *   Arbitrary data to be associated with the new task in the queue.
     *
     * @return
     *   A unique ID if the item was successfully created and was (best effort)
     *   added to the queue, otherwise FALSE. We don't guarantee the item was
     *   committed to disk etc, but as far as we know, the item is now in the
     *   queue.
     */
    public function createItem($data);
    
    /**
     * Retrieves 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 int
     *   An integer estimate of the number of items in the queue.
     */
    public function numberOfItems();
    
    /**
     * Claims an item in the queue for processing.
     *
     * @param $lease_time
     *   How long the processing is expected to take in seconds, defaults to an
     *   hour. After this lease expires, the item will be reset and another
     *   consumer can claim the item. For idempotent tasks (which can be run
     *   multiple times without side effects), shorter lease times would result
     *   in lower latency in case a consumer fails. For tasks that should not be
     *   run more than once (non-idempotent), a larger lease time will make it
     *   more rare for a given task to run multiple times in cases of failure,
     *   at the cost of higher latency.
     *
     * @return
     *   On success we return an item object. If the queue is unable to claim an
     *   item it returns false. This implies a best effort to retrieve an item
     *   and either the queue is empty or there is some other non-recoverable
     *   problem.
     *
     *   If returned, the object will have at least the following properties:
     *   - data: the same as what what passed into createItem().
     *   - item_id: the unique ID returned from createItem().
     *   - created: timestamp when the item was put into the queue.
     */
    public function claimItem($lease_time = 3600);
    
    /**
     * Deletes a finished item from the queue.
     *
     * @param $item
     *   The item returned by \Drupal\Core\Queue\QueueInterface::claimItem().
     */
    public function deleteItem($item);
    
    /**
     * Releases an item that the worker could not process.
     *
     * Another worker can come in and process it before the timeout expires.
     *
     * @param $item
     *   The item returned by \Drupal\Core\Queue\QueueInterface::claimItem().
     *
     * @return bool
     *   TRUE if the item has been released, FALSE otherwise.
     */
    public function releaseItem($item);
    
    /**
     * Creates 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();
    
    /**
     * Deletes a queue and every item in the queue.
     */
    public function deleteQueue();

}