Return the modified item for further processing
	 * in the next pass through. Or, return false to remove the
	 * item from the queue.
	 *
	 * @param mixed $task Queue item to iterate over.
	 *
	 * @return mixed
	 */
	abstract protected function task( $task );

	private function is_active_instance( $instance_id ) {
		return $instance_id === $this->get_active_instance_id();
	}

	/**
	 * Save the unique ID of the process we are presuming to be dead, so we can prevent it from coming back.
	 *
	 * @param $instance_id
	 *
	 * @return void
	 */
	private function set_active_instance_id( $instance_id ) {
		update_site_option( $this->get_active_instance_option_id(), $instance_id );
	}

	private function get_active_instance_id() {
		return get_site_option( $this->get_active_instance_option_id(), '' );
	}

	private function get_active_instance_option_id() {
		return $this->identifier . '_active_instance';
	}

	private function set_process_id( $instance_id ) {
		update_site_option( $this->get_process_id_option_key(), $instance_id );
	}

	public function get_process_id() {
		return get_site_option( $this->get_process_id_option_key() );
	}

	private function delete_process_id() {
		delete_site_option( $this->get_process_id_option_key() );
	}

	private function get_process_id_option_key() {
		return $this->identifier . '_process_id';
	}

	public function set_logger( $logger ) {
		$this->logger_container->set_logger( $logger );
	}

	/**
	 * @return Background_Logger_Container
	 */
	private function logger() {
		return $this->logger_container;
	}

	public function get_status() {
		return $this->status;
	}

	/**
	 * @param $tasks array
	 *
	 * @return void
	 */
	public function start( $tasks ) {
		$this->do_action( 'before_start' );

		$total_items = count( $tasks );
		$this->status->start( $total_items );
		$this->update_queue( $tasks );
		// Generate ID for the whole process.
		$this->set_process_id( $this->generate_unique_id() );

		$this->logger()->info( "Starting new process with $total_items tasks" );

		// Trigger the started event before dispatching the request to ensure it is called before the completed event.
		$this->do_action( 'started' );

		$this->spawn();
	}

	private function mutex( $operation ) {
		$mutex = new Mutex( $this->get_handler_mutex_id() );
		$mutex->set_break_on_timeout( true ) // Let the previous handler do its thing 
		      ->set_timeout( $this->get_lock_duration() )
		      ->execute( $operation );
	}

	private function get_handler_mutex_id() {
		return $this->identifier . '_handler_lock';
	}

	private function get_time_limit() {
		return apply_filters( $this->identifier . '_default_time_limit', 20 ); // 20 seconds
	}

	private function get_lock_duration() {
		$lock_duration = ( property_exists( $this, 'queue_lock_time' ) ) ? $this->queue_lock_time : 60; // 1 minute

		return apply_filters( $this->identifier . '_queue_lock_time', $lock_duration );
	}

	protected function get_instance_expiry_duration_seconds() {
		return MINUTE_IN_SECONDS * 2;
	}

	private function get_last_run_transient_key() {
		return $this->identifier . '_last_run';
	}

	private function clear_last_run_timestamp() {
		delete_site_transient( $this->get_last_run_transient_key() );
	}

	private function cleanup() {
		// Delete options and transients
		$this->delete_queue();
		delete_site_option( $this->get_active_instance_option_id() );
		$this->delete_process_id();
		$this->delete_revival_count();
		$this->clear_last_run_timestamp();

		// Cancel all events
		$this->clear_scheduled_event();
	}

	private function task_limit_reached( $processed_tasks_count ) {
		if ( $this->get_tasks_per_request() === self::TASKS_PER_REQUEST_UNLIMITED ) {
			return false;
		}

		return $processed_tasks_count >= $this->get_tasks_per_request();
	}

	public function get_tasks_per_request() {
		return $this->tasks_per_request;
	}

	/**
	 * @param int $tasks_per_request
	 */
	public function set_tasks_per_request( $tasks_per_request ) {
		$this->tasks_per_request = $tasks_per_request;
	}

	private function do_action( $action ) {
		do_action( "{$this->identifier}_$action", $this->identifier, $this );
	}

	private function get_cron_interval_seconds() {
		$minutes = property_exists( $this, 'cron_interval' )
			? $this->cron_interval
			: 5;

		$interval = apply_filters( $this->identifier . '_cron_interval', $minutes );

		return $interval * MINUTE_IN_SECONDS;
	}

	public function get_identifier() {
		return $this->identifier;
	}

	protected function should_update_queue_after_task() {
		return false;
	}

	private function increment_revival_count() {
		$revival_count = $this->get_revival_count();
		$this->set_revival_count( $revival_count + 1 );
	}

	private function set_revival_count( $instance_id ) {
		update_site_option( $this->get_revival_count_option_key(), $instance_id );
	}

	public function get_revival_count() {
		return (int) get_site_option( $this->get_revival_count_option_key(), 0 );
	}

	private function delete_revival_count() {
		delete_site_option( $this->get_revival_count_option_key() );
	}

	private function get_revival_count_option_key() {
		return $this->identifier . '_revival_count';
	}

	protected function get_revival_limit() {
		return apply_filters( $this->identifier . '_revival_limit', 5 );
	}

	protected function is_revival_limit_reached() {
		return $this->get_revival_count() >= $this->get_revival_limit();
	}
}