Idézet a címbeli könyv 564-565. oldaláról:
A batch set can have the following keys in its associative array. Only the operations key is required.
(...)
* file: If the callback functions for operations and finished are not in scope during a normal Drupal request, the path of the file containing these functions must be given. The path is relative to the base_path() of the Drupal installation and can be conveniently built using drupal_get_path(). It is unnecessary to define file if the functions are already in scope.
Nos, ez a legutóbbi mondat nem (vagy nem teljesen így) igaz - folytatás a KYbest blogján; hozzászólásokat is oda kérném, ha vannak.