Items of an item set may be stored by an item server (e.g., mail messages comprising a mailbox stored by a mail server) that supports the application of actions, but only within a constraint (e.g., a maximum duration or computational resources consumed by the action). The application of complex actions to large item sets may exceed the constraint if performed in one request. Moreover, the request may be made by a user interface of a user device, but the user interface may stall while interoperating with the item server to perform the action. Instead, a background process may issue a sequence of requests applying the action to respective batches of items within the constraint. This architecture may enable the user interface to remain responsive to the user while the action is performed, and even to be terminated without jeopardizing the completion of the action upon the item set.