A method for fulfilling requests in an inventory system with a management module, comprising: receiving, in the management module, a request indicating an inventory item to retrieve; selecting, with the management module, from a plurality of holders containing inventory items, a holder containing the inventory item; selecting, with the management module, from a plurality of inventory stations, at least one inventory station at which to fulfill the request, wherein each of the plurality of inventory stations is associated with a queue comprising a plurality of queue spaces; selecting, with the management module, from a plurality of self-powered mobile drive units, a mobile drive unit to move the selected holder from a storage location of the selected holder to the selected inventory station; moving the mobile drive unit to the storage location and docking the mobile drive unit with the selected holder; moving the selected holder with the mobile drive unit from the storage location towards the selected inventory station; determining with the management module that a threshold number of queue spaces in a queue associated with the selected inventory station are already assigned to holders; in response to determining that the threshold number of queue spaces are already assigned, selecting one of a plurality of unoccupied drift spaces with the management module, wherein a drift space is a predetermined location in a workspace remote from the inventory stations utilized by the inventory system to allow a selected holder to progressively move closer to the selected inventory station; moving the selected holder to the selected drift space with the selected mobile drive unit; determining with the management module that one or more of the queue spaces in the queue associated with the selected inventory station are no longer assigned to a holder; in response to determining that one or more of the queue spaces are no longer assigned to a holder, moving the selected holder from the selected drift space to a queue space of the queue associated with the selected inventory station with the selected mobile drive unit. receiving, in the management module, a request indicating an inventory item to retrieve; selecting, with the management module, from a plurality of holders containing inventory items, a holder containing the inventory item; selecting, with the management module, from a plurality of inventory stations, at least one inventory station at which to fulfill the request, wherein each of the plurality of inventory stations is associated with a queue comprising a plurality of queue spaces; selecting, with the management module, from a plurality of self-powered mobile drive units, a mobile drive unit to move the selected holder from a storage location of the selected holder to the selected inventory station; moving the mobile drive unit to the storage location and docking the mobile drive unit with the selected holder; moving the selected holder with the mobile drive unit from the storage location towards the selected inventory station; determining with the management module that a threshold number of queue spaces in a queue associated with the selected inventory station are already assigned to holders; in response to determining that the threshold number of queue spaces are already assigned, selecting one of a plurality of unoccupied drift spaces with the management module, wherein a drift space is a predetermined location in a workspace remote from the inventory stations utilized by the inventory system to allow a selected holder to progressively move closer to the selected inventory station; moving the selected holder to the selected drift space with the selected mobile drive unit; determining with the management module that one or more of the queue spaces in the queue associated with the selected inventory station are no longer assigned to a holder; in response to determining that one or more of the queue spaces are no longer assigned to a holder, moving the selected holder from the selected drift space to a queue space of the queue associated with the selected inventory station with the selected mobile drive unit.