Abstract: A facility for determining availability status for item groups each containing one or more items is described. For each item whose availability status changes, for each item group containing the item, the facility adds the item group to a list of item groups if the item group is not already present in the list. The facility periodically removes a plurality of item groups from the list. For each item group it removes from the list, the facility determines an availability status for the item group based upon availability statuses of items contained in the group.