16. A system for assigning a document identification tag to a new document, the new document to be added to a collection of documents, the system comprising: at least one central processing unit; and a communications bus for connecting the central processing unit to a computer readable medium; the computer readable medium comprising: a data structure representing a subdivision of a predetermined set of monotonically ordered document identification tags into a plurality of tiers, wherein the plurality of tiers are monotonically ordered with respect to a query-independent document importance metric, wherein each tier is associated with a respective subset of the set of document identification tags; an interface configured to receive query-independent information about the new document, the information including the query-independent document importance metric; and a tag assignment module configured to select, based at least on the query-independent information, one of the tiers; assign to the new document a document identification tag from the respective subset of document identification tags associated with the selected tier, the assigned document identification tag not previously assigned to any of the documents in the collection of documents; and store an assignment of the document identification tag from the respective subset of document identification tags associated with the selected tier to the new document in the computer readable medium.
26. The system of claim 16 , wherein the tag assignment module is further configured to detect satisfaction of a flush condition, when the flush condition is satisfied the tag assignment module performing a flush operation, the flush operation including building a first sorted map and a second sorted map; wherein the first sorted map is keyed and sorted by globally unique identifiers, and includes for each globally unique identifier a corresponding document identification tag; and wherein the second sorted map is keyed and sorted by document identification tags assigned to documents since a prior flush operation, and includes for each such document identification tag a corresponding globally unique identifier.