9. The method of claim 1 , further comprising: when a flush condition is satisfied, 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 identifier; and wherein the second sorted map is keyed and sorted by document identifiers assigned to documents since a prior flush operation, and includes for each such document identifier a corresponding globally unique identifier.
9. The method of claim 1 , further comprising: when a flush condition is satisfied, 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 identifier; and wherein the second sorted map is keyed and sorted by document identifiers assigned to documents since a prior flush operation, and includes for each such document identifier a corresponding globally unique identifier. 10. The method of claim 9 , wherein the globally unique identifiers are URL fingerprints.