1. One or more computer hardware storage media having computer-executable instructions embodied thereon, that when executed, cause a computing device to perform a method for maintaining a user inventory for digital advertising, the method comprising: receiving a request for user data associated with a user identification, the user identification including identification signals; determining that user data associated with the user identification is not available in an in-cache memory due to a loss or a purge of one or more cookies stored therein; requesting from a user identification mapping service a list of alternative identifications corresponding to the user, each alternative identification including at least one identification signal in common with the identification signals of the user identification, the at least one identification signal stored in a user identification mapping store separate from the in-cache memory; sorting the list of alternative identifications to identify a match to the user identification; associating the match, identification signals associated with the match, and user data associated with the match with the user identification for which user data is not available in the in-cache memory; and communicating the user data associated with the match in response to an advertising call.