1. A method, comprising: receiving publisher data specifying slot inventory that is available for allocation, the slot inventory including one or more advertisement slots on one or more web pages provided for a publisher; receiving advertiser data from two or more advertisers, the advertiser data specifying first allocation constraints for a first set of advertisements that are provided by a first advertiser and second allocation constraints for a second set of advertisements that are provided by a second advertiser, the first allocation constraints specifying a first minimum number of impressions that must be allocated, independent of an auction, to the first advertiser and a first budget, the second allocation constraints specifying a second minimum number of impressions that must be allocated, independent of an auction, to the second advertiser and a second budget; reserving, independent of an auction, a first set of impressions for the first advertiser and a second set of impressions for the second advertiser, the first set of impressions including the first minimum number of impressions, and the second set of impressions including the second minimum number of impressions; determining, by one or more computers, that an estimated number of available impressions includes additional impressions beyond the first minimum number of impressions and the second minimum number of impressions; releasing, from reservation and for allocation through an auction, at least one impression from the first set of impressions that were reserved for the first advertiser; reserving, for the first advertiser, one or more of the additional impressions to replace at least the released at least one impression, the additional impressions being reserved for the first advertiser to prevent violation of the first allocation constraints following the release of the at least one impression; and allocating the at least one impression through the auction, the allocation being performed without violating the first allocation constraints or the second allocation constraints.