An ad collision machine can be configured to evaluate collision queries for possible ad collisions and is associated with an ad datacenter configured to evaluate and respond to bid requests on behalf of a plurality of advertisers. The ad collision machine can comprise a plurality of nodes and a data cache containing a plurality of user ID-campaign ID keys representing recently submitted bids in response to bid requests. Once a selected node receives a collision query, a user ID-campaign ID key is retrieved from the collision query. If the first key is not found in the data cache, it is written to the data cache by the node and the ad collision machine returns that user ID-campaign ID pair as available to be bid on.