13. The non-transitory machine-readable storage medium of claim 8 , wherein receiving the request for the bucketing token from the ranking service comprises receiving a request for a content item of the plurality of associated content items, wherein the bucketing token is associated with the content item.
13. The non-transitory machine-readable storage medium of claim 8 , wherein receiving the request for the bucketing token from the ranking service comprises receiving a request for a content item of the plurality of associated content items, wherein the bucketing token is associated with the content item. 14. The non-transitory machine-readable storage medium of claim 13 , wherein providing the bucketing token to the ranking service comprises providing the requested content item to the ranking service, wherein the bucketing token is embedded in a metadata of the content item.