An aggregation service acts as an intermediary between a template (or template processor) and a set of two or more services that supply data for generating web pages or other types of content items. The aggregation service stores dependency information in connection with the set of services, and uses this information to control the order and timing with which service requests are made on behalf of the template/template processor. The aggregation service aggregates the retrieved service data for incorporation into the template, and in some cases, formats the service data for display.