1. A system, comprising: a data processing apparatus including one or more computers; a data store storing advertising campaign data used by an advertising service, the advertising campaign data conforming to a flat Application Programming Interface (API), and including a plurality of campaign management entities including an advertising campaign entity that defines an advertising campaign; and a computer storage system storing instructions that when executed by the data processing apparatus cause the data processing apparatus to perform actions comprising: accessing a plurality of rules, each rule associated with at least one of the advertising campaign entities, each rule defining an operation and a corresponding event, the event being an occurrence of a condition defined for the advertising campaign defined by the advertising campaign entity associated with the rule, and the operation being executed in response to detection of the corresponding event and specifying an entity change for a campaign management entity associated with the advertising campaign entity associated with the rule, wherein the rules utilize a hierarchical data-model API to access the campaign management entities; converting, during execution of a given rule, each hierarchical data-model API call for campaign management entities specified in the given rule to a flat API call for the campaign management entities specified in the given rule, each hierarchical data-model API call being a call that uses only data objects to specify campaign management entities, and each flat API call being a call that uses a service request and a data object to access campaign management entities, the converting comprising: determining, from at least one hierarchical data-model API call specified in the given rule, a corresponding service request and a data object for use in each flat API call; and generating each flat API call using the service call and the data object determined from the at least one data-model API call; and accessing advertising campaign data for the advertising campaign defined by the advertising campaign entity associated with the given rule using each flat API call.