1. A method performed by data processing apparatus, the method comprising: receiving request data indicating that a user device has requested a content item, the request data specifying one or more other data processing apparatus to which user interactions with the content item are to be reported, the content item including a reference to a resource that is requested in response to user interaction with the content item; providing response data in response to the request data, the response data including data that cause presentation of the content item; receiving interaction data specifying that user interaction with the content item occurred; providing redirect data that cause the user device to be redirected to the resource; and providing reporting data to each of the one or more other data processing apparatus, the reporting data specifying that the user interaction with the content item occurred, wherein the reporting data is provided asynchronously relative to the redirect data.