1. A non-transitory computer-readable medium embodying a program executable in at least one computing device, wherein, when executed, the program causes the at least one computing device to at least: in response to receiving a request for a network page from a client, the request indicating a network page associated with a network site of a first party, fetch page generation code supplied by the first party, the network site being hosted by a second party on behalf of the first party; aggregate first party data from a plurality of data sources, wherein individual ones of the plurality of data sources provide a respective plurality of data items using a respective multipart-encoded data stream; determine that a subset of the first party data is received, the subset of the first party data corresponding to data items that are provided to the page generation code to generate at least a portion of the network page; and execute the page generation code supplied by the first party to generate the at least a portion of the network page; and facilitate access by the page generation code to the subset of the first party data as a plurality of predefined variables.