1. A non-transitory computer-readable medium embodying a program executable in a computing device, the program comprising: code that, in response to receiving a request for a network page from a client, receives customer data from a plurality of data sources, wherein the network page is associated with a network site hosted by a hosting provider on behalf of a customer, and individual ones of the plurality of data sources provide a respective plurality of data items using a respective multipart-encoded data stream; code that executes a first page generation code supplied by the customer to generate a first portion of the network page in response to receiving all of a first subset of the customer data which is used by the first page generation code, the first page generation code being executed before all of a second subset of the customer data is received, wherein the code that executes the first page generation code is configured to facilitate access by the first page generation code to the first subset of the customer data as a first plurality of predefined variables; code that executes a second page generation code supplied by the customer to generate a second portion of the network page in response to receiving all of the second subset of the customer data which is used by the second page generation code, wherein the code that executes the second page generation code is configured to facilitate access by the second page generation code to the second subset of the customer data as a second plurality of predefined variables; code that generates the network page, the network page including the first portion of the network page at a first predefined location and the second portion of the network page at a second predefined location, wherein the first portion of the network page is generated before the second portion of the network page; code that sends the network page to the client in response to the request, wherein the second portion of the network page is sent to the client before the first portion of the network page according to a predefined sequence; and code that takes an action in response to detecting that the first page generation code or the second page generation code at least attempts to access the plurality of data sources directly, wherein the action includes refraining from executing the first page generation code or the second page generation code, terminating the first page generation code or the second page generation code, or replacing the first page generation code or the second page generation code with default page generation code.