1. A computer-implemented method in a content delivery network (CDN), and on a device comprising hardware including memory and at least one processor, the computer-implemented method comprising: (A) receiving a request for a CD service of a particular service type, wherein a CD service of said particular service type defines a particular number of configurable layers of request processing associated with said CD service of said particular service type, sequentially from a particular first layer to a particular last layer; (B) processing said request, starting at said particular first layer, said processing being based on a modifiable runtime environment, said processing continuing conditionally through each of said particular layers in turn until either said request is terminated by one of said layers or said particular last layer processes said request, wherein said runtime environment comprises a modifiable control environment and a modifiable request environment, said modifiable request environment being based on said request, wherein said modifiable control environment comprises a modifiable global control environment, and wherein said modifiable control environment is distinct from said modifiable request environment, wherein the first layer determines the modifiable request environment from information associated with the request initially received in (A), and wherein each subsequent layer obtains the modifiable request environment from the previous layer, wherein, in processing of said request, at least one of said layers modifies said modifiable control environment to produce a modified control environment, and wherein processing of said request by a subsequent layer is based on the modified control environment, wherein, in processing of said request, at least one of said layers modifies said runtime environment.