The concept of S variable streams is based on the interaction between client and server. As a variation on the classical client/server model, there are the following three connections in the SDF-P context:
On the client side, the user sends S variables (created either by the system, or by a DECLARE-VARIABLE or SET-VARIABLE command) to the server, and/or receives variables sent back by the server.
The server - FHS or SDF-P itself - processes these S variables as necessary for the client’s requirements. Thus, for example, FHS can be requested to output the S variables received from the client to screen masks, and to update them. If, in addition, the client expects an answer, FHS will later send these or other variables back to the client.
SDF-P has - in addition to the optional task of server - the role of controller or router. Thus, for example, SDF-P is responsible for establishing the server’s link to the client at runtime, because this link is not a fixed one; instead it can be chosen dynamically, to ensure that the client code has the maximum possible independence from the server.