1. A method of processing a data stream (DS), comprising: selecting a Stream Source Description Language (SSDL) file and a Service Unit (SU), the SSDL file and the SU being assigned by a Service Unit Activation Language (SUAL) file; transmitting a query written by the selected SUAL file to a corresponding stream provider based on a stream provider address written by the selected SSDL file; preparing a buffer based on schema information about a result of the transmitted query; generating an instance of the selected SU; initializing the selected SU into an execution mode assigned by the SUAL file; updating, in real time, data stored in the prepared buffer by using the DS provided by the stream provider; and processing the data stored in the buffer according to the execution mode in response to a user request to provide a result of the user request to a user.