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 selected SSDL file; preparing a buffer based on a schema information about a result of the transmitted query; generating an instance of the selected SU; intializing 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 by a service operator (SO) module, wherein the SUAL file comprises the SSDL file, the SU, the query and the execution mode, the execution mode comprises a push mode and a pull mode, one instance of the selected SU per user request is generated and preserved during a requested time duration in the push mode, and a predetermined number of the instances of the selected SU is generated and managed in an instance pool in the pull mode, wherein the SO module comprises: a user request handler (URH) that registers the user request and transmits the result of the user request to the user; a scheduler that controls a processing schedule of the user request registered in the URH in response to a resource usage state; a service unit instance manager (SUIM) that processes the user request by executing the SUI by processing the SUI in response to the processing schedule from the scheduler according to the execution mode and provides the result of the user request to the URH; and a runtime moniter (RM) that monitors the resource usage state of each component and the SUI that is currently being executed, and provides the resource usage state to the scheduler, wherein the SUIM generates one SUI per user request and the generated SUI becomes extinct after a requested time duration when the execution mode corresponds to the push mode.