The applications of ubiquitous sensor networks require database system to support the following three functions in addition with conventional database functions. (1) continual event monitoring. Since control systems such as robots perform accurately, event monitoring must be executed in strict real-time. (2) signal processing. To recognize events in the physical world, sensor data must be processed by non traditional way such as similar sequence retrievals. (3) fast signal stream persisting. All of sensor data should be stored to consider the reason of illegal events after accidents or offline data mining. To support the requirements, we propose a new database system KRAFT. To realize (1), KRAFT controls user-level threads on FreeBSD KSE scheduler. To realize (2), KRAFT provides similar sequence retrieval operators. The operators' distance functions are Euclidean and dynamic time warping. To realize (3), KRAFT provides direct persisting, which does not execute the write ahead logging process. We describe preliminary results of experiments and show the performance of KRAFT.