During their navigation web users leave many records of their activity. This huge amount of data can be a useful source of knowledge. Sophisticated mining processes are needed for this knowledge to be extracted, understood and used. In this paper we propose a Web Usage Mining (WUM) system, called \suggest, designed to efficiently integrate the WUM process with the ordinary web server functionalities. It can provide useful information to make easier the web user navigation and to optimize the web server performance. Two quantities are introduced in order to give a measure of the quality of our WUM system.