Using simulations of a complex system such as a Web Application Server provides us with a tool that allows the running of tests without using the resources needed in a real system; thus enabling a "what-if" analysis platform that gives support in the decisions taking process. This paper goes through the methodology and the construction of this light and fast simulation, showing the points to take care of and that have made this approach successful. Our evaluation shows some of the results obtained with simulations, compares them with the real system and provides new information to investigate hypotheses; as for instance the effect of modifying the backlog queue component on overloaded systems, to increase the number of sessions finished. We have used several results from research on the effect of admission control over SSL connections to verify our simulated approach.