Next, I present an article about the basic philosophy that's useful when building a system based on message passing — the fundamental Inter Process Communications (IPC) model used by all QNX operating systems. We'll take a hypothetical security system example, and examine how the design is derived to meet various goals. We'll look at the design of the individual pieces (things like swipe card readers and door lock actuators) and see how they fit into a larger system. We'll also discuss things like scalability — how to make your software design scale from a small security system to a large, multi-site security system.