The CMS data acquisition system is made of two major subsystems: event building and event filter. The presented paper describes the architecture and design of the software that processes the data flow in the currently operating experiment. The central DAQ system relies heavily on industry standard networks and processing equipment. Adopting a single software infrastructure in all subsystems of the experiment imposes, however, a number of different requirements. High efficiency and configuration flexibility are among the most important ones. The XDAQ software infrastructure has matured over an eight years development and testing period and has shown to be able to cope well with the CMS requirements. We provide performance figures and report on the initial experience with the system at hand.