Design and performance of the COMPASS online event filter

With the start-up of the LHC still at the horizon, COMPASS is the experiment most actively taking data at CERN. The high trigger rate creates a demanding environment for the implementation of an online event filter. This paper presents the design principles and key features as well as the performance during the previous beam times, which had been devoted to deep inelastic muon scattering measurements. An outlook on the second phase of COMPASS, devoted to hadron spectroscopy, will be given in the end.