[en] The Electromagnetic Calorimeter (ECAL) of the Compact Muon Solenoid (CMS) detector at the CERN Large Hadron Collider (LHC) is a crystal homogeneous calorimeter made of about 76 000 lead tungstate crystals. The detector was installed in the CMS experimental cavern in 2007 and 2008 and was commissioned with cosmic rays and with LHC beams in 2008. The trigger and data acquisition system of the CMS ECAL comprises 35 000 Front End ASICs and 170 Off Detector VME Boards. The operation of the system, performed by the ECAL online software, requires the configuration of O(10⁷) parameters and the realtime monitoring of O(10⁵) registers. In this paper we discuss the design and architecture of the ECAL online software and its performances in cosmic ray runs and with the first LHC beams.