Software Trigger Systems in High Energy Physics

In order to efficiently collect and store experimental data, the High Energy Physics experiments need a trigger system to separate --- in real-time --- interesting experimental events (signal) from uninteresting ones (background). In the aim of optimising the performance, a modern trigger system splits the data selection task in two or more stages, where the highest level one is usually implemented as a great deal of software processes (software trigger) running in parallel on the nodes of a computer farm which can reach the size of 2000-5000 PCs. Data link technologies and operating environment used in the most challenging High Level Triggers of the recent High Energy Physics experiments are here reviewed. In particular, the High Level Trigger of the LHCb experiment at CERN, which has the highest input event rate (1.1 MHz) among the forthcoming experiments, is taken as an example.