Prototype of a File-Based High-Level Trigger in CMS

The DAQ system of the CMS experiment at the LHC is upgraded during the\naccelerator shutdown in 2013/14. To reduce the interdependency of the DAQ\nsystem and the high-level trigger (HLT), we investigate the feasibility of\nusing a file-system-based HLT. Events of $\\sim$1\\,MB size are built at the level-1\ntrigger rate of 100\\,kHz. The events are assembled by $\\sim$50 builder units\n(BUs). Each BU writes the raw events at $\\sim$2\\,GB/s to a local file system shared\nwith $\\cal O$(10) filter-unit machines (FUs) running the HLT code. The FUs read the\nraw data from the file system, select $\\cal O$(1\\%) of the events, and write the\nselected events together with monitoring meta-data back to a disk. This data is\nthen aggregated over several steps and made available for offline\nreconstruction and online monitoring. We present the challenges, technical\nchoices, and performance figures from the prototyping phase. In addition, the\nsteps to the final system implementation will be discussed.