Design, deployment and functional tests of the on-line event filter for the ATLAS experiment at LHC

The event filter selection stage is a fundamental component of the ATLAS trigger and data acquisition architecture. Its primary function is the reduction of data flow and rate to values acceptable by the mass storage operations and by the subsequent off-line data reconstruction and analysis steps. The computing instrument of the EF is generally organized as a set of independent sub-farms, each connected to one output of the event builder switch fabric. Each sub-farm comprises a number of processors analyzing several complete events in parallel. This paper describes the design of the ATLAS EF system, its deployment in the 2004 ATLAS combined test beam together with some examples of integrating selection and monitoring algorithms. Since the processing algorithms are not specially designed for EF but are inherited as much as possible from the off-line ones, special emphasis is reserved to system reliability and data security, in particular for the case of failures in the processing algorithms. Another key design element has been system modularity and scalability. The EF shall be able to follow technology evolution and should allow for using additional processing resources possibly remotely located.