Flexible processing framework for online event data and software triggering

The BABAR experiment is the particle detector at the new PEP-II "B factory" facility at the Stanford Linear Accelerator Center. The experiment's goal is the detailed study of the fundamental phenomenon of charge-parity symmetry breaking in B meson decays. Within the detector's online system, the Online Event Processing subsystem (OEP) provides for near-real-time processing of data delivered from the data acquisition system's event builder. It supports four principal tasks: the final "Level 3" software trigger, rapid-feedback fast data quality monitoring, event displays, and the final steps in online calibrations. In order to accommodate the expected 2000 Hz event rate, the OEP subsystem runs on computing farm, receiving events on each node into a shared memory buffer system and coordinating processes performing analysis on this data. Analysis tasks are assigned administrative characteristics providing for protection of data integrity and ensuring that indispensable tasks such as triggering are always performed, while others are performed as resources are available. Data access in OEP uses BABAR's standard offline reconstruction framework. Thus, notably, the Level 3 trigger software could be developed in advance in the full offline simulation environment and yet be suitable for near-real-time online running with minimal changes.