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 symmet
ry breaking in B meson decays. Within the detector's online system, the Onl
ine Event Processing subsystem (OEP) provides for near-real-time processing
of data delivered from the data acquisition system's event builder, It sup
ports four principal tasks: the final "Level 3" software trigger, rapid-fee
dback 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 pr
otection of data integrity and ensuring that indispensable tasks such as tr
iggering 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 i
n the full offline simulation environment and yet be suitable for near-real
-time online running with minimal changes.