  Among the many challenges presented by the future ATLAS detector at the LHC
are the high data taking rate and volume and the derivation of a rapid trigger
decision with limited resources. To address this challenge within the ATLAS
second level trigger system, a Region-of-Interest mechanism has been adopted
which dramatically reduces the relevant fiducial volume necessary to be readout
and processed to small regions guided by the hardware-based first level
trigger. Software has been developed to allow fast translation between
arbitrary geometric regions and identifiers of small collections of the event
data. This facilitates on-demand data retrieval and collection building. The
system is optimized to minimize the amount of data transferred and unnecessary
building of complex objects. Details of the design and implementation are
presented along with preliminary performance results.
