  ATLAS event data processing requires access to non-event data (detector
conditions, calibrations, etc.) stored in relational databases. The
database-resident data are crucial for the event data reconstruction processing
steps and often required for user analysis. A main focus of ATLAS database
operations is on the worldwide distribution of the Conditions DB data, which
are necessary for every ATLAS data processing job. Since Conditions DB access
is critical for operations with real data, we have developed the system where a
different technology can be used as a redundant backup. Redundant database
operations infrastructure fully satisfies the requirements of ATLAS
reprocessing, which has been proven on a scale of one billion database queries
during two reprocessing campaigns of 0.5 PB of single-beam and cosmics data on
the Grid. To collect experience and provide input for a best choice of
technologies, several promising options for efficient database access in user
analysis were evaluated successfully. We present ATLAS experience with scalable
database access technologies and describe our approach for prevention of
database access bottlenecks in a Grid computing environment.
