  The ATLAS EventIndex system comprises the catalogue of all events collected,
processed or generated by the ATLAS experiment at the CERN LHC accelerator, and
all associated software tools to collect, store and query this information.
ATLAS records several billion particle interactions every year of operation,
processes them for analysis and generates even larger simulated data samples; a
global catalogue is needed to keep track of the location of each event record
and be able to search and retrieve specific events for in-depth investigations.
Each EventIndex record includes summary information on the event itself and the
pointers to the files containing the full event. Most components of the
EventIndex system are implemented using BigData open-source tools. This paper
describes the architectural choices and their evolution in time, as well as the
past, current and foreseen future implementations of all EventIndex components.
