OBK - An Online High Energy Physics' Meta-Data Repository

ATLAS will be one of the four detectors for the LHC (Large Hadron Collider) particle accelerator currently being built at CERN, Geneva. The project is expected to start production in 2006 and during its lifetime (15-20 years) to generate roughly one petabyte per year of particle physics' data. This vast amount of information will require several meta-data repositories which will ease the manipulation and understanding of physics' data by the final users (physicists doing analysis). Metadata repositories and tools at ATLAS may address such problems as the logical organization of the physics data according to data taking sessions, errors and faults during data gathering, data quality or terciary storage meta-information. The OBK (Online Book-Keeper) is a component of ATLAS' Online Software - the system which provides configuration, control and monitoring services to the DAQ (Data AQquisition system). In this paper we will explain the role of the OBK as one of the main collectors and managers of meta-data produced online, how that data is stored and the interfaces that are provided to access it - merging the physics data with the collected metadata will play an essential role for future analysis and interpretion of the physics events observed at ATLAS. We also provide an historical background to the OBK by analysing the several prototypes implemented in the context of our software development process and the results and experience obtained with the various DBMS technologies used.