Data Bookkeeping Service 3 - Providing event metadata in CMS

The Data Bookkeeping Service 3 provides a catalog of event metadata for Monte Carlo and recorded data of the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) at CERN, Geneva. It comprises all necessary information for tracking datasets, their processing history and associations between runs, files and datasets, on a large scale of about $200,000$ datasets and more than $40$ million files, which adds up in around $700$ GB of metadata. The DBS is an essential part of the CMS Data Management and Workload Management (DMWM) systems, all kind of data-processing like Monte Carlo production, processing of recorded event data as well as physics analysis done by the users are heavily relying on the information stored in DBS.