Views of CMS Event Data: Objects, Files, Collections, Virtual Data Products

The CMS data grid system will store many types of data maintained by the CMS collaboration. An important type of data is the event data, which is defined in this note as all data that directly represents simulated, raw, or reconstructed CMS physics events. Many views on this data will exist simultaneously. To a CMS physics code implementer this data will appear as C++ objects, to a tape robot operator the data will appear as files. This note identifies different views that can exist, describes each of them, and interrelates them by placing them into a vertical stack. This particular stack integrates several existing architectural structures, and is therefore a plausible basis for further prototyping and architectural work. This document is intended as a contribution to, and as common (terminological) reference material for, the CMS architectural efforts and for the Grid projects PPDG, GriPhyN, and the EU DataGrid.