The implementation of persistency in the Compact Muon Solenoid (CMS) Software Framework uses the core I/O functionality of ROOT. We will discuss the current ROOT/IO implementation, its evolution from the prior Objectivity/DB{trademark} implementation, and the plans and ongoing work for the conversion to ''POOL'', provided by the LHC Computing Grid (LCG) persistency project. The CMS experiment [1] is one of the four approved LHC experiments. Data taking is scheduled to begin in 2007, and will last at least ten years. The CMS software and computing task [2] will be 10-1000 times larger than that of current HEP experiments. Therefore it is essential that software must be modular, flexible, and maintainable as well as providing high performance and quality. One of the technologies utilized has been a C++ based object oriented database management system (ODBMS). Originally, the specific implementation used for object persistency was a commercial product, Objectivity/DB [3]. In 2001, it became apparent that Objectivity was not the optimal long term solution for data persistency, and that it was necessary to abandon Objectivity with a very short time scale. A decision was made to directly use ROOT/IO [4] as a component of an interim persistency implementation. In the very near future, the LHC computing grid persistency project will provide POOL [5] as an implementation for persistency. This paper primarily covers the conversion from Objectivity/DB to ROOT/IO. Also briefly discussed is the ongoing transition to POOL.
