XROOTD - A highly scalable architecture for data access

When dealing with the concurrent access from a multitude of clients to petabyte-scale data repositories, high performance, fault tolerance, robustness, and scalability are four very important issues. This work describes the architecture and the choices done in designing the xrootd file access system. The first goal of the system was to provide access to over 10⁷ files representing several petabytes of experimental physics data. This work addresses the high demand data access needs of modern physics experiments, such as the BaBar experiment at SLAC, and covers method and tools useful to any other field in which reliability, performance and scalability in data access are a primary issue.