**5.1 File Catalogue (FC)**

The File Catalogue is one of the key components of the AliEn suite. It provides a hierarchical structure (like a UNIX File system) and is designed to allow each directory node in the hierarchy to be supported by different database engines, running on different hosts. This building on top of several databases allows to add another database to expand the catalogue namespace and assures scalability of the system and allow growth of the catalogue as the files accumulate over the years.

Unlike real file systems, the FC does not own the files; it is a metadata catalogue on the Logical File Names (LFN) and only keeps an association/mapping between the LFNs and (possibly multiple) Physical File Names (PFN) of real files on a storage system. PFNs describe the physical location of the files and include the access protocol (rfio, xrootd), the name of the AliEn Storage Element and the path to the local file. The system supports file replication and caching.

The FC provides also a mapping between the LFNs and Globally Unique Identifiers (GUID). The labeling of each file with the GUID allows for the asynchronous caching. The write-once strategy combined with GUID labeling guarantees the identity of files with the same GUID label in different caches. It is possible to automatically construct PFNs : to store only the GUID and Storage Index and the Storage Element builds the PFN from the GUID. There are two independent catalogues: LFN->GUID and GUID->PFN. A schema of the AliEn FC is shown in Figure 15.

The FC can also associate metadata to the LFNs. This metadata is a collection of user-defined key value pairs. For instance, in the case of ALICE, the current metadata is the software version used to generate the files, number of events inside a file, or calibration files used during the reconstruction.
