The EDG Replica Manager manages data held in files. In a Grid environment the data files are replicated, possibly on a temporary basis, to many different sites depending on where the data is needed. The users or applications do not need to know where the data is located. Having a logical name for the data file they should be able to use the data management services to locate and access the data.
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Users and applications can use the Grid Unique IDentifier (GUID) to locate the data; these are assigned at data registration time and are based on the UUID standard to guarantee unique IDs. GUIDs are immutable. All the replicas of the data will share the same GUID.
In order to locate a Grid accessible file, the human user will normally use a Logical File Name (LFN). LFNs are usually more intuitive, human-readable strings as opposed to the cryptic GUIDs, and are allocated by the user. Within the Grid LFNs must be globally unique as well, each LFN being an alias for exactly one GUID. The logical name space can be partitioned at will by the users. A GUID may have many LFNs as aliases. In this document we also call LFNs 'GUID aliases'.
Given either a LFN or GUID, the Metadata and Replica Location Catalogs can be contacted to obtain a Storage URL (SURL) for the physical instance(s) of the required file. The SURL is a URI having the Storage Resource Manager as its hostname which takes responsibility for the storage of the physical file. This Storage Resource Manager (SRM) should be able to accept the SURL and return a valid URL, a Transport URL (TURL) in our terminology, to the application. This TURL provides all the information required (i.e. protocol, host, port and path) so that the application can open and retrieve the required physical file.
Note that Reptor requires both catalogs, Replica Location Service and Replica Metadata Catalog, but an end-user should not need to distinguish between the two of them. In the remainder of the document we use the terms Replica Catalog or Catalog to refer to both of them regardless of their exact functionality.