A data processing apparatus, comprising a metadata store storing information about files that are stored in a distributed data storage system, and comprising a class database; one or more processing units; logic configured for receiving and storing in the class database a definition of a class of data storage servers comprising one or more subclasses each comprising one or more server selection criteria; associating the class with one or more directories of the data storage system; in response to a data client storing a data file in a directory, binding the class to the data file, determining and storing a set of identifiers of one or more data storage servers in the system that match the server selection criteria, and providing the set of identifiers to the data client.