The Name Service cannot really be considered a layer, as it exists as an external service to the XML Store. The XML Store never uses the Name Service - it is the client that employs the service, to associate value references with human-readable names. The simple proposed name service has interface Directory, is implemented in subclass DirectoryImpl. All classes related to the directory are located in package edu.it.xmlstore.directory. See Building applications using the XML Store for a more detailed description of how to use Directory.