1. An enterprise computing system comprising: a backend server designed to store directory information based on a relational model, and to provide directory information as corresponding responses to queries based on said relational model, said relational model defining a plurality of tables, each table having corresponding columns; and a directory server to process directory requests based on a first hierarchical model, said first hierarchical model defining a plurality of attributes, said directory server comprises at least a processor and a memory, wherein execution of instructions stored in said memory causes said directory server to perform the actions of: generate a set of classes representing structure of said plurality of tables in said relational model, said set of classes being according to a second hierarchical model, each column of said plurality of tables corresponding to a field of one of said set of classes, wherein said second hierarchical model is different from said first hierarchical model; receive a first directory request for values corresponding to a set of attributes in said first hierarchical model; identify a set of tables, and a set of columns of said set of tables, in said relational model corresponding to said set of attributes; invoke an interface of a mapping module with an input data, said input data being specified in the form of a subset of said set of classes and fields of said subset, respectively representing said identified set of tables and said set of columns, wherein said mapping module, upon invocation of said interface, is designed to construct a relational query corresponding to said input data by employing said subset of said set of classes and fields according to said second hierarchical model, said relational query being based on said relational model, whereby said relational query based on said relational model is constructed upon said invocation; execute said relational query in said backend server to retrieve a set of values of said directory information, said set of values corresponding to said set of attributes; and send a first reply to said first directory request, said first reply containing said set of values retrieved from said backend server, said set of values being presented according to said first hierarchical model and corresponding to said set of attributes in said first reply.