the DBMS to communicate with other DBMS or not. Execution autonomy allows a component DBMS to control the operations requested by local and external operations. Association autonomy gives a power to component DBS to disassociate itself from a federation which means FDBS can operate independently of any single DBS. The ANSI/X3/SPARC Study Group outlined a three level data description architecture, the components of which are the conceptual schema, internal schema and external schema of databases. The three level architecture is however inadequate to describing the architectures of an FDBS. It was therefore extended to support the three dimensions of the FDBS namely Distribution, Autonomy and Heterogeneity. The five level schema architecture is explained below. Concurrency control The Heterogeneity and Autonomy requirements pose special challenges concerning concurrency control in an FDBS, which is crucial for the correct execution of its concurrent transactions (see also Global concurrency control). Achieving global serializability, the major correctness criterion, under these requirements has been characterized as very difficult and unsolved. Commitment ordering, introduced in 1991, has provided a general solution for this issue (See Global serializability; See Commitment ordering also for the architectural aspects of the solution). Five level schema architecture for FDBSs The five level schema architecture includes the following: Local Schema is basically the conceptual model of a component database expressed in a native data model. Component schema is the subset of the local schema that the owner organisation is willing to share with other users of the FDBS and it is translated into a common data model. Export Schema represents a subset of a component schema that is available to a particular federation. It may include access control information regarding its use by a specific federation user. The export schema helps in managing flow of control of data. Federated Schema is an integration of multiple export schemas. It includes information on data distribution that is generated when integrating export schemas. External schema is extracted from a federated schema, and is defined for the users/applications of a particular federation. While accurately representing the state of the art in data integration, the Five Level Schema Architecture above does suffer from a major drawback, namely IT imposed look and feel. Modern data users demand control over how data is presented; their needs are somewhat in conflict with such bottom-up approaches to data integration. See also Enterprise Information Integration (EII) Data Virtualization Master data management (MDM) Schema