This paper presents a reference architecture for distributed database management systems from both system and schema perspectives, and discusses how various federated database system (FDBS) architectures can be developed. It defines a methodology for developing one of the popular FDBS architectures and discusses critical issues related to developing and operating an FDBS. The paper also explores the characteristics of database systems, the taxonomy of multi-DBMS and federated database systems, and the scope and organization of the paper. A federated database system is a collection of cooperating but autonomous component database systems. These systems may be heterogeneous, meaning they can use different data models, query languages, and transaction management capabilities. The federated database management system (FDBMS) is the software that provides controlled and coordinated manipulation of the component DBSs. The concept of federation exists in many contexts, including political entities like the United Nations and the Soviet Union. In both cases, there is varying levels of autonomy and heterogeneity among the components. The key characteristic of a federation is the cooperation among independent systems. In terms of an FDBS, this is reflected by controlled and sometimes limited integration of autonomous DBSs. The paper discusses the characteristics of database systems, including distribution, heterogeneity, and autonomy. It also discusses the taxonomy of multi-DBMS and federated database systems, and the scope and organization of the paper. The paper defines a reference architecture for distributed database management systems, which includes system components, processor types, and schema types. It also discusses various FDBS architectures, including loosely coupled and tightly coupled FDBSs, and the development of FDBSs. The paper discusses the evolution process of FDBSs, the tasks involved in developing an FDBS, and the operation of an FDBS. It also discusses future research and unsolved problems in the field of federated database systems. The paper concludes with an acknowledgment, references, a bibliography, a glossary, and an appendix comparing some features of relevant prototype efforts.This paper presents a reference architecture for distributed database management systems from both system and schema perspectives, and discusses how various federated database system (FDBS) architectures can be developed. It defines a methodology for developing one of the popular FDBS architectures and discusses critical issues related to developing and operating an FDBS. The paper also explores the characteristics of database systems, the taxonomy of multi-DBMS and federated database systems, and the scope and organization of the paper. A federated database system is a collection of cooperating but autonomous component database systems. These systems may be heterogeneous, meaning they can use different data models, query languages, and transaction management capabilities. The federated database management system (FDBMS) is the software that provides controlled and coordinated manipulation of the component DBSs. The concept of federation exists in many contexts, including political entities like the United Nations and the Soviet Union. In both cases, there is varying levels of autonomy and heterogeneity among the components. The key characteristic of a federation is the cooperation among independent systems. In terms of an FDBS, this is reflected by controlled and sometimes limited integration of autonomous DBSs. The paper discusses the characteristics of database systems, including distribution, heterogeneity, and autonomy. It also discusses the taxonomy of multi-DBMS and federated database systems, and the scope and organization of the paper. The paper defines a reference architecture for distributed database management systems, which includes system components, processor types, and schema types. It also discusses various FDBS architectures, including loosely coupled and tightly coupled FDBSs, and the development of FDBSs. The paper discusses the evolution process of FDBSs, the tasks involved in developing an FDBS, and the operation of an FDBS. It also discusses future research and unsolved problems in the field of federated database systems. The paper concludes with an acknowledgment, references, a bibliography, a glossary, and an appendix comparing some features of relevant prototype efforts.