171、As we all know, databases were originally constructed as a means of consolidating or centralizing information.The more modem perspective, however, is to view databases as a means of integrating information that may be stored on different machines throughout a network or an internet.For instance, an international corporation may be stored and maintain local employee records at local sites yet link these records via a network to create a distributed database——a single large integrated database that consists of data residing on different machines.A distributed database may contain fragmented and/or replicated data.The first case is exemplified by the previous employee-records example in which different fragments of the database are stored in different locations.In the second case, duplicates of the same database component are stored at different locations.Such replication may occur as a means of reducing information retrieval time.Both cases pose problems not present in more traditional centralized systems - such as how to disguise the distributed nature of the database so that it functions as a coherent system or how to ensure that replicated portions of a database remain duplicates of each other as updates occur.In turn, the study of distributed databases is a current area of research.(5)、What is the passage mainly about?