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Complexity. A DDBMS that hides the distributed nature from the user and provides an acceptable level of performance, reliability, and availability is inherently more complex than a centralized DBMS. Replication also adds an extra level of complexity, which if not handled adequately, will lead to degradation in availability, reliability, and performance compared with the centralized system, and the advantages we cited above will become disadvantages.
Cost. Increased complexity means that we can expect the procurement and maintenance costs for a DDBMS to be higher than those for a centralized DBMS. Furthermore, a DDBMS requires additional hardware to establish a network between sites. There are ongoing communication costs incurred with the use of this network. There are also additional manpower costs to manage and maintain the local DBMSs and the underlying network.
Security. In a centralized system, access to the data can be easily controlled. However, in a DDBMS not only does access to replicated data have to be controlled in multiple locations, but the network itself has to be made secure. In the past, networks were regarded as an insecure communication medium. Although this is still partially true, significant developments have been made recently to make networks more secure.
Integrity control more difficult. Enforcing integrity constraints generally requires access to a large amount of data that defines the constraint, but is not involved in the actual update operation itself. In a DDBMS, the communication and processing costs that are required to enforce integrity constraints may be prohibitive.
Lack of standards. Although DDBMSs depend on effective communication, we are only now starting to see the appearance of standard communication and data access protocols. This lack of standards has significantly limited the potential of DDBMSs. There are also no tools or methodologies to help users convert a centralized DBMS into a distributed DBMS.
Lack of experience. General-purpose DDBMSs have not been widely accepted, although many of the protocols and problems are well understood. Consequently, we do not yet have the same level of experience in industry as we have with centralized DBMSs. For a prospective adopter of this technology, this may be a significant deterrent.
Database design more complex. Besides the normal difficulties of designing a centralized database, the design of a distributed database has to take account of fragmentation of data, allocation of fragments to specific sites, and data replication.
kalvinspartan answered the question on July 4, 2018 at 16:47
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