Ans :
The primary disadvantage of distributed database systems is the added complexity
required to ensure proper coordination among the sites. This increased complexity
takes the form of:
Higher Software development cost: Distributed database systems are
complex to implement and, thus, more costly. Increased complexity implies
that we can expect the procurement and maintenance costs for a DDBMS to be
higher than those for a centralised DBMS. In addition to software, a distributed
DBMS 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 maintenance costs to manage and maintain the local
DBMSs and the network.
Greater potential for bugs: Since the sites of a distributed system operate
concurrently, it is more difficult to ensure the correctness of algorithms. The art
of constructing distributed algorithms is an active and important area of
research.
Increased processing overhead: The exchange of messages and the additional
computation required to achieve coordination among the sites is an overhead
that does not arise in centralized systems.
Complexity: A distributed DBMS that is reliable, available and secure is
inherently more complex than a centralised DBMS. Replication of data
discussed in the next section, also adds to complexity of the distributed DBMS.
However, adequate data replication is necessary to have availability, reliability,
and performance.
Security: In a centralized system, access to the data can be easily controlled.
However, in a distributed DBMS not only does access to replicated data have to
be controlled in multiple locations, but also the network needs to be secured.
Networks over a period have become more secure, still there is a long way to
go.
Lack of standards and experience: The lack of standards has significantly
limited the potential of distributed DBMSs. Also, there are no tools or
methodologies to help users convert a centralized DBMS into a distributed
DBMS.
General Integrity control more difficult: Integrity is usually expressed in
terms of constraints or the rules that must be followed by database values. In a
distributed DBMS, the communication and processing costs that are required to
enforce integrity constraints may be very high as the data is stored at various
sites. However, with better algorithms we can reduce such costs.
Purpose: General-purpose distributed DBMSs have not been widely accepted.
We do not yet have the same level of experience in industry as we have with
centralized DBMSs.
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. The designer of distributes systems must balance the advantages
against the disadvantages of distribution of data.
