Database Reference
In-Depth Information
Several factors have led to the development of DDBS:
Distributed nature of some database applications
Increased reliability and availability
Allowing data sharing while maintaining some measure of local control To improved
performance
Promises of Distributed DBMS
It provides transparent management of distributed, fragmented, and replicated data.
Transparency refers to separation of the higher-level semantics of a system from lower-
level implementation details.
It provides improved reliability and availability through distributed transactions. The
users can still access part of the distributed database with “proper care” even though some
of the data is unreachable. Distributed transactions facilitate maintenance of consistent
database state even when failures occur.
Performance can be improved. Since each site handles only a portion of a database, the
contention for CPU and I/O resources is not that severe. Data localization reduces commu-
nication overheads.
It has ability to add new sites, data, and users over time without major restructuring.
Disadvantages of DDBSs
No operating true distributed database systems in existence
DDBS problems are inherently more complex than centralized DBMS ones More hard-
ware, software and people costs
Problems of synchronization and coordination to maintain data consistency
remote database fragments must be secured, and they are not centralized so the remote sites
must be secured as well. The infrastructure must also be secured.
Database design more complex - besides of the normal difficulties, the design of a dis-
tributed database has to consider fragmentation of data, allocation of fragments to specific
sites and data replication.
2.9.5 Client/Server Model
The client/server model includes one or more shared computers, called servers, that are
connected by a network to the individual users' workstations, called clients. Client/ server
computing started in the 1980s. The original model used is now called the two-tier client/
