The distributed databases are needed because of the
following reasons:
Organisation
and Economic Reasons: With the recent development ,of
computer technology, the economy of scale motivation for having largege,
centralised computer centre is becoming questionable.
Interconnection
of Existing Database: Distributed
database are the natural solution when
several database already exist in
an organisation and the necessity of performing global, application arises.
In this case, the distributed database is created
bottom-up from the pre-existing local database. This process may require a
certain degree of local restructuring.
Incremental
Growth: Distributed database easily used in new branches and new warehouse.
With a centralised approach, either the initial dimensions
of the system take care of future expansion, which is difficult foresee and
expensive to implement, or the growth has a major impact not only on the new
application but also on the existing ones.
Reduced
Communication Overhead: In a geographically distributed
database, we use local application, which reduces communication overhead.
Therefore, the maximisation of the locality of applications is one of the
primary objectives in distributive database design.
Performance:
Due to high degree of parallelism it gives autonomous result. This
consideration can be applied to any multiprocessor system, and not only to
distribute databases but also so allows parallelism, but is not considered a
distributed database. However, distributed database have the advantage that the
decomposition of data reflects application criteria which depends on maximising
the application locality, in this way the mutual interference between different
processors is minimised. The load is shared between the different processors
and critical bottlenecks such as the communication network itself services of
the whole or common system, are avoided.
Reliability
and Availability: The distributed database approach especially with
redundant data can be used also in order to obtain higher reliability and
availability.
However,
obtaining this goal is not straightforward and requires the use of technique,
which is still not completely understood. The autonomous processing capability
of different site does not by itself guarantee a higher overall reliability of
the system, but it ensures a graceful degradation property.
