by Dinesh Thakur

The distribution of data and applications has potential advantages over traditional centralized database systems. Unfortunately, there are also disadvantages; in this section we review the advantages and disadvantages of DDBMS.

Advantages

There are following advantages of DDBMs:

Reflects organizational structure

Many organizations are naturally distributed over several locations. For example, a bank has many offices in different cities. It is natural for databases used in such an application to be distributed over these locations. A bank may keep a database at each branch office containing details such things as the staff that work at that location, the account information of customers etc.

The staff at a branch office will make local inquiries of the database. The company headquarters may wish to make global inquiries involving the access of data at all or a number of branches.

Improved share ability and local autonomy

The geographical distribution of an organization can be reflected in the distribution of the data; users at one site can access data stored at other sites. Data can be placed at the site close to the users who normally use that data. In this way, users have local control of the data, and they can consequently establish and enforce local policies regarding the use of this data. A global database administrator (DBA) is responsible for the entire system. Generally, part of this responsibility is assigned the local level, so that the local DBA can manage the local DBMS.

Improved availability

In a centralized DBMS, a computer failure terminates the applications of the DBMS. However, a failure at one site of a DDBMS, or a failure of a communication link making\ some sites inaccessible, does not make the entire system in opera bite. Distributed DBMSs are designed to continue to function despite such failures. If a single node fails, the system may be able to reroute the failed node's requests to another site.

Improved reliability

As data may be replicated so that it exists at more than one site, the failure of a node or a communication link does not necessarily make the data inaccessible.

Improved Performance

As the data is located near the site of 'greatest demand', and given the inherent parallelism of distributed DBMSs, speed of database access may be better than that achievable from a remote centralized database. Furthermore, since each site handles only a part of the entire database, there may not be the same contention for CPU and I/O services as characterized by a centralized DBMS.

Economics

It is now generally accepted that it costs much less to create a system of smaller computers with the equivalent power of a single large computer. This makes it more cost effective for corporate divisions and departments to obtain separate computers. It is also much more cost-effective to add workstations· to a network than to update a mainframe system.

The second potential cost saving occurs where database are geographically remote and the applications require access to distributed data. In such cases, owing to the relative expense of data being transmitted across the network as opposed to the cost of local access, it may be much more economical to partition the application and perform the processing locally at each site.

Modular growth

In a distributed environment, it is much easier to handle expansion. New sites can be added to the network without affecting the operations of other sites. This flexibility allows an organization to expand relatively easily. Adding processing and storage power to the network can usually handle the increase in database size. In a centralized DBMS, growth may entail changes to both hardware (the procurement of a more powerful system) and software (the procurement of a more powerful or more configurable DBMS).

Disadvantages of DDBMS

There are following disadvantages of DDBMSs:

Complexity

A distributed DBMS that hides the distributed nature from the user and provides an acceptable level of performance, reliability, availability is inherently more complex then a centralized DBMS. The fact that data can be replicated also adds an extra level of complexity to the distributed DBMS. If the software does not handle data replication adequately, there wi1l be degradation in availability, reliability and performance compared with the centralized system, and the advantages we cites 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 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 labor 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 distributed DBMS not only does access to replicated data have to be controlled in multiple locations but also 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 to make networks more secure.

Integrity control more difficult

Database integrity refers to the validity and consistency of stored data. Integrity is usually expressed in terms of constraints, which are consistency rules that the database is not permitted to violate. Enforcing integrity constraints generally requires access to a large amount of data that defines the constraints. In a distributed DBMS, the communication and processing costs that are required to enforce integrity constraints are high as compared to centralized system.

Lack of Standards

Although distributed DBMSs 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 distributed DBMSs. There are also no tools or methodologies to help users convert a centralized DBMS into a distributed DBMS

Lack of experience

General-purpose distributed DBMSs 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 fragmentation to specific sites, and data replication.