RDbms5.1

Unit-I		Unit-II	Unit-III	Unit-IV	Unit-V
Part-I	Part-II	Part-I	Part-I	Part-I	Total
Part-III	Part-IV	Part-II	Part-II	Part-II
Part-V		Part-III

 

NEED FOR DISTRIBUTED DATABASE SYSTEMS

1.Explain need of distributed database systems?

Ans:

A distributed database is a set of database stored on multiple computers that appears to applications as a single database.

·        As a result, an application can simultaneously access and modify the data in several databases in a network.

·        Each database in the system is controlled by its local server but cooperates to maintain the consistency of the global distributed database.

·        The computers in a distributed system communicate with each other through various communication media, such as high-speed buses or telephone line.

·        They don’t share main memory, nor a clock, although, to work properly many applications on different computers might have to synchronise their clocks.

·        The processors in a distributed system may vary in size and function such as small microcomputers, workstation, minicomputers, and large general-purpose computer system.

·        These processors are referred to by sites, nodes, computers, and so on.

·        A distributed database system consists of a collection of sites, each of which may participate in the execution of transactions, which access data at one site, or several sites.

Distributed Database Systems can be considered a system connected to intelligent remote devices each of which can itself act as a local database repository.

Ø All data is accessible from each site.

Ø The distributed system increases the efficiency of access because multiple of sites can co-ordinate efficiently to respond to a query and control & processing is limited to this DBMS.

STRUCTURE OF DISTRIBUTED DATABASE

2.Explain the structure of Distributed Database ?

Ans:

Figure 1: Three different database system architectures.

·        A distributed database system consists of a collection of sites, each of which maintains a local databases system.

·        Each site is able to process local transactions, those transactions that access data only in that single site.

·        In addition, a site may participate in the execution of global transactions, those transactions that access data at several sites.

·        The architecture of Distributed Database systems is given in Figure 1

The sites in a distributed system can be connected physically in a variety of ways. The various topologies are represented as graphs whose nodes correspond to sites. An edge from node A to node B corresponds to a direct connection between the two sites.

The DDBMS configurations involve:

• Installation cost. The cost of physically linking the sites in the system

• Communication cost. The cost in time and money to send a message from site A to site B.

• Reliability. The frequency with which a link or site fails.

• Availability. The degree to which data can be accessed despite failure of some links or sites.

          The sites of a distributed database system may be distributed physically either over a large geographical area (such as all over India), or over a small geographical area such as a single building or a number of adjacent buildings).

Typical wide area links are telephone lines, microwave links, and satellite channels. The most common links are twisted pair, base band coaxial, broadband coaxial, and fiber optics.

 

Figure 2: Some interconnection Networks

          The sites of a distributed database system may be distributed physically either over a large geographical area (such as all over India), or over a small geographical area such as a single building or a number of adjacent buildings).

Typical wide area links are telephone lines, microwave links, and satellite channels. The most common links are twisted pair, base band coaxial, broadband coaxial, and fiber optics.

ADVANTAGES AND DISADVANTAGES OF DDBMS

3.Explain Advantages and Disadvantages Of DDBMS?

(Or) Explain Advantages and Disadvantages of Data Distribution ?

Ans:

There are several reasons for building distributed database systems, including sharing of data, reliability and availability, and speedup of query processing. However, along with these advantages come several disadvantages, including software development cost, greater potential for bugs, and increased processing overheads.

Advantages of Data Distribution

The primary advantage of distributed database systems is the ability to share and access data in a reliable and efficient manner.

Data sharing and Distributed Control

The geographical distribution of an organization can be reflected in the distribution of the data; if a number of different sites are connected to each other, then a user at one site may be able to access data that is available at another site.

Data can be placed at the site close to the users who normally use that data. The local control of data allows establishing and enforcement of local policies regarding use of local data.

A global database administrator (DBA) is responsible for the entire system. Generally, part of this responsibility is given to the local administrator, so that the local DBA can manage the local DBMS. Thus in the distributed banking system, it is possible for a user to get his/her information from any branch office.

Reflects organisational structure

Many organizations are distributed over several locations. If an organisation has many offices in different cities, databases used in such an application are distributed over these locations. Such an organisation may keep a database at each branch office containing details of the staff that work at that location, the local properties that are for rent, etc. The staff at a branch office will make local inquiries to such data 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 Reliability

In DDBMS failure at one site or  failure of a communication link making some sites inaccessible, does not make the entire system inoperable.

Distributed DBMSs are designed to continue to function despite such failures. In particular, if data are replicated in several sites, a transaction needing a particular data item may find it at several sites. Thus, the failure of a site does not necessarily imply the shutdown of the system.

Improved availability

The data in a distributed system may be replicated so that it exists at more than one site. Thus, the failure of a node or a communication link does not necessarily make the data inaccessible. The ability of most of the systems to continue to operate despite the failure of one site results in increased availability which is crucial for database systems used for real-time applications.

Improved performance

The data is located near the site of its demand, and stored multiple copies, speed of database access may be better for distributed databases .

Speedup Query Processing

A query that involves data at several sites can be split into sub-queries. These sub-queries can be executed in parallel by several sites. Such parallel sub-query evaluation allows faster processing of a user's query. In those cases in which data is replicated, queries may be sent to the least heavily loaded sites.

Economics

It is now generally accepted that it costs less to create a system of smaller computers with the equivalent power of a single large computer. It is more cost-effective to obtain separate computers.

It may be more economical to partition the application and perform the processing locally at application site.

Modular growth

In distributed environments, it is easier to expand. New sites can be added to the network without affecting the operations of other sites, as they are somewhat independent. This flexibility allows an organisation to expand gradually.

 

Disadvantages of Data Distribution

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 .. In addition to software, a distributed DBMS requires additional hardware to establish a network between sites.

• 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 centralised systems.

Complexity: A distributed DBMS that is reliable, available and secure is inherently more complex .

• Security: 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.

• Lack of standards and experience: The lack of standards has significantly limited the potential of distributed DBMSs.

• 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.

• Database design more complex: The design of a distributed database has to take account of fragmentation of data, allocation of fragments to specific sites, and data replication.

CLIENT SERVER DATABASES

4.     Explain Client server databases?

(Or) Explain Emergence of Client Server Architecture?

(Or) Explain Need for Client Server Computing ?

Ans:

It is an approach that presents a single systems view from a user's viewpoint. It involves processing on multiple, interconnected machines. It provides coordination of activities in a manner transparent to end-users. Remember, client-server database is distribution of activities into clients and a server. It may have a centralised or distributed database system at the server backend. It is primarily a very popular commercial database implementation model.

 

Emergence of Client Server Architecture

Some of the pioneering work that was done by some of the relational database vendors allowed the computing to be distributed on multiple computers on network using contemporary technologies involving:

• Low Cost, High Performance PCs and Servers

• Graphical User Interfaces

• Open Systems

• Object-Orientation

• Workgroup Computing

• EDI and E-Mail

• Relational Databases

• Networking and Data Communication.

 

Need for Client Server Computing

·        Client machine is basically a PC or a workstation that provides presentation services and the appropriate computing, connectivity and interfaces.

·        Server machine provides database services, connectivity and computing services to multiple users.

·        Both client machines and server machines are connected to the same network.

·        As the number of users grows, client machines can be added to the network while as the load on the database server increases more servers can be connected through the network.

·        Server machines are more powerful machines. Database services to multiple client requests.

·        The client-server systems are connected though the network.

·        This network may be Local Area Network (LAN), Wide Area Network (WAN) .

·        The client and server machines communicate through standard application program interfaces (called API), and remote procedure calls (RPC).

·        The language through which RDBMS based C/S environment communicate is the structured query language (SQL).