CSE 1021 - BCA - Lecture 1 - Additional Notes[1]

7/29/2019 CSE 1021 - BCA - Lecture 1 - Additional Notes[1]

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Limitations of File-Based System

Separation and isolation of data

When data is isolated in separate files, it is more difficult to retrieve theappropriate data. E.g. there is the need retrieve data from both the mkt. dept

and the sales dept.

Duplication of data

This is wasteful as it costs time and money to enter data more than once.

It takes up additional storage space and hence associated with cost.

Data may be inconsistent thus leading to loss of data integrity.

Data dependence

Since the structure and storage of data files and records are defined in the

application code, changes to an existing structure are difficult to make. E.g.

to modify the size of a field in a file requires a one-off program to convert the

file to the new format. E.g. the maximum size of textbox from 50 to 255 in

application.

Incompatible file formats

Since the structure of files are embedded in application programs, the

structures are dependent on the programming language used. Thus, the

structure of files generated by different application programs may be

different, making them difficult to be processed jointly. A software would be

required to convert the files to some format common. This is time-consuming

and expensive.

Fixed queries

Such systems are dependent on the application developer who is responsible

to write queries and reports. Thus the type of query or report that can be

produced is fixed. Unplanned queries are not possible.


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DBMS Other Facilities

A security system which prevents unauthorised access to the database.

An integrity system which maintains the consistency of stored data.

A concurrency control system which allows shared access of the database.A recovery control system which restores the database to a previous

consistent state following a hardware or software failure.

A user-accessible catalogue which contains descriptions of the data in the

database.

Components of the DBMS Environment

Hardware - Ranging from a single PC to a single mainframe, to a network of

computers.

Software - The DBMS software, the application programs, the operating

system

Data - The most important component acting as a bridge between the

machine components and the user components.

Procedures - Refers to the instructions and rules governing the design and use

of the database.

People - Database Administrators, Database Designers, Application

Developers, End-Users

ADVANTAGES:

Control of data redundancyIn contrast to traditional file-based systems that waste space by storing

the same information in more than one file, the database approach attempts

to eliminate redundancy by integrating files and hence keeping a single

copy of each data. Although, not eliminating redundancy entirely, it controls

the amount of redundancy inherent in the database.

Data consistency


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Since a data item is stored only once in the database any update to its

value needs to be done only once and the new value is immediately

available to every users.

More information from the same amount of dataWith the integration of operational data, it may be possible to derive

additional information from the same data, e.g. users from a department may

have access to data of other departments which was previously not shared.

Sharing of dataNew applications can build on existing data in the database being

shared among all authorised users and add only data that is not currently

stored rather than having to define all the data requirements again. The new

applications can also rely on the functions of the DBMS, such as data

definition and manipulation and concurrency and recovery control, rather

than having to provide these functions themselves.

Improved data integrityDatabase integrity, i.e. validity and consistency of stored data, is

expressed in terms of constraints (consistency rules) that the database is not

allowed to violate.

Improved securityDatabase security is the protection of the database from unauthorised

users. It may be through users names and passwords as an identification

method. The type of access allowed may also be controlled, e.g. retrieval,

insert, update or delete.

Enforcement of standardsIntegration allows the definition and enforcement of necessary

standards, e.g. standards for data formats to facilitate exchange of data

between systems, naming conventions, documentation standards.


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Economy of scaleAll operational data being combined into one database and a set of

applications created to work on this one source results in cost savings. The

budget that would normally be allocated to each departments can be

combined, possibly resulting in a lower total cost leading to an economy of

scale.

Balance of conflicting requirementsEach user or department may have conflicting needs. The DBA can

make decisions about design and operational use of the database to provide

an optimum use of resources and hence performance.

Improved data accessibility and responsivenessSince the database is shared, data is directly accessible to users. Many

DBMSs provide query languages or report writers that allow users to ask ad

hoc questions and obtain the required information almost immediately.

Increased productivityThe DBMS provide many of the standard functions that would have to

be written by a programmer in a file-based system. At a basic level, the DBMSprovides all the lower level file-handling routines that are typical in application

programs. This allows programmers to concentrate on specific functionalities

required by users.

Improved maintenance through data independenceDBMS, in contrast to file-based systems separate data description from

the applications thus making the applications immune to changes in the data

descriptions. This is known as data independence.

Increased concurrencyDBMSs manage concurrent database access and ensure that there is

no loss of information or loss of integrity.


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Improved backup and recovery servicesIn file-base systems, nightly backup of data may be used to protect

data from failures to the computer system or application program. DBMSs

provide facilities to minimise loss following a failure.

DISADVANTAGES:

ComplexityProviding the functionality expected of a good DBMS makes the DBMS

an extremely complex piece of software.

SizeThe complexity and breadth of functionality makes the DBMS an

extremely large piece of software occupying huge amount of disk space and

requiring substantial amount of memory to run efficiently.

Cost of DBMSsThe cost varies significantly depending on the environment and

functionality provided. The DBMS also needs to be maintained.

Additional hardware costsAdditional storage may be required for the DBMS and database. Also

a larger or dedicated machine may be required to run the DBMS to achieve

the required performance.

Cost of conversionCost of converting existing applications to run on the new DBMS and

hardware may be significant. This may include the cost of training staff to use

the new system or employing specialist staff to help convert and run the new

system.

Performance


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Unlike file-based systems, the DBMS is written to cater for many

applications. Some applications may thus not run as fast as they used to.

Higher impact of a failureCentralisation of resources increases the vulnerability of the system.

Since all users and applications rely on the availability of the DBMS, failure of

any component can bring operation to a halt.

Documents

CSE 1021 - BCA - Lecture 1 - Additional Notes[1]