Explain Dbms Architecture

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Database Users and User Interfaces

There are four different types of database-system users, differentiated by the way

they expect to interact with the system. Different types of user interfaces have been

designed for the different types of users.

Naive users are unsophisticated users who interact with the system by invoking

one of the application programs that have been written previously. For

example, a bank teller who needs to transfer $50 from account A to account B

invokes a program called transfer. This program asks the teller for the amount

of money to be transferred, the account from which the money is to be transferred,

and the account to which the money is to be transferred.

As another example, consider a user who wishes to find her account balance

over the World Wide Web. Such a user may access a form, where she

enters her account number. An application program at the Web server then

retrieves the account balance, using the given account number, and passes

this information back to the user.

The typical user interface for naive users is a forms interface, where the

user can fill in appropriate fields of the form. Naive users may also simply

read reports generated from the database.

• Application programmers are computer professionals who write application

programs. Application programmers can choose from many tools to develop

user interfaces. Rapid application development (RAD) tools are tools that enable

an application programmer to construct forms and reports without writing

a program. There are also special types of programming languages that

combine imperative control structures (for example, for loops, while loops

and if-then-else statements) with statements of the data manipulation language.

These languages, sometimes called fourth-generation languages, often

include special features to facilitate the generation of forms and the display of

data on the screen. Most major commercial database systems include a fourthgeneration

language.

• Sophisticated users interact with the system without writing programs. Instead,

they form their requests in a database query language. They submit

each such query to a query processor, whose function is to break down DML

statements into instructions that the storage manager understands. Analysts

who submit queries to explore data in the database fall in this category.

Online analytical processing (OLAP) tools simplify analysts’ tasks by letting

them view summaries of data in different ways. For instance, an analyst

can see total sales by region (for example, North, South, East, andWest), or by

product, or by a combination of region and product (that is, total sales of each

product in each region). The tools also permit the analyst to select specific regions,

look at data in more detail (for example, sales by city within a region)

or look at the data in less detail (for example, aggregate products together by

category).

Another class of tools for analysts is data mining tools, which help them

find certain kinds of patterns in data.

We study OLAP tools and data mining in Chapter 22.

• Specialized users are sophisticated users who write specialized database

applications that do not fit into the traditional data-processing framework.

Among these applications are computer-aided design systems, knowledgbase and expert systems, systems that store data with complex data types (for

example, graphics data and audio data), and environment-modeling systems.

Chapters 8 and 9 cover several of these applications.

Database Administrator

One of the main reasons for using DBMSs is to have central control of both the data

and the programs that access those data. A person who has such central control over

the system is called a database administrator (DBA). The functions of a DBA include:

• Schema definition. The DBA creates the original database schema by executing

a set of data definition statements in the DDL.

• Storage structure and access-method definition.

• Schema and physical-organization modification. The DBA carries out changes

to the schema and physical organization to reflect the changing needs of the

organization, or to alter the physical organization to improve performance.

• Granting of authorization for data access. By granting different types of

authorization, the database administrator can regulate which parts of the database

various users can access. The authorization information is kept in a

special system structure that the database system consults whenever someone

attempts to access the data in the system.

• Routine maintenance. Examples of the database administrator’s routine

maintenance activities are:

􀀀 Periodically backing up the database, either onto tapes or onto remote

servers, to prevent loss of data in case of disasters such as flooding.

􀀀 Ensuring that enough free disk space is available for normal operations,

and upgrading disk space as required.

􀀀 Monitoring jobs running on the database and ensuring that performance

is not degraded by very expensive tasks submitted by some users.

The Query Processor

The query processor components include

• DDL interpreter, which interprets DDL statements and records the definitions

in the data dictionary.

• DML compiler, which translates DML statements in a query language into an

evaluation plan consisting of low-level instructions that the query evaluation

engine understands.

A query can usually be translated into any of a number of alternative evaluation

plans that all give the same result. The DML compiler also performs

query optimization, that is, it picks the lowest cost evaluation plan from among

the alternatives.

• Query evaluation engine, which executes low-level instructions generated by

the DML compiler.

Storage Manager

A storage manager is a program module that provides the interface between the lowlevel

data stored in the database and the application programs and queries submitted

to the system. The storage manager is responsible for the interaction with the file

manager. The raw data are stored on the disk using the file system, which is usually

provided by a conventional operating system. The storage manager translates

the various DML statements into low-level file-system commands. Thus, the storage

manager is responsible for storing, retrieving, and updating data in the database.

The storage manager components include:

• Authorization and integrity manager, which tests for the satisfaction of integrity

constraints and checks the authority of users to access data.

• Transaction manager, which ensures that the database remains in a consistent

(correct) state despite system failures, and that concurrent transaction executions

proceed without conflicting.

• File manager, which manages the allocation of space on disk storage and the

data structures used to represent information stored on disk.

• Buffer manager, which is responsible for fetching data from disk storage into

main memory, and deciding what data to cache in main memory. The buffer

manager is a critical part of the database system, since it enables the database

to handle data sizes that are much larger than the size of main memory.

The storage manager implements several data structures as part of the physical

system implementation:

• Data files, which store the database itself.

• Data dictionary, which stores metadata about the structure of the database, in

particular the schema of the database.

• Indices, which provide fast access to data items that hold particular values.

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