Internal or Physical View of Schema, Data Independence, Funct ions of DBMS

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Database Management System (CS403)

Lecture No. 04

Reading Material

"Database

Systems

Principles,

Design

and

4.1.3,

Implementation" written by Catherine Ricardo, 4.1.4

Maxwell Macmillan.

Hoffer

Chapter 2

Overview of Lecture

o Internal Schema of the Database Architecture

o Data Independence

o Different aspects of the DBMS

Internal or Physical View / Schema

This is the level of the database which is responsible for the storage

of data on the storage media and places the data in such a format

that it is only readable by the DBMS. Although the internal view and

the physical view are so close that they are generally referred to a

single layer of the DBMS but there lays thin line which actually

separated the internal view from the physical view. As we know that

data when stored onto a magnetic media is stored in binary format,

because this is the only data format which can be represented

electronically, No matter what is the actual format of data, either

text, images, audio or video. This binary storage mechanism is

always implemented by the Operating System of the Computer.

DBMS to some extent decides the way data is to be stored on the

disk. This decision of the DBMS is based on the requirements

specified by the DBA when implementing the database. Moreover

the DBMS itself adds information to the data which is to be stored.

For example a DBMS has selected a specific File organization for the

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storage of data on disk, to implement that specific file system the

DBMS needs to create specific indexes. Now whenever the DBMS will

attempt to retrieve the data back form the file organization system

it will use the same indexes information for data retrieval. This

index information is one example of additional information which

DBMS places in the data when storing it on the disk. At the same

level storage space utilization if performed so that the data can be

stored by consuming minimum space, for this purpose the data

compression can be performed, this space optimization is achieved

in such a way that the performance of retrieval and storage process

is not compromised. Another important consideration for the storage

of data at the internal level is that the data should be stored in

such a way that it is secure and does not involve any security risks.

For this purpose different data encryption algorithms may be used.

Lines below detail further tidbits of the internal level.

The difference between the internal level and the external level

demarcates a boundary between these two layers, now what is that

difference, it in fact is based on the access or responsibility of the

DBMS for the representation of data. At the internal Level the

records are presented in the format that are in match with schema

definition of the records, whereas at the physical level the data is

not strictly in record format, rather it is

in character format.,

means the rules identified by the schema of the record are not

enforced at this level. Once the data has been transported to the

physical level it is then managed by the operating system. Operating

system at that level uses its own data storage utilities to place the

data on disk.

Inter Schema Mapping:

The mechanism through which the records or data at one level is

related to the changed format of the same data at another level is

known as mapping. When we associate one form of data at the

external level with the same data in another form is know as the

external/conceptual mapping of the data. (We have seen examples

of external/conceptual mapping in the previous lecture)

In the

same way when data at the conceptual level is correlated with the

same data at the internal level, this is called the conceptual/Internal

mapping.

Now the question arises that how this mapping is performed. Means

how is it possible to have data at one level in date format and at a

higher level the same data show us the age. This hidden mechanism,

conversion system or the formula which converts the date of birth

of an employee into age is performed by the mapping function and

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it is defined in the specific ext/con mapping, for example, when the

data at the conceptual level is presented as the age of the employee

is done by the external schema of that specific user. Now in this

scenario the ext/con mapping is performing the mapping with the

internal view and is retrieving the data in desire format of the user.

In the same way the mapping between an internal view and

conceptual view is performed.

The figure below gives a clear picture of this mapping process and

informs where the mapping between different levels of the database

is performed.

Fig:

Mapping

between

External/Conceptual

and

Conceptual/Internal levels

In Figure-1 we can see clearly where the mapping or connectivity is

performed between different levels of the database management

system. Figure-1 is showing another very important concept that the

internal layer and the physical layers lie separately the Physical

layer is explicitly used for data storage on disk and is the

responsibility of the Operating system. DBMS has almost no concern

with the details of the physical level other than that it passes on the

data along-with necessary instructions required to the store that

data to the operating system.

Figure-2 on the next page shows how data appears on different

levels of the database architecture and also at that of physical level.

We can clearly see that the data store on the physical level is in

binary format and is separate from the internal view of data in

location and format. Separation of the physical level from the

internal level is of great use in terms of efficiency of storage and

data retrieval.

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Fig: 2. Representation of data at different levels of data base

Architecture and at the physical level at bottom

At the internal level we can see that data is prefixed with Block

Header and Record header RH, the Record header is prefixed to

every record and the block header is prefixed to a group of records;

because the block size is generally larger than the record size, as a

result when an application is producing data it is not stored record

wise on the disk rather block wise which reduces the number of disk

operations and in-turn improves the efficiency of writing process.

Data Independence:

Data Independence is a major feature of the database system and

one of the most important advantages of the Three Level Database

Architecture. As it has been discussed already that the file

processing system makes the application programs and the data

dependent on each other, I-e if we want to make a change in the

data we will have to make or reflect the corresponding change in

the associated applications also.

The Three Level Architecture facilitates us in such a way that data

independence is automatically introduced to the system. In other

words we can say the data independence is major most objective of

the Three Level Architecture. If we do not have data independence

then whenever there will be a change made to the internal or

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physical level or the data accessing strategy the applications

running at the external level will demand to be changed because

they will not be able to properly access the changed internal or

physical levels any more. As a result these applications will stop

working and ultimately the whole system may fail to operate.

The Data independence achieved as a result of the three level

architecture proves to be very useful because once we have the

data , database and data applications independent of each other we

can easily make changes to any of the components of the system,

without effecting the functionality and operation of other

interrelated components.

Data and program independence is on advantage of the 3-L

architecture the other major advantage is that ant change in the

lower level of the 3-L architecture does not effect the structure or

the functionality on upper levels. I-e we get external/conceptual

and conceptual/internal independence by the three levels

Architecture.

Data independence can be classified into two type based on the

level at which the independence is obtained.

Logical Data Independence

Physical Data Independence

Logical data independence

Logical data independence provides the independence in a way that

changes in conceptual model do not affect the external views. Or

simply it can be stated at the Immunity of external level from

changes at conceptual level.

Although we have data independence at different levels, but we

should be careful before making a change to anything in database

because not all changes are accepted transparently at different

levels. There may be some changes which may cause damage or

inconsistency in the database levels. The changes which can be

done transparently may include the following:

o Adding a file to the database

o Adding a new field in a file

o Changing the type of a specific field

But a change which may look similar to that of the changes stated

above could cause problems in the database; for example: Deleting

an attribute from the database structure,

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This could be serious because any application which is using this

attribute may not be able to run any more. So having data

independence available to us we still get problem after a certain

change, it means that before making a certain change its impact

should also be kept in mind and the changes should be made while

remaining

the

limits

the

data

independence.

Fig:3. The levels where

the

Conceptual

and

Physical

data

independence are effective

Physical Data Independence

Physical data independence is that type of independence that

provides us changes transparency between the conceptual and

internal levels. I-e the changes made to internal level shall not

affect the conceptual level. Although the independence exist but as

we saw in the previous case the changes made should belong to a

specific domain and should not exceed the liberty offered by the

physical data independence. For example the changes made to the

file organization by implementing indexed or sequential or random

access at a later stage, changing the storage media, or simply

implement a different technique for managing file indexes or hashes.

Functions of DBMS

o Data Processing

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A user accessible Catalog

Transaction Support

Concurrency Control Services

Recovery Services

Authorization Services

Support for Data Communication

Integrity Services

DBMS lies at the heart of the course; it is the most important

component of a database system. To understand the functionality of

DBMS it is necessary that we understand the relation of database

and the DBMS and the dissection of the set of functions the DBMS

performs on the data stored in the database.

Two important functions that the DBMS performs are:

User management

Data Management

The detailed description of the above two major activities of DBMS

is given below;

o Data Processing

By Data management we mean a number of things it may include

certain operations on the data such as: creation of data, Storing of

the data in the database, arrangement of the data in the databases

and data-stores, providing access to the data in the database, and

placing of the data in the appropriate storage devices. These action

performed on the data can be classified as data processing.

o A User Accessible Catalog

DBMS has another very important task known as access proviso to

catalog. Catalog is an object or a place in the DBMS which stores

almost all of the information of the database, including schema

information, user information right of the users, and many more

things about the database. Modern relational DBMS require that the

Administrative users of the database should have access to the

catalog of the database.

o Transaction Support

DBMS is responsible for providing transaction support. Transaction

is an action that is used to perform some manipulation on the data

stored in the database. DBMS is responsible for supporting all the

required operations on the database, and also manages the

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transaction execution so that only the authorized and allowed

actions are performed.

o Concurrency Support

Concurrency support means to support a number of transactions to

be executed simultaneously, Concurrency of transactions is managed

in such a way that if two or more transaction is making certain

processing on the same set of data, in that case the result of all the

transactions should be correct and no information should be lost.

o Recovery Services

Recovery services mean that in case a database gets an inconsistent

state to get corrupted due to any invalid action of someone, the

DBMS should be able to recover itself to a consistent state, ensuring

that the data loss during the recovery process of the database

remains minimum.

o Authorization Services

The database is intended to be used by a number of users, who will

perform a number of actions on the database and data stored in the

database, The DBMS is used to allow or restrict different database

users to interact with the database. It is the responsibility of the

database to check whether a user intending to get access to

database is authorized to do so or not. If the user is an authorized

one than what actions can he/she perform on the data?

o Support for Data Communication

The DBMS should also have the support for communication of the

data indifferent ways. For example if the system is working for such

an organization which is spread across the country and it is

deployed over a number of offices throughout the country, then the

DBMS should be able to communicate to the central database

station. Or if the data regarding a product is to be sent to the

customers worldwide it should have the facility of sending the data

of the product in the form of a report or offer to its valued

customers.

o Integrity Services

Integrity means to maintain something in its truth or originality. The

same concept applies to the integrity in the DBMS environment.

Means the DBMS should allow the operation on the database which

are real for the specific organization and it should not allow the

false information or incorrect facts.

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DBMS Environments:

o Single User

o Multi-user

· Teleprocessing

· File Servers

· Client-Server

o Single User Database Environment

This is the database environment which supports only one user

accessing the database at a specific time. The DBMS might have a

number of users but at a certain time only one user can log into the

database system and use it. This type of DBMS systems are also

called Desktop Database systems.

o Multi-User Database systems

This is the type of DBMS which can support a number of users

simultaneously interacting with the database indifferent ways. A

number of environments exist for such DBMS.

· Teleprocessing

This type of Multi user database systems processes the user

requests at a central computer, all requests are carried to the

central computer where the database is residing, transactions are

carried out and the results transported back to the terminals

(literally dumb terminals). It has become obsolete now.

· File Servers

This type of multi-user database environment assumes another

approach for sharing of data for different users. A file server is used

to maintain a connection between the users of the database system.

Each client of the network runs its own copy of the DBMS and the

database resides on the file server. Now whenever a user needs

data from the file server it makes a request the whole file

containing the required data was sent to the client. At this stage it

is important to see that the user has requested one or two records

from the database but the server sends a complete file, which might

contain hundreds of records. Now if the client after making the

desired operation on the desired data wants to write back the data

on the database he will have to send the whole file back to the

server, thus causing a lot of network overhead. The Good thing

about this approach is that the server does not have lots of actions

to do rather it remains idle for lots of the time in contrast with that

of the teleprocessing systems approach.

· Client-Server

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This type of multi-user environment is the best implementation of

the network and DBMS environments. It has a DBMS server machine

which runs the DBMS and to this machine are connected the clients

having application programs running for each user. Once a users

wants to perform a certain operation on data in the database it

sends its requests to the DBMS through its machine's application

software; the request is forwarded to the DBMS server which

performs the required operation on data in the database stored in

the dame computer and then passes back the result to the user

intending the result. This environment is best suited for large

enterprises where bulk of data is processed and requests are very

much frequent.

This concludes the topics discusses in the lecture No4.In the next

lecture Database application development process will be discussed

Exercises:

- Extend the format of data from the exercise of previous

lecture to include the physical and internal levels.

Complete your exercise by including data at all three

levels

- Think of different nature of changes at all three levels of

database architecture and see, which ones will have no

effect on the existing applications, which will be

adjusted in the inter-schema mapping and which will

effect the existing applications.

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