Database Concepts 1. 1 CSE2132 Database Systems Week 1 Lecture Database Concepts.

Database Concepts 1. 1

CSE2132 Database Systems

Week 1 Lecture Database Concepts


Data - An Informal DefinitionData are raw facts. Facts concerning people, places, events, or other

objects/concepts. They can be of any type:

qualitative or quantitativehistorical or predictionabstract or subjective

Sources(1)External to an organization: telephone calls, turnaround

documents, remote workstation (point of sale) transactions.(2)Internal to an organization: financial and management accounting

production and operation management, payroll and personnel.(3)From “strange” places: metal sensor on the street, digitizing of an

image, hardware failure messages.


Data and the Organization• Data by itself is useless unless it is some how aggregated,

organized and prepared in a form convenient for decision making or other organizational activities.

DATA INFORMATION ACTION

A lack of data leads to inadequate information and thus ill-informeddecisions and business collapse.


Data as Resource

Why use database?

Data is a valuable corporate resource which needs adequate, accuracy, consistency and security controls.

The centralized control of data means that for many applications the data will already exist, and facilitate quicker development.

Data will no longer be related by application programs, but by the structure defined in the database.

Easier to maintain systems


Basic Definitions

Database: A collection of related data

Schema: Some part of the real world about which data is stored in the database.

Database Management System(DBMS):A software package to facilitate the creation, maintenance and protection of a computerized database.


A DBMS and Interfaced Utilities

Query Language(e.g.SQL)

Data Dictionary

ApplicationGenerator

ReportGenerator

Graphics

DataentryScreens

ApplicationPrograms

DBMS

Data

(Frontend)


Traditional File Systems Databases are often contrasted to traditional file systems

though they are now rarely used. But as the problems that existed were the impetus for the development of the “Database Concept” it is worth noting some of these.

- Applications were often considered in relative isolation.

- Data that should have been together was not.

- The potential for flexible enquiry and reporting was limited.

- All validations were in the programs.

- Procedures were required for backup and recovery.

- All programmers had access to all records.

- There was limited concurrent access.


Objectives of Database Technology

· Program-Data Independence· Minimal Data Redundancy· Increased Data Sharing· Improved Data Quality · Improved Security of Data· Improved Access to Data· Reduced Program Maintenance· Inter-relate Data Through Model


Program-Data Independence

· Data is stored independently of the programs· The degree to which descriptions of data are embedded in

application programs.· Can the database structure be changed with no impact on

programs ?· Role of the database catalog or dictionary.· Maintenance costs are high !


Degrees of Data Independence

· Device Characteristics· Blocking Factors· Data Access Organization· Physical Record Location· Logical Views (Local)· Virtual Data Items· Virtual Records· Data Value Characteristics· Data Element Name Only

GreaterIndependence


Inter-Related Data

CLAIMS

RENEWALS

AGENCY

DBMS

RENEWALS

CLAIMS AGENCY

QUERY

Data related by structure

Flexible Inquiry easier LOCALVIEWS


Application Development

· Some companies use database technology as just a multi-key access method:

CLAIMS

PREMIUMS

DBMS CLAIMS DB

PREMIUMS DBDBMS


· Applications & their data needs are not considered in isolation.

· Centralized control of one or several databases takes place. i.e database administration.

· Data administration is seen as an important part of system development.

CLAIMS

PREMIUMS

CLAIMS

PREMIUMS

DBMS

Application Development


Semantic Data Integrity

· Validation or integrity rules may be defined and automatically invoked at run time by the DBMS regardless of the source of update i.e. application program, 4GL screen or query language.

· Significant variation exists among DBMS in the level of support for semantic data integrity.

· ISO suggest that 100% of all enterprise rules should be held in the conceptual schema, and specifically none in application programs.

· An area of significant development during the 1990's.


CATALOGUEData Definitions &

Integrity Rules

STORED DATA

DBMS

ApplicationPrograms

4GL Screens &Stored Procs.

InteractiveQuery

Language

Semantic Data Integrity


Database Transactions

EMP# SALARY DEPT

E1 50,000 D1E2 18,000 D1

DEPT# T0TAL SALARY

D1 68,000D2 90,000

A database transaction may be defined as a group (one or more) of database operations which need to be treated as one atomic unit which may either succeed or fail. That is all the database operations must succeed or all the database operations must fail.

If this is not the case tasks can be half complete and the database loses it’s integrity.


Concurrency Control

Part 2 QOH 10

Part 2 QOH 10

Part 2 QOH 5

Part 2 QOH 20Part 2 QOH 10QOH=QOH+10

QOH=QOH-5

Transaction 1 Transaction 2

Problems such as the “lost update” can arise if no measures are taken to support concurrent transactions. The DBMS should support multiple concurrent transactions potentially using the same data and ensure that the data remains consistent at all times.

readread

write

write


Record LockingTo avoid concurrency problems, transactions must be made

logically serial. One common technique used is record locking. That is, a transaction can lock a record, preventing update by another transaction, until completed. Although TX2 waits a little while both complete without error.

TX1 TX2

Part 10 lock

Part 20 lock

Part 30 lock

Part 301 lock

Part 20 wait

Part 101 lock

timeend trans.

end trans.

Part 20 lock


DeadlockThe technique of locking introduces it’s own problems.DBMS

packages with record locking must also be able to handle "deadlock" situations.

TRANSACTION A TRANSACTION B lock 10 lock 20

request 20 wait request 10 wait

Note that both transactions above could wait forever!Deadlock detection algorithms can be used to detect this situation amongst locked records. Usually the least expensive transaction is aborted.


Restart and Recovery· Many DBMS maintain redundant copies of data in a log file to

support recovery from failures. This may be on disk or tape.

· Copies of record images after update , together with periodic dumping of the database allow for rollforward recovery where the database contents are not intact. A dump of the database is an exact copy of the content of the database at some point in time.

· Copies of record images before update allow rollback recovery of changes made by incomplete or failed transactions.

· Many DBMS take periodic checkpoints to reduce recovery time in the event of failure. A rollback recovery then goes back to the checkpoint rather than the start of the transaction.


Rollforward Recovery

A log file of After Images of recordschanged since the last dump was takenare applied in a forward order thru time.

A dump of the databasefrom the previous period.

The restored database shows no effect of the major disaster such as a “disk crash”.


Rollback Recovery

A Log file of Before Imagesof records which are changed are kept. If a transaction fails, the before images are written back to the database in reverse order thru time. When the start of the failed transaction is reached the rollback stops.

The restored databasein a state as if the transaction never occurredso no effects of the transaction failure are apparent.


Security

· Each user may require identification with a user-id and password.

· Users may be limited in the data they can see and what actions they can perform on that data.

· The DBMS may encrypt and decrypt data as it is stored and retrieved.

· Many systems now provide data value sensitive security.


Easy Access to Data· High level query languages provided by DBMS provide

easy access to data

20 READ #1, CUSTNO, NAME, BAL30 IF END #1 GO TO 8040 IF BAL > 20050 PRINT CUSTNO, NAME, BAL60 ENDIF70 etc

VERSUS

SELECT CUSTNO, NAME, BAL FROM CUSTDB WHERE BAL > 200;


Data Models

A way of viewing or abstracting the data.

There are a number of data models on which current generation DBMS are based:

Hierarchical, Network, Relational,

Extended Relational and Object-Oriented


The Costs of DBMSThe initial purchase cost

Planning and design

Database education

Application conversion

System overheads (response)

Complexity of support

Database Concepts 1. 1 CSE2132 Database Systems Week 1 Lecture Database Concepts.

Documents

Transcript of Database Concepts 1. 1 CSE2132 Database Systems Week 1 Lecture Database Concepts.