System design : files

Data Design Concepts

Data StructuresA file or table contains data about people, places or

events that interact with the systemFile-oriented systemFile processingDatabase system

Data processing

1-Input data 2-process 3-Output data

Physically this data consists of alphanumeric characters grouped into data items or fields: for example, a customer name or address

Related fields are grouped into records A file is an organized collection of related

records

Types of data

Master data is critical to the system and its users.

System Telecom, for example, might hold data about customers, about the company’s own employees, and about call-logging stations.

Types of data

Transaction data is data relating to business activities. such as telephone calls logged.

For example, a new set of records may be created each day and written to file between system start-up and system shut-down.

The next morning, that data will be used to update the master data, and a new transaction file will be created for that day’s transactions.

Types of Data

Transaction files are sometimes known as transaction logs, log files, update files or change files

Output files contain information for output from the system, such as data for printing as a report. They are usually generated by processing master data and transaction data.

Types of Data

Security or dump files contain copies of data held in the computer at a particular moment. Their purpose is to provide a back-up, to

permit recovery in case data is lost or damaged.

Archive files contain archive information for long-term storage. System Telecom might want to archive details of

payments made by customers over past years.

Library files contain library routines such as utility programs and system software. The term can encompass any file containing any

compiled computer program.

Audit files are used by a computer auditor to check that the programs are functioning correctly, and to trace any change to master files. Such a file contains copies of all transactions

that have been applied to the permanent system files.

Data Design Concepts

Overview of File ProcessingCan be more efficient and

cost-effective in certain situations

Potential problems Data redundancy Data integrity Rigid data structure

Data Design Concepts

The Evolution from File Systems to Database SystemsAdvantages

Scalability Better support for client/server systems Economy of scale Flexible data sharing Enterprise-wide application – database administrator

(DBA)

Data Design Concepts

The Evolution from File Systems to Database SystemsAdvantages

Stronger standards Controlled redundancy Better security Increased programmer productivity Data independence

Figure 8-5: Data Structure Hierarchy

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File Organization

Serial OrganisationThe main disadvantage of serial organization

is that it does not cater for direct access to records.

If the required record is in the fifteenth position in the file, the first 14 must be read prior to accessing record 15.

File Organization

Sequential OrganisationThe advantages of sequential organisation

are: It is a simple method of writing data to disk. It is the most efficient organisation if the records

can be processed in the order in which they are read.

It can be used for variable-length records as well as fixed-length records.

File Organization

Indexed Sequential OrganisationThe main advantage of indexed sequential

organization is its versatility. It combines direct access to a group of records

with rapid sequential scanning of the group to obtain the record required.

What is disadvantage??

Random Organization A mathematical formula is derived that, when applied

to each record key, generates an answer that is used to position the record at a corresponding address.

The main advantages of random organization are: No indexes are required. It permits the fastest access times. It is suitable for volatile data: records can be inserted or

deleted indefinitely without reorganizing the dataset.

What are disadvantagesGive an example of such mathematical

formula Full Index Organization Chained Data

reports

Write about Access methodsFactors Influencing File Design

Systems Analysis and Design 2nd EditionBy:

Questions

What are the main issues to be considered when choosing a file organization?

What circumstances would make an indexed sequential organization appropriate for a master file?