System design : files

Data Design Concepts  Data Structures  A file or table contains data about people, places or events that interact with the system  File-oriented system  File processing  Database 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 Processing  Can 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 Systems  Advantages 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 Systems  Advantages Stronger standards Controlled redundancy Better security Increased programmer productivity Data independence

Figure 8-5: Data Structure Hierarchy 15

File Organization Serial Organisation  The 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 Organisation  The 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 Organisation  The 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 disadvantages  Give an example of such mathematical formula Full Index Organization Chained Data

reports Write about  Access methods  Factors Influencing File Design Systems Analysis and Design 2nd Edition By:

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?