The History Of Logical System Design

Published Date: 02 Nov 2017

Include a description of the key points in the models.

Discuss the tradeoffs and weaknesses encountered in the designs.

Summarize any important points for the developers regarding the design.

Database Design Concept:

The aim of the unit is to demonstrate the role databases play in the development of information systems and that; as a result, a thorough understanding of such is vital. Databases are an essential part of the IT industry; as the applications that integrate with them diversify and expand in functionality, so does the need for Database Systems.

The aim of this unit is to provide a broad base of knowledge that relates to the design principles, development techniques, and implementation of database systems for both the system designer and software engineer. There is a strong focus on structured query languages, modeling entities, modeling relationships, normalization, and mapping.

Entity Relationship (ER) modelling

is a design tool

is a graphical representation of the database system

provides a high-level conceptual data model

supports the user's perception of the data

is DBMS and hardware independent

had many variants

is composed of entities, attributes, and relationships

Database designs also include ER (Entity-relationship model) diagrams. An ER diagram is a diagram that helps to design databases in an efficient way.

Attributes in ER diagrams are usually modeled as an oval with the name of the attribute, linked to the entity or relationship that contains the attribute.

Within the relational model the final step can generally be broken down into two further steps, that of determining the grouping of information within the system, generally determining what are the basic objects about which information is being stored, and then determining the relationships between these groups of information, or objects. This step is not necessary with an Object database

IV) System Design

1. Logical System Design

This step of the phase involves preparation of design specification for converting input data into output results. Its scope includes.

a) Understanding the used requirements

b) Identify data requirements.

c) Suggesting a logical organization of data

d) Suggesting a logical procedure to produce the desired outputs from available inputs.

Its is very important that we understand the requirement of various users in the organization. Users at different levels in an organization have different information requirements for decision making.

After estimating user’s requirements, a system designer works backwards to identify data requirements. This includes identifying the data source, the nature and type of data that is available and data gaps. The next step is to establish process logic to involve a

Data Flow Analysis and a Data Processing Analysis. Data flow analysis helps us to arrive at an organization of data into computer files.

A logical representation of data flow analysis and data processing analysis can be effectively provided through structured system design tools. These are:

Data Flow Diagrams

Data Dictionary

Decision Table

Decision Tree

Structured English or data processing analysis.

2. Physical System Design

While a logical design provides an estimate of processing requirements , a physical design involves mapping the logical design into the physical hardware of a computer system. Upgrading the existing hardware or acquiring a new system. If required to meet the processing requirements is also undertaken at this stage.

A system flowchart is used to present the physical system design. Various activities for which analyst are responsible at this stage are:

a) to specify design philosophy.

b) to prepare a detailed written description of the proposed system.

c) to prepare the system flowcharts, procedure charts, data flow diagrams , decision tables and data structure charts etc.

d) to define the reports to produced by the system.

e) to define the master files to be used.

f) to specify the checks and controls to be built into the system.

g) to define the primary input and the output documents.

h) to specify arrangements for converting current files.

i) to present proposal to the management.

j) to implement management decision either to abort project or select and alternative design philosophy.

V) Implementation

Actual programming is undertaken at this stage to implement the proposed system. This activity includes.

1. Selection of conversion software

First of all , Data Processing Department Decides which programming language or pre-written software is to be used to the implementation of system. For elementary system and small size organization, even spreadsheet package , Lotus 1-2-3 , MS Excel etc are sufficient.

For complex systems and medium size organizations , there numerous options available . one can go for small Database Management Software package like Ashton Tate’s d Base series of products and its peers like Fox inc’s Fox-base and fox-base plus, Microsoft ‘ s Fox-pro.

For Larger Organizations Options available are the programming languages and RDBMS

2. System Coding / programming

After the data processing, staff have decided about the language/software to be used , programs corresponding to the design is written. Coding approach might be one of the following:

Top down

Bottom up

Structures

Defensive

Modular

3. File Conversion

Before having an actual run of the program written during programming phase , one have prepare the database structure chalked out during file design phase.

Moreover, data are to be filled in these database files beforehand. These data are readily available from paper documents lying with accounts department and personal department.

4. System Debugging

In this phase one’s endeavor is to make the code prepared during coding phase error free. If the programs to debugged are not voluminous, then one can go for manual debugging. Otherwise one may use the special purpose debugging utilities. Various types of error occurred during are:

Lexical Error

Syntactical Error

Semantic Error

Logical/run time Error

5. System Testing

During this phase of system development it is ensured that the system stick to the management constraints and fulfills user requirements.

6. Training

System is to be designed to interact with user in account in jargon. It is suggested that minimal use of computer specific terminology be made. These terms are borrowed from daily life. Any account personal needing very little training in order to be conversant with new system.

7. System Changeover

Once above phases are conducted successfully the newly developed system can be put in operation. A time tested approach here is that for some time both, the present system and the new system are operated parallel. Their outputs are compared. The old system is removed with the reliable new system.

VI) System Maintenance and Review

The newly designed and implemented system is reviewed by the analyst after some time. He critically examines whether the system in use is suitable to meet the changing conditions of not. He also ensures that the system fully meets the objectives of the system for which it has been created.

(e) System Design

Based on the user requirements and the detailed analysis of the existing system, the new system must be designed. This is the phase of system designing. It is the most crucial phase in the developments of a system. The logical system design arrived at as a result of systems analysis is converted into physical system design. Normally, the design proceeds in two stages:

Preliminary or General Design: In the preliminary or general design, the features of the new system are specified. The costs of implementing these features and the benefits to be derived are estimated. If the project is still considered to be feasible, we move to the detailed design stage.

Structured or Detailed Design: In the detailed design stage, computer oriented work begins in earnest. At this stage, the design of the system becomes more structured. Structure design is a blue print of a computer system solution to a given problem having the same components and inter-relationships among the same components as the original problem. Input, output, databases, forms, codification schemes and processing specifications are drawn up in detail.

In the design stage, the programming language and the hardware and software platform in which the new system will run are also decided. There are several tools and techniques used for describing the system design of the system. These tools and techniques are:

Flowchart

Data flow diagram (DFD)

Data dictionary

Structured English

Decision table

Decision tree

Each of the above tools for designing will be discussed in detailed in the next lesson.

The system design involves:

i. Defining precisely the required system output

ii. Determining the data requirement for producing the output

iii. Determining the medium and format of files and databases

iv. Devising processing methods and use of software to produce output

v. Determine the methods of data capture and data input

vi. Designing Input forms

vii. Designing Codification Schemes

viii. Detailed manual procedures

ix. Documenting the Design

(f) Coding

The system design needs to be implemented to make it a workable system. This demands the coding of design into computer understandable language, i.e., programming language. This is also called the programming phase in which the programmer converts the program specifications into computer instructions, which we refer to as programs. It is an important stage where the defined procedures are transformed into control specifications by the help of a computer language. The programs coordinate the data movements and control the entire process in a system. It is generally felt that the programs must be modular in nature. This helps in fast development, maintenance and future changes, if required.

(g) Testing

Before actually implementing the new system into operation, a test run of the system is done for removing the bugs, if any. It is an important phase of a successful system. After codifying the whole programs of the system, a test plan should be developed and run on a given set of test data. The output of the test run should match the expected results. Sometimes, system testing is considered a part of implementation process.

Using the test data following test run are carried out:

Program test: When the programs have been coded, compiled and brought to working conditions, they must be individually tested with the prepared test data. Any undesirable happening must be noted and debugged (error corrections)

System Test: After carrying out the program test for each of the programs of the system and errors removed, then system test is done. At this stage the test is done on actual data. The complete system is executed on the actual data. At each stage of the execution, the results or output of the system is analysed. During the result analysis, it may be found that the outputs are not matching the expected output of the system. In such case, the errors in the particular programs are identified and are fixed and further tested for the expected output. When it is ensured that the system is running error-free, the users are called with their own actual data so that the system could be shown running as per their requirements.

(h) Implementation

After having the user acceptance of the new system developed, the implementation phase begins. Implementation is the stage of a project during which theory is turned into practice. The major steps involved in this phase are:

Acquisition and Installation of Hardware and Software

Conversion

User Training

Documentation

The hardware and the relevant software required for running the system must be made fully operational before implementation. The conversion is also one of the most critical and expensive activities in the system development life cycle. The data from the old system needs to be converted to operate in the new format of the new system. The database needs to be setup with security and recovery procedures fully defined.

During this phase, all the programs of the system are loaded onto the user’s computer. After loading the system, training of the user starts. Main topics of such type of training are:

How to execute the package

How to enter the data

How to process the data (processing details)

How to take out the reports

After the users are trained about the computerized system, working has to shift from manual to computerized working. The process is called ‘Changeover’. The following strategies are followed for changeover of the system.

(i) Direct Changeover: This is the complete replacement of the old system by the new system. It is a risky approach and requires comprehensive system testing and training.

(ii) Parallel run: In parallel run both the systems, i.e., computerized and manual, are executed simultaneously for certain defined period. The same data is processed by both the systems. This

strategy is less risky but more expensive because of the following:

Manual results can be compared with the results of the computerized system.

The operational work is doubled.

Failure of the computerized system at the early stage does not affect the working of the organization, because the manual system continues to work, as it used to do.

(iii) Pilot run: In this type of run, the new system is run with the data from one or more of the previous periods for the whole or part of the system. The results are compared with the old

system results. It is less expensive and risky than parallel run approach. This strategy builds the confidence and the errors are traced easily without affecting the operations. The documentation of the system is also one of the most important activity in the system development life cycle. This ensures the continuity of the system. There are generally two types of documentation prepared for any system. These are:

User or Operator Documentation

System Documentation

The user documentation is a complete description of the system from the users point of view detailing how to use or operate the system. It also includes the major error messages likely to be encountered by the users. The system documentation contains the details of system design, programs, their coding, system flow, data dictionary, process description, etc. This helps to understand the system and permit changes to be made in the existing system to satisfy new user needs.

(i) Maintenance

Maintenance is necessary to eliminate errors in the system during its working life and to tune the system to any variations in its working environments. It has been seen that there are always some errors found in the systems that must be noted and corrected. It also means the review of the system from time to time. The review of the system is done for:

knowing the full capabilities of the system

knowing the required changes or the additional requirements

studying the performance.

If a major change to a system is needed, a new project may have to be set up to carry out the change. The new project will then proceed through all the above life cycle phases.