System Design

Dr. Sangeeta Arora

(Head Of Deptt)

PG Deptt of Computer Science And IT

Introduction: System Design is the process of designing the architecture, components, and interfaces for a system so that it meets the end-user requirements.�It’s a wide field of study in Engineering and includes various concepts and principles that will help you in designing scalable systems. �

Software Design Levels�Software design yields three levels of results:��Architectural Design - The architectural design is the highest abstract version of the system. It identifies the software as a system with many components interacting with each other. At this level, the designers get the idea of proposed solution domain.��High-level Design- The high-level design breaks the ‘single entity-multiple component’ concept of architectural design into less-abstracted view of sub-systems and modules and depicts their interaction with each other. High-level design focuses on how the system along with all of its components can be implemented in forms of modules. It recognizes modular structure of each sub-system and their relation and interaction among each other.�

Detailed Design- Detailed design deals with the implementation part of what is seen as a system and its sub-systems in the previous two designs. It is more detailed towards modules and their implementations. It defines logical structure of each module and their interfaces to communicate with other modules.�

Modularization:��Modularization is a technique to divide a software system into multiple discrete and independent modules, which are expected to be capable of carrying out task(s) independently. ��These modules may work as basic constructs for the entire software. Designers tend to design modules such that they can be executed and/or compiled separately and independently.�

Advantage of modularization:��Smaller components are easier to maintain�Program can be divided based on functional aspects�Desired level of abstraction can be brought in the program�Components with high cohesion can be re-used again�Concurrent execution can be made possible�Desired from security aspect�

Concurrency:�

• In software design, concurrency is implemented by splitting the software into multiple independent units of execution, like modules and executing them in parallel. In other words, concurrency provides capability to the software to execute more than one part of code in parallel to each other.

Example

• The spell check feature in word processor is a module of software, which runs along side the word processor itself

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Coupling :�When a software program is modularized, its tasks are divided into several modules based on some characteristics. As we know, modules are set of instructions put together in order to achieve some tasks. They are though, considered as single entity but may refer to each other to work together. There are measures by which the quality of a design of modules and their interaction among them can be measured.

Cohesion :�

• Cohesion is a measure that defines the degree of intra-dependability within elements of a module. The greater the cohesion, the better is the program design.
• There are seven types of cohesion, namely –
• Co-incidental cohesion - It is unplanned and random cohesion, which might be the result of breaking the program into smaller modules for the sake of modularization. Because it is unplanned, it may serve confusion to the programmers and is generally not-accepted.
• Logical cohesion - When logically categorized elements are put together into a module, it is called logical cohesion.

Temporal Cohesion - When elements of module are organized such that they are processed at a similar point in time, it is called temporal cohesion.�Procedural cohesion - When elements of module are grouped together, which are executed sequentially in order to perform a task, it is called procedural cohesion.�Communicational cohesion - When elements of module are grouped together, which are executed sequentially and work on same data (information), it is called communicational cohesion.�Sequential cohesion - When elements of module are grouped because the output of one element serves as input to another and so on, it is called sequential cohesion.�Functional cohesion - It is considered to be the highest degree of cohesion, and it is highly expected. Elements of module in functional cohesion are grouped because they all contribute to a single well-defined function. It can also be reused.�Coupling�

Design Verification :��The output of software design process is design documentation, pseudo codes, detailed logic diagrams, process diagrams, and detailed description of all functional or non-functional requirements.�The next phase, which is the implementation of software, depends on all outputs mentioned above.�It is then becomes necessary to verify the output before proceeding to the next phase.

The early any mistake is detected, the better it is or it might not be detected until testing of the product. If the outputs of design phase are in formal notation form, then their associated tools for verification should be used otherwise a thorough design review can be used for verification and validation.�By structured verification approach, reviewers can detect defects that might be caused by overlooking some conditions. A good design review is important for good software design, accuracy and quality.�

Software analysis and design includes all activities, which help the transformation of requirement specification into implementation. Requirement specifications specify all functional and non-functional expectations from the software. These requirement specifications come in the shape of human readable and understandable documents, to which a computer has nothing to do.

Data Flow Diagram�Data flow diagram is graphical representation of flow of data in an information system. It is capable of depicting incoming data flow, outgoing data flow and stored data. The DFD does not mention anything about how data flows through the system.�Types of DFD :�Data Flow Diagrams are either Logical or Physical.�Logical DFD - This type of DFD concentrates on the system process, and flow of data in the system.For example in a Banking software system, how data is moved between different entities.�Physical DFD - This type of DFD shows how the data flow is actually implemented in the system. It is more specific and close to the implementation.�

DFD Components�DFD can represent Source, destination, storage and flow of data using the following set of components -�

• Entities - Entities are source and destination of information data. Entities are represented by a rectangles with their respective names.
• Process - Activities and action taken on the data are represented by Circle or Round-edged rectangles.
• Data Storage - There are two variants of data storage - it can either be represented as a rectangle with absence of both smaller sides or as an open-sided rectangle with only one side missing.
• Data Flow - Movement of data is shown by pointed arrows. Data movement is shown from the base of arrow as its source towards head of the arrow as destination.

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Structure Charts

• Structure chart is a chart derived from Data Flow Diagram. It represents the system in more detail than DFD. It breaks down the entire system into lowest functional modules, describes functions and sub-functions of each module of the system to a greater detail than DFD.
• Structure chart represents hierarchical structure of modules. At each layer a specific task is performed.
• Here are the symbols used in construction of structure charts -
• Module - It represents process or subroutine or task. A control module branches to more than one sub-module. Library Modules are re-usable and invokable from any module.

Software Design Approaches :

Here are two generic approaches for software designing:

Top Down Design

• We know that a system is composed of more than one sub-systems and it contains a number of components. Further, these sub-systems and components may have their on set of sub-system and components and creates hierarchical structure in the system.
• Top-down design takes the whole software system as one entity and then decomposes it to achieve more than one sub-system or component based on some characteristics. Each sub-system or component is then treated as a system and decomposed further. This process keeps on running until the lowest level of system in the top-down hierarchy is achieved.
• Top-down design starts with a generalized model of system and keeps on defining the more specific part of it. When all components are composed the whole system comes into existence.

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Bottom-up Design

• The bottom up design model starts with most specific and basic components. It proceeds with composing higher level of components by using basic or lower level components. It keeps creating higher level components until the desired system is not evolved as one single component. With each higher level, the amount of abstraction is increased.
• Bottom-up strategy is more suitable when a system needs to be created from some existing system, where the basic primitives can be used in the newer system.
• Both, top-down and bottom-up approaches are not practical individually. Instead, a good combination of both is used.

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Structured Design :��Structured design is a conceptualization of problem into several well-organized elements of solution. It is basically concerned with the solution design. �Benefit of structured design is, it gives better understanding of how the problem is being solved. Structured design also makes it simpler for designer to concentrate on the problem more accurately.�Structured design is mostly based on ‘divide and conquer’ strategy where a problem is broken into several small problems and each small problem is individually solved until the whole problem is solved.�

These modules are arranged in hierarchy. They communicate with each other. A good structured design always follows some rules for communication among multiple modules, namely -�Cohesion - grouping of all functionally related elements.�Coupling - communication between different modules.�A good structured design has high cohesion and low coupling arrangements.�

Function Oriented Design��In function-oriented design, the system is comprised of many smaller sub-systems known as functions. These functions are capable of performing significant task in the system. The system is considered as top view of all functions.�Function oriented design inherits some properties of structured design where divide and conquer methodology is used.�

Design Process :�The whole system is seen as how data flows in the system by means of data flow diagram.�DFD depicts how functions changes data and state of entire system.�The entire system is logically broken down into smaller units known as functions on the basis of their operation in the system.�Each function is then described at large.

UI is broadly divided into two categories:��1.Command Line Interface��2.Graphical User Interface�

Command Line Interface (CLI) :��CLI has been a great tool of interaction with computers until the video display monitors came into existence.� CLI is first choice of many technical users and programmers. CLI is minimum interface a software can provide to its users.��CLI provides a command prompt, the place where the user types the command and feeds to the system.

Graphical User Interface :�Graphical User Interface provides the user graphical means to interact with the system. GUI can be combination of both hardware and software. Using GUI, user interprets the software.�Typically, GUI is more resource consuming than that of CLI. With advancing technology, the programmers and designers create complex GUI designs that work with more efficiency, accuracy and speed.�

Example�Mobile GUI, Computer GUI, Touch-Screen GUI etc. Here is a list of few tools which come handy to build GUI:�1.FLUID�2.AppInventor (Android)�3.LucidChart�4.Wavemaker�5.Visual Studio�

THANK YOU�