Software Engineering II

Lecture 1: Software Design Concepts and Principles

Software Design

It encompasses the set of principles, concepts, and practices that lead to the development of a high-quality system or product.

Design principles establish and override the philosophy that guides the designer as the work is performed.

Design concepts must be understood before the mechanics of design practice are applied.

practices change continuously as new methods, better analysis, and broader understanding evolve

Data/Class Design

transform analysis classes into implementation classes and data structures.

Architectural Design

defines relationships among the major software structural elements

Interface Design 

defines how software elements, hardware elements, and end-users communicate

Component-level design 

transform structural elements into a procedural description of software components 

Design and Quality

The design must implement all of the explicit requirements contained in the analysis model, and it must accommodate all of the implicit requirements

Design and Quality

It should be a readable, understandable guide for those who generate code and for those who test and subsequently support the software.

Quality Guidelines

A design should be modular

Quality Guidelines

A design should contain distinct representations of data, architecture, interfaces and components

Quality Guidelines

A design should lead to data structures that are drawn from recognizable data patterns

Quality Guidelines

A design should contain functionally independent components

Quality Guidelines

A design should lead to interfaces that reduce the complexity of connections between the components and the external environment.

Quality Guidelines

A design should be derived using a repeatable method that is driven by software requirements analysis

Quality Guidelines

A design should be represented using meaningful notation.

Abstraction

data (named collection of data describing a data object) and procedural (named sequence of instructions with a specific and limited function).

Architecture 

data (named collection of data describing a data object), procedural (name sequence of instructions with a specific and limited function). 

Design Patterns

Describe a design structure that solves a well-defined design problem within a specific context.

Separation of concerns 

Any complex problem can be more easily handled if it’s subdivided into pieces. 

Modularity

Compartmentalization of data and function

Information Hiding

controlled interfaces that define and enforce access to the component procedural

detail and any local data structure used by the component.

Functional Independence

single-minded (high cohesion) components with aversion to excessive interaction with other components (low coupling).

Stepwise Refinement 

incremental elaboration of detail for all abstractions 

Refactoring

A reorganization technique that simplifies the design without changing functionality.

Design Classes

provide design detail that will enable analysis classes to be implemented.

Complete

includes all necessary attributes and methods, and is sufficient (contains only thos

Primitiveness 

each class method focuses on providing one service

High cohesion

small, focused, single-minded classes

Low coupling

class collaboration to kept to minimum

Principle #1

Design should be traceable to the requirements model

Principle #2

Always consider the architecture of the system to be built

Principle #3

Design of data is as important as the design of processing functions. 

Principle #4

Interfaces (both internal and external) must be designed with care

Principle #5

User interface design should be tuned to the needs of the end-user and stress ease of use

Principle #6

Component-level design should be functionally independent

Principle #7

Components should be loosely coupled to each other rather than the environment

Principle #8

Design representation(models)should be easily understandable.

Principle #9

The design should be developed iteratively.

Principle #10 

Creation of a design model does not preclude using an agile approach.

Data Model

data objects and database architectures. It also examines data objects independently of processing.

Data Object

can be an external entity, a thing, an event, a place, a role, an organizational unit, or a structure. 

Interface

A set of operations that describes the externally observable behavior of a class and provides access to its public operations.

UI/User Experience (UX)

is a major engineering action to ensure the creation of usable software products. 

Deployment Design Elements 

Indicates how software functionality and subsystems will be allocated within the physical computing environment. 

Descriptor form deployment diagrams

Shows the computing environment but does not indicate configuration details

Instance form deployment diagrams 

Identify specific hardware configurations that are developed in the latter stages of design. 

Software Architecture

The architecture is not the operational software; it is a representation that enables a software engineer to: 
> Analyze the effectiveness of the design in meeting its stated requirements 

>Consider architectural alternatives at a stage when making design changes is still relatively easy, and 

>Reduce the risks associated with the construction of the software. 

set of components

that performs a function required by a system

set of connectors

that enable “communication, coordination and cooperation” among components

constraints 

that define how components can be integrated to form the system 

semantic models

that enable a designer to understand the overall properties of a system by analyzing the known properties of its constituent parts.

Data Centered Architecture

Data Flow Architecure

Call Return Architecture

Object-Oriented Architecture

Layered Architecture

Model View Controller Architecture

Economy

software is uncluttered and relies on abstraction to reduce unnecessary detail

Visibility

architectural decisions and their justifications should be obvious to software engineers who review

Spacing

separation of concerns in a design without introducing hidden dependencies

Symmetry

Architectural symmetry implies that a system is consistent and balanced in its attributes

Emergence 

Emergent, self-organized behavior and control are key to creating scalable, efficient, and economic software architectures. 

Archetype

is an abstraction (similar to a class) that represents one element of system behavior.

User Experience Design 

We’re not just referring to how the software looks, but also how it feels and functions from the user perspective 

“by accident”

UX ensures that no part of the software exists ____.

Strategy

identifies user needs and customer business goals, which form the basis for all UX design work.

Scope

includes both the functional and content requirements needed to realize a feature set consistent with the project strategy.

Functional Requirements

These describe what the system should do — the specific features, actions, or behaviours it must perform.

Non-functional requirements

These describe how the system should perform its tasks.

Structure

consists of the interaction design

interaction design

how the system reacts to user actions

information architecture

how the information is organized

Skeleton

compromising three components: information design, interface design, navigation design. Think of this as your wireframe

Surface

presents visual design, or the appearance of the finished project, to its users. The final visual design— the overall look and feel of your app.