Nov. 4, 2013
cs561 Software Engineering Notes

Software architecture and design

Investigate general principles of design
patterns to construct high-level architecture of the system

In particular we will learn to
	Design as a series of decisions
	Look at different types of design
		top down  - starting with architecture
		bottom up - starting with utilities

Learn about
	Design principles
		Divide and conquer
		striving for high cohesion and low coupling
		Using good abstraction to hide details

	Evaluate trade offs
		cost-benefit analysis

	SW architectures for high level design

	Writing effective design documents

What is design for SW?

	Problem solving process

	Objective - find and describe a way to implement functional 
        requirements within constraints

	Constraints are imposed by  quality, platform, and process requirements
           - Response time
           - Throughput
           - Resource use
           - Reliability
           - Availability
           - Maintenance

Recall question
	What is a platform requirement? Process requirement?
          Platform - technology to use or computing platform
          Process requirement - development methods, cost, delivery date,...

During design have many subproblems to deal with and many options to deal with 
them

Need to choose best options for each design decision
To do so consult the following:

	Current design
	Requirements
	Technology available
	Knowledge of design principles and best practices
	Your own past knowledge

Definitions of system parts

Subsystems, components, and modules

Component - any piece of SW that has a clear role that can be isolated and replaced a with different component having similar functionality
	Ex. source code
	executable
	DLL
	DB

Module - component defined at programming language level
	Methods, classes, and packages

System - logical entity having set of definable responsibilities or objectives 
and consisting of HW, SW or both.
	Implemented by group of components

Subsystem - system that is part of a larger system with a definite interface
	Ex. packages in java used to implement subsystems or individual classes

Draw class diagram


Design

What is top down?

Start with high level structure work down toward decisions about high level 
constructs

If I want to build a First Person Shooter what is the top level design?
Game --> Components -->  Levels , Storyline, Characters ---> Character Design, 
...

What is bottom up design?

Start with low level utilities then figure out how to put them together to get 
high level constructs

UI, Character Class, Physics for character --> Character Design, … -> levels -> components -> game

Lots of times we use both

Why?
Have ideas for some specifics and ideas for high level too


Lots of kinds of design
	Architecture - division of SW into subsystems and components
		Deciding how these will be connected
	Class design - design of features and classes (associations, 
          attributes, interactions, states)
	UI Design
	DB design
	Algorithm design
	Protocol design - comm networks


Want good design resulting in
	Increased profit, decreased cost, increased revenue
		What is revenue? what is cost? what is profit?
	Conform to requirements to give customer what he/she wants
	Speed up development
	Increased quality

How to get good design?
Follow general design principles

1. Divide and conquer
	Work on smaller manageable parts of a SW system
	Do development in parallel
	Allows for experts to focus on content they are comfortable with
	Able to have replaceable or interchangeable parts
	Can make components reusable (add incentive for SW engineer)
	
	Many ways to divide up a project
		Distributed system divided into clients and servers
		System divided into subsystems
		Subsystem divided into packages
		Packages divided into classes
		Classes divided into methods and attributes

2. Increase cohesion where possible
	Cohesion - stick or stay together
	Want to keep things together that belong together especially w.r.t. 
        divide and conquer
	Subsystems/modules have high cohesion if related classes/modules are 
        kept together and unrelated ones are not included
	Many types of cohesion
		Functional - module performs a single computation and returns a 
                value without side
		effects (leaves system in same state)
		File, DB, user updates are side effects

		Layer - related services are kept together, everything else is 
                        kept out
			-there is a strict hierarchy in which higher level 
                         services can access only lower
			level services
			-accessing a service may result in a side effect
		Communicational - facilities for operating on the same data are 
                                  kept together and everything else is kept 
                                  out.  Good classes exhibit communicational 
                                  cohesion

		Sequential - set of procedures that work in sequence to perform 
                             a computation are kept together.  Output from one 
                             is input to the next.  Everything else is kept out.

		Procedural - Set of procedures that are called one after 
                             another is kept together.

		Temporal - procedures used in the same general phase of 
                           computation (e.g. init or termination) are kept 
                           together

		Utility - related utilities are kept together when there is no 
                          way to group them using a stronger form of cohesion