design

Design

COP 3538 Summer 2012

© Lethbridge/Laganière 2001Chapter 9: Architecting and

designing software 2

The Process of Design •Definition:

– Design is a problem-solving process whose objective is to find and describe a way:

• To implement the system’s functional requirements...• while respecting the constraints imposed by

non-functional requirements...– Such as performance, maintainability, security, persistence,

cost, reliability, portability, etc…..(long list)– including also the budget, technologies, environment, legal

issues, deadlines, ...• and while adhering to general principles of good quality

– Consider this perspective:

3

Object-Oriented Analysis

•An investigation of the problem (rather than how a solution is defined)

•During OO analysis, there is an emphasis on finding and describing the objects (or concepts) in the problem domain.

– For example, concepts in a Library Information System include Book, and Library.

4

Object-Oriented Design•Emphasizes a conceptual solution that fulfills the requirements.

•Need to define software objects and how they collaborate to fulfill (satisfy) the requirements.

– For example, in the Library Information System, a Book software object may have a title attribute, an ISBN number, and a getChapter method.

•Designs are implemented in a programming language.– In the example, we will have a Book class in Java.

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From Design to Implementation

Book

titleprint()

public class Book{

public void print();private String title;

}

Book(concept)

Analysisinvestigationof the problem

Designlogical solution

Constructioncode

Domain concept Representation inanalysis of concepts

Representation in anobject-orientedprogramming language.



Process of Design•Design is all about decisions.•Approaches: Top down – start with the architecture • Bottom up – start with utilities• There are a number of very serious design principles that lead to maintainable software that may persist for years• We will look at satisfying functional requirements while accommodating portability, reuse potential, performance

• There are always TRADEOFFS! There is no free lunch!!!--

• Will look at several of the tradeoffs

• Will look at a software architecture to support your high-level design



Design as a Series of Decisions

•A designer is faced with a series of design issues – These are sub-problems of the overall design problem. – Always several alternative solutions: design options. – Designer makes a design decisions to resolve issues.

• This process involves choosing the best option from among alternatives.

– Recognize that there may be a number of solutions – in fact, there may be a number of good solutions for the problem to be solved.

– We would like the ‘best’ one.



Making decisions•To make each design decision, the software engineer uses:

– Knowledge of• the requirements (use cases, UI prototype, feature lists,

supplementary specification document, class diagrams, interaction diagrams …)

• the design as created ‘so far’• Available technologies (RMI, RPC, xml, jsp, servlets,

html, jdbc, etc.) given a development environment• software design principles and ‘best practices’ • what has worked well in the past

– Sometimes there is no single, best solution.

– Sometimes they conflict – each presenting pros and cons



‘Design space’ – example Consider choice of thin client vs fat client options:

•The space of possible designs that could be achieved by choosing different sets of alternatives is often called the design space

(more efficient use of CPU and of network resources) Why???

(simpler software)

Adv of fat client: bandwidth, networking services, reduced need for powerful server…Adv of thin client: simpler client devices; maintaining services; central bus logic…Disadvantages? Know! Cost, reliability, maintenance, security, bandwidth, network traffic; Never a single answer for all cases! Design!

client-server

monolithic

separate user interface layer for client

no separate user interface layer for client

fat-client

thin-client

programmed in Java

programmed in .Net

programmed in C++; C#?



Criticality of Certain Design Decisions• Some design decisions are critical; others not so. • Example: architectural design decision to separate the user interface module from rest of system.

– Yes: easier to develop and maintain, internationalize, employ reuse.

– No: Likely not as efficient; (disadvantage. why?)

– Recommend iterating User Interface as a part of iteration planning!

– As increments of value are produced, so too should the interface evolve.

Modeling – Design Model• Note that the design model (assumes a good domain model depicting important concepts in the business domain) really is a set of diagrams showing logical design.

• This will include software class diagrams, design level sequence diagrams (as opposed to analysis level sequence diagrams using conceptual objects…. and lots of abstraction) as well as package diagrams. (Larman)

•The software architecture model will include a summary of design ideas and their motivations. Why???



© Lethbridge/Laganière 2001Chapter 9: Architecting and designing

software 12

Features of Top-down and Bottom-up Design

•Top-down design– First design the very high level structure of the system.– Then gradually work down to detailed decisions about low-level

constructs.– Finally arrive at detailed decisions such as:

• the format of particular data items;• the individual algorithms that will be used.

– Start with the software architecture and the type of database that will be used (not ‘which’ database).

– Ultimately arrive at specific data items and detailed algorithms.



Top-down and Bottom-up Design

•Bottom-up design– Make decisions about reusable low-level utilities.– Then decide how these will be put together to

create high-level constructs.

Hybrid

•A mix of top-down and bottom-up approaches is normally used:

– Top-down design is almost always needed to give the system a good structure (architecture).

– Bottom-up design is normally useful so that reusable components can be created.



Different Aspects of Design – Very important!!

– All kinds of ‘design’ This is where the decisions are made!!!!

– Architecture design: • The division into subsystems and components,

– How these will be connected and interact. Interfaces. – Class design:

• The various features of classes.– User interface design – Algorithm design:

• The design of computational mechanisms.– Protocol design:

• The design of communications protocol.•For a while, now, we will emphasize Architectural Design – after we discuss class design.



Principles Leading to Good Design •Overall goals of Good Design:

– Increase profit by reducing cost and increasing revenue – Ensure design accommodates requirements – Speed up development for use / competing in marketplace– Increase qualities such as

• Usability – learnability; ease of use; on-line help…• Efficiency• Reliability• Maintainability• Reusability to reduce cost and increase revenues



Design Principle 1: Divide and conquer

•Trying to deal with something big all at once is normally much more difficult than dealing with a series of smaller, managable, understandable things

–Hence the iterative approach to software development)• A software engineer/software developer can specialize.

– Specialize in network, distribution, database, algorithms, searching / sorting techniques…

• Individual components smaller, easier to understand.

• Parts can be replaced or changed without having to replace or extensively change other parts.



Ways of Dividing a Software System

– A distributed system is divided up into clients and servers

– A system is divided up into subsystems

– A subsystem can be divided up into one or more packages

– A package is divided up into classes

– A class is divided up into attributes and methods



Design Principle 2: Increase (High) Cohesion where possible

•Divide and Conquer says split things up. Smaller parts, easier to grasp.

•A subsystem or module has high cohesion if it keeps together things that are related to each other, and keeps other things out!

– Makes system as a whole easier to understand / change

– Type of cohesion:• Functional, Layer, Communicational, Sequential, Procedural,

Temporal, Utility



Functional Cohesion

• Achieved when all the code that computes a particular result is kept together - and everything else is kept out

– i.e. when a module only performs a single computation, and returns a result, without having side-effects.• No changes to anything but the computation

• Normally implemented via parameters

• Can call other methods, if cohesion is preserved.– (Recall: Call by Value; Call by Reference… as examples)– Avoid things like common, global data, more

– Benefits to the system:• Easier to understand• More reusable• Easier to replace• Example: pass an array of integers; sort the array and return sorted array.

Can change algorithms if interface remains unchanged…



Layer Cohesion• All facilities for providing or accessing a set of related services are kept together, and everything else is kept out

– The layers should form a hierarchy• (Sample Layers – presentation (interface); business (domain) logic;

application logic; technical services…)• Higher layers can access services of lower layers, • Lower layers do not access higher layers

– Will talk about architectural layers a great deal very soon…

– The set of procedures through which a layer provides its services is the application programming interface (API)

– Specification of API says how to use it.– You can replace a layer without having any impact on the other layers

because you know that it does not access upper layers.• E.g. Replace the UI layer….



Example of the Use of Layers

Screen displayfacilities

User accountmanagement

Filesystem

Kernel(handling processes

and swapping)

Application programs

Userinterface

Applicationlogic

Databaseaccess

Networkcommunication

Transmittingand receiving

Dealing withpackets

Dealing withconnections

Dealing withapplication protocols

a) Typical layers in an application program

b) Typical layers in an operating system

c) Simplified view of layers in a communication system

Operating systemaccess

Examples: services for computations; transmissions of messages; storage of data; managing security, interacting with users; accessing the operating system; interacting with hardware, and more

Sometimes we have a business services and then a domain (more general) layer…Will have a middleware layer often!



Communicational Cohesion

•All the modules that access or “manipulate certain data” are kept together (e.g. in the same class) - and everything else is kept out

– A class would have good communicational cohesion • if all the system’s facilities for storing and manipulating its

data are contained in this class.• if the class does not do anything other than manage its

data.– Main advantage: When you need to make changes to

the data, you find all the code in one place – Keep methods where the data is, if possible.– Talk about this extensively in Data Structures course!!



Other Measures of Cohesion• Sequential Cohesion•Procedures, in which one procedure provides input to the next, are kept together – and everything else is kept out

– You should achieve sequential cohesion, only once you have already achieved the preceding types of cohesion.

•Procedural Cohesion•Keep together several procedures that are used one after another

– Even if one does not necessarily provide input to the next. Weaker than sequential.

•Temporal Cohesion•Operations that are performed during the same phase of the execution of the program are kept together, and everything else is kept out

– For example, placing together the code used during system start-up or initialization – Weaker than procedural cohesion.

•Utility Cohesion Related utilities which cannot be logically placed in other cohesive units are kept together

– A utility is a procedure or class that has wide applicability to many different subsystems and is designed to be reusable.

– For example, the java.lang.Math class.

•



Design Principle 3: Reduce (Low) Coupling where possible

•Coupling occurs when there are interdependencies between one module and another

– When interdependencies exist, changes in one place will require changes somewhere else.

– A network of interdependencies makes it difficult to see at a glance how some component works.

– Type of coupling: (in decreasing order of avoidance!)• Content, Common, Control, Stamp, Data, Routine Call, Type use,

Inclusion/Import, External



Content Coupling: The Worst of the Bad!

•Occurs when one component surreptitiously modifies data (or instructions!) that is/are internal to another component

– To reduce content coupling you should therefore encapsulate all instance variables

• declare them private • and provide get and set methods

– A worse form of content coupling occurs when you directly modify an instance variable from outside the object.

– Discuss: how easy it is to do this and how/why it has been

done in the past! (especially in non-object-oriented systems)• Assembler; ‘Alter’ verb in Cobol



Common Coupling Avoid where possible and practical!!!

•Occurs whenever you use a global variable– All the components using global variables become

coupled to each other

– Can be acceptable for creating global variables that represent system-wide default values

– Clearly, when a value is changed, it may be very difficult to trace the source of the change!



Control Couplingnot as bad, but still pretty strong coupling!

if you are able to.•Occurs when one procedure calls another using a ‘flag’ or ‘command’ that explicitly controls what the second procedure does (passing a switch….)

– To make a change you have to change both the calling and called method; that is, to avoid the flags…

– The use of polymorphic operations is normally the best way to avoid control coupling

– One way to reduce the control coupling could be to have a look-up table

• commands are then mapped to a method that should be called when that command is issued



Example of control couplingpublic routineX(String command){ if (command.equals("drawCircle") { drawCircle(); } else { drawRectangle(); }}

See? Flag is passed (command) whose value is used to control flow!Can be handled better through polymorphism…



Other Forms of Coupling:

• Stamp Coupling• Data Coupling• Routine-call Coupling• Type Use Coupling• Inclusion or import Coupling• External Coupling



Design Principle 4: Keep Level of Abstraction as High as Possible

•Ensure that your designs allow you to hide or defer consideration of details, thus reducing complexity

– A good abstraction is said to provide information hiding – Abstractions allow you to understand the essence of a

subsystem without having to know unnecessary details – Allows us to grasp the essentials!

• Defer the less important grunt-level items until later• We understand stacks, queues, trees, heaps…. Sufficient!!

– Allows us to deal with complexity!

© Lethbridge/Laganière 200126 Chapter 9: Architecting and


Design Principle 5: Increase Reusability where Possible

• Design for Reusability (in other contexts) – Goal is to increase reusability; – Actively reuse the work of others.– Generalize your design as much as possible

• Layers, packages, subsystems help greatly!• Some address the GUI; some general business domain;

others: application specifics. • Design your system to contain hooks

– Essentially this means to design your system where some things are provided, but can be modified. Others must be added…

– Simplify your design as much as possible • Divide and conquer; low coupling; high cohesion…• Separation of ‘concerns’ (subsystems, packages) etc.



Design Principle 6: Reuse existing designs and code where possible

• Design with Reuse is Complementary to Design for Reusability

– Actively reusing designs or code allows you to take advantage of the investment you or others have made in reusable components



Design Principle 7 Design for Flexibility

•Actively anticipate changes that a design may have to undergo in the future, and prepare for them

– Reduce coupling and increase cohesion of design elements– Create abstractions like interfaces or super classes.

• These actions readily supports extensions and polymorphism– Do not hard-code anything

• Remember, programming is the realization of design!!!• This is not the time…

– Use reusable code and make code reusable (where practical…)



Design Principle 8: Anticipate Obsolescence

• Plan for changes in the technology or environment so the software will continue to run or can be easily changed

– Avoid using early releases of technology• NEVER a good idea to use unproven technologies…

– Avoid using software libraries specific to particular environments – may not be supported in the future…

– Avoid using undocumented features or little-used features of software libraries

– Avoid using software or special hardware from companies less likely to provide long-term support

– Use standard languages and technologies supported by multiple vendors



Design Principle 9: Design for Portability

• Have software run on as many platforms as possible – Avoid use of facilities specific to one particular environment

• e.g. a library only available in Microsoft Windows– Remember: we develop systems that will hopefully be used ‘for a long

time.’ Platforms change!

– But: more and more technologies are here and coming along which support portability very nicely….

• Don’t want to care how many bits there are in an integer or key definitions that apply only in a special environment.

• Be aware of these in your design decisions / choices.• Document design decisions!!



Design Principle 10: Design for Testability

•Take Steps to make Testing Easier– Map your tests to use-cases– Map the validation of your prototype to use cases.– Design each test back to– demonstrate satisfaction of functional / non-

functional requirements – One may design a program to automatically test the software– Study the testing discipline and be aware of the many faces of

testing…many techniques.



Design Principle 11: Design Defensively

• Never trust how others will try to use a component you are designing

– Handle all cases where other code might attempt to use your component inappropriately

– “Robustness” – Discuss tradeoffs and criticality…– Check the validity of all inputs to your components

• the preconditions• Unfortunately, over-zealous defensive design can result in

unnecessarily repetitive checking– More on Robustness:

• The 80-20 rule• Many ‘absolutely necessary’ features are never used;

rather, test for ‘desired outcomes!’

design

Documents

design decisions

design designcop

design options

overall design problem

series of design issues

process of design definition

serious design principles

book class