Software Design
Requirements
Reqs. may be from customer or “home invented”
Design
Code-Implementation
Test Case Design
Testing
Builds
Rel.
Maintenance
Important “End-Results” of Design
• “Well-Designed” software results in final
solution that is:
• Easy to learn/understand
• Easy to use
Clear, Consistent, Complete?
• Easy to code
• Easy to maintain (Extensible)
•
•
•
•
Which ones do the “users” care?
Affordable (economical)
Reusable
Dependable
Valuable (increase user productivity, life saving, etc. – meets
customer requirements )
Any other property that you may want to add? -- easy to integrate or test
--- secure ---?
Design of Software Product
• Designers are interested not only in the code,
but the complete product which includes:
–
–
–
–
–
–
Documentation and User Instructions
Code (functionality, data, security, reliability, etc.)
Interfaces (both systems and user)
Packaging and Delivery Scheme
Installation
Service (including maintenance)
This seems like a lot of topics that software designers must cover !!
So, there are specializations within the field of software design.
Design as an “Activity” (verb)
Software “Product” Design is the activity of
specifying:
- the nature and
- composition
of software products that satisfy the clients’
- needs and
- desires
subject to certain set of constraints
What are the concepts behind “nature of” versus “composition of” ?
What about “needs” versus “desire” ?
Understanding the Problem
•
Design occurs “after” requirements (which identifies the
“problems”)
– “after” is fuzzy time boundary – there is no clean boundary
•
Think of Design as both a) understanding & b) solving
“problems”
1. Customer (user) identification of problem (requirements)
sometimes borders on solution --- but is really a constraint. Thus
we should thoroughly “understand” problems
2. For each problem there may be several solutions or designs that
apply
3. Use some “time-honored” problem solving techniques
•
Modifying the problem slightly to
– a) facilitate easier/cheaper solution or
– b) to match previously solved problem
•
•
Trial and error (e.g. iterative prototyping is the fancy term)
Group brainstorming (I only recommend a “small” group)
Two Basic Design Techniques
• Abstraction
– Focusing on the “relevant” properties of an entity
by suppressing and ignoring some other properties
and the details ----- trick is deciding on the
“relevant.”
• Modeling
– Constructing a model “entity” to represent the real
entity of interest through:
• Associating : parts of the model to the parts of the real
entity
• Associating: the relationships among parts of the model to
the relationships among parts of the entity of interest.
2 aspects of Abstraction
• A) Problem/Solution “Simplification”:
concentrating on only the key property
– (For example): In drawing a figure, we choose to first focus on
shape of the figure (e.g. circle, oval, rectangle, diamond, etc.)
before worrying about the details ( color, the line width,
solid/dotted line, etc.) ---- based on past experiences?
• B) Problem/Solution “Refinement: simplify first
and then enhance the solution with more details
– Top-down refinement (Decomposition): partition the problem
into several key components first and then solve the details of
each component
• Bottom-up aggregation: solving pieces of a complex problem and
then aggregate the solutions
• Note: Bottom-up solution usually requires a top-down framework of
key components.
Modeling
• Modeling is based on problem/solution “simplification” aspect of
Abstraction.
– Decide on the key components (abstraction) of the entity of interest.
– Decide on the key relationships that exist among the components of
the entity of interest.
– Construct the components of the model to represent the
components of the entity of interest.
– Construct the relationships among the components of the model to
reflect the relationships among the components of the original entity
of interest.
• In doing so:
– There may be parts in the model that do not match the original entity
– There may be relationships in the model that do not match that in the
original entity
• Note: If everything matched, then the model would just be a
“copy,” not an abstraction of the original entity.
Modeling: “components” and “relations”
Part B
Part A
“Thing-X” that
modeling
Needs
modeling
Part D
Part C
Part F
Another Definition of Design (from K. Lano)
• Design is the activity of (i) “constructing”
components and (ii) “organizing” their
interactions in order to achieve the system that
will satisfy the requirements.
– Components:
• Identifying/specify -- the components
• Specifying --- their functionalities
• Specifying --- any constraints (performance, security, etc.)
– Interactions:
• Identify and Specify Component relations (for OO classes ---- inheritance, aggregation, etc.)
• Identify and Specify Component dependencies/interactions
(interfaces, joint responsibilities, sequences of interactions,
etc.)
Compare this with previous definition --- is this a better refinement?
Develop a Software Model for A “Solar System”
• Start the discussion of this in class
• The remaining parts are given as an
assignment.
(see Assignment 1)
For this assignment also think about what
Is a “well constructed” model or design
e.g.
easy to understand
easy to implement
etc.
See earlier slide
“Advantages” of Modeling
• Allows us to understand the problem or requirements
better (model of the problem)
• Allows us to create an “initial rough design”
– “Model” of problem develops
“Model” of solution
• Allows us to investigate and study both the problem
and the solution in more detail
• Allows us to have a documentation of the
problem/solution descriptions (models)
Two Types of Design Models
• Static model:
– Represents the parts, relationships, and attributes
that do not really change (during execution of the
software )
• Dynamic model:
– Represents what happens during the execution of
the software.
In this course we will be using both static and dynamic models of software
Software Product Designer &
Engineering Designer
• A software product designer is concerned with:
– Product features and capabilities
– Visually appealing and easy to use user-interfaces
– Fitting into users’ business processes and operability
– Product packaging
** ( a closer tie to what is specified in the product requirements )
(similarities and differences?)
• A software engineering designer is concerned with:
–
–
–
–
Individual functionalities and performance
Internal structure of the software system
Individual programs and their interfaces & interactions
The data flow through the software system (including inputs and
outputs)
– System maintenance and evolution
** (a closer tie to the internal technical design )
Software Design Team
• We Need both:
– Software Product Designer
– Software Engineering Designer
• Different sets of skills needed:
– Product Designer – knowledge of: user interface, communicating the
requirements/product design,
marketing, business processes, applications
domain knowledge
– Engineering Designer – knowledge of: architecture, programming,
database and data structures, development
platforms, networking, operating
systems
Software Design
product designer
Requirements
engineering designer
Design
product designer
Code-Implementation
Test Case Design
Testing
Builds
Rel.
Maintenance
(product and
engineering
redesign)
Software Product Design
.vs.
Software Engineering Design
• Product Design Activity – output is a software requirements
specification document
– Answers mostly “what” the user and customer needs and wants --formulates the problem in an organized way :
• Functionalities & features
• Looks and interfaces
• Constraints
organize: relating these
* * * Information/data needs to be folded in also * * *
• Engineering Design Activity – output is a design specification
document
– Answers mostly “how” the software will be put together to respond
to the users’ needs and wants --- formulates the solution in an
organized way
• High level software:
– Components and
– How they interact
– Rationale behind your design decisions ( * my addition )
• Low level specifications of programs and data and their behavior
Where is the analysis / innovation / creativity? ---- what do you think?
Is There a Design Method?
• There is no ONE design method in the form of a
“recipe” for design.
Note: There are cooking recipes! But most software designs are too large
and complex compared to food dishes; so there are no design “recipes” .
• There are several methods that all speak to:
– Design process: a set of steps that starts with “some inputs”
and transform that input to some output and eventually results
in a design specification (e.g. structured method, OO-method,
etc.)
– Design notation: a set of symbolic representations and how
the symbols are to be used (e.g. UML, DFD, etc.)
– Design “heuristics”: a set of guiding rules (e.g. low coupling
and high cohesion)
More importantly, the mental activities that perform the design (analysis/innovation), I
believe, is not easily explainable or understood ---- thus it would be hard to “teach” it!
Brief History of Major Design “Methods”
• Niklaus Wirth; E. Dijkstra (early 1970’s) : Stepwise
Refinement
functional
–
abstraction
top-down approach that repeatedly decomposed procedures
into smaller ones until we reach programming level.
• Stevens, Myers, and Constantine (mid-1970’s):
Structured Design: (also DeMarco; Ross; Mills; etc.)
functional–
& data
–
abstraction
Also top-down and procedural in nature
Included dataflow diagram and entity-relationship
considerations
– Started people on using CASE tools and associated notations
provided by the tools
• Object Oriented Design (late 1980’s- mid 1990’s)
“object”
–
abstraction
Based on “Objects/ Classes” (inheritance, polymorphism, etc.)
– Uses standard UML design notation (Booch, Jacobson,
Rumbaugh)
- Recently, we are pushing ------ “design with patterns”