1. Informatics 121 Software Design I
Lecture 5
Duplication of course material for any commercial purpose without the explicit written permission of the professor is prohibited.

2. Today’s lecture
Design failures
Practice

3. Intermezzo: what experts do
Experts prefer solutions that they know work
Experts look around
Experts take inspiration from wherever they can
Experts use analogy
Experts use design methods (selectively)
Experts network

4. Two fundamental challenges
The nature of software
The nature of people

5. Nature of software (Brooks)
Complexity
software is among the most complex people-made artifacts
Conformity
software has no laws of nature that simplify its existence; rather, it lives in a world of designed artifacts to which it much conform
Changeability
software is subject to continuous pressure to change
Invisibility
because the reality of software is not embedded into space, it is inherently unvisualizable

6. Nature of people Diversity Indiscernibility Familiarity Volatility
people differ in how they experience the world
Indiscernibility
experiences are distinctly mental in nature, with tangible reactions and signs not always matching their actual experience
Familiarity
people tend to be risk averse, sticking to role, organizational, and societal norms and values
Volatility
with every new exposure, people reinterpret and modify their opinions and expectations

7. Software design is a wicked problem
The problem is not understood until after the formulation of a solution
Wicked problems have no stopping rule
Solutions to wicked problems are not right or wrong
Every wicked problem is essentially novel and unique
Every solution to a wicked problem is a “one shot operation”
Wicked problems have no given alternative solutions
Worse, software design is a ‘hard’ wicked problem

8. Seven difficulties every software designer faces
Prediction
Tradeoffs
Change
Bias
Longevity
Quality, cost, and effort
Source code as a design notation

9. Difficulty #1: predicting people’s experiences
Software design is a predictive activity that must anticipate the experiences of people
account for differing experiences
account for shifting experiences
Furthermore, when people share their vision for a future change in the world, they tend to be conservative
The difficulty lies in how to balance what people perceive they need and what they actually need

10. Difficulty #2: tradeoffs
Tradeoffs must be made continuously over the course of a design project
a design solution is judged relative to other possible solutions that could have been made
it is frequently impossible to quantify the relative virtues of each alternative
Furthermore, any given design solution inevitably satisfies certain stakeholders more than others
The difficulty lies in how to balance the decisions across the needs and experiences of the various stakeholders

11. Difficulty #3: change Change happens and must be accommodated
changing client demands
changing context of people, hardware, and software
changing understanding of the design problem
changing attitudes of the audience
Furthermore, the more complete a design solution already is, the more difficult it will be to change it
The difficulty lies in how to balance accommodating change with making sufficient progress on the design solution

12. Difficulty #4: bias
Bias exists within all stakeholders, and can be a positive or negative influence
audience
designer
client
others
Furthermore, bias is typically difficult to detect and address
The challenge lies in how to balance benefiting from positive bias while avoiding the effects of negative bias

13. Difficulty #5: longevity
Longevity is expected of most software design solutions
software does not wear out
software fulfills critical societal services
Furthermore, it very likely will need to accommodate new needs as well as deviations from present needs
The difficulty lies in how to balance addressing today’s design problem with accommodating future, unknown needs

14. Difficulty #6: quality, cost, and effort
Quality, cost, and effort are three constraining factors in design, not all of which can be optimized at the same time
cost constraints typically set by other stakeholders
cost constraints typically set before the design project is truly understood
Furthermore, there is no stopping rule, since no optimal design solution exists
The difficulty lies in how to balance a design solution that is of good enough quality with expectations on cost and effort

15. Difficulty #7: source code as a design notation
Source code is a terrible design notation for expressing goals, constraints, assumptions, decisions, and ideas
Furthermore, other software design notations typically do not map easily to source code
while they certainly help make progress, much manual effort remains in transforming findings stemming from the use of these notations to the source code
The difficulty lies in how to balance use of other design notations with the need to ultimately express the design solution in source code

16. Practice Virtual campus visit
Disney electronic queue management system
Mountain climbing 3D guide system
Social restaurant table

17. Disney electronic queue management system