Design

• On small and mid-sized projects, design and construction are often combined.

Levels of Design

1. System
2. Subsystem
3. Classes/ Modules
4. Data and Functions/ Methods
5. Code

• This indicates the level of details.
• Used to organize the system.
• You need to understand that different design methods are required at each level.
• People who design the overall system may not be the same as those who build classes/functions.

Key Design Concepts

1. Manage complexity.
2. Aim for desirable characteristics in the software design.
3. Respect the different levels of design.

Principles of Design

Design is a Wicked Problem

• Means that a problen can only be fully defined while solving it.
• Requires you to solve the problem a second time after you understand it to create a good solution.

Design is a Sloppy Process

• The finished design may look organized but the steps to reach that point involve many mistakes.
• The differences between a good and bad solution can be small.
• It can be difficult to know when a design is good enough.

Prioritization

• Designer must develop a system which selects between often conflicting characteristics/ priorities.
• ie. memory use, performance, development time, etc.

Restrictions

• Restricting choices is part of design.
• Restrictions simplify solutions and lead to better results.
• Without restrictions a design can become too broad and unwieldy.

Design is Nondeterministic

• You often cannot predict the results of a design.
• Different people will find different solutions; more than one can be valid.

Design is a Heuristic Process

• Design tends to use ‘rules of thumb’; things that generally work but are not guaranteed.
• Bad idea to attempt to rigidly define a process to try and guarantee results.

Design is Emergent

• Design emerges (evolves) through process.
• Design does not appear suddenly in one step.