A Philosophy of Software Design, 2nd Edition

1. Complexity is the root of software design challenges

Complexity comes from an accumulation of dependencies and obscurities.

Complexity accumulates incrementally. As software systems grow, they tend to become more complex due to the gradual accumulation of dependencies between components and obscure code sections. This complexity manifests in three primary ways:

• Change amplification: Small changes require modifications in many places
• Cognitive load: Developers need to understand large amounts of information to make changes
• Unknown unknowns: It's unclear what code needs to be modified or what information is relevant

Simplicity is the antidote. To combat complexity, software designers should focus on creating simple, obvious designs that minimize dependencies and obscurities. This involves:

• Modular design: Dividing systems into independent modules
• Information hiding: Encapsulating implementation details within modules
• Clear abstractions: Providing simple interfaces that hide underlying complexity

2. Strategic programming trumps tactical approaches

The best approach is to make lots of small investments on a continual basis.

Long-term thinking yields better results. Strategic programming focuses on creating a great design that happens to work, rather than just making code work. This approach involves:

• Investing time in design upfront
• Continually making small improvements
• Refactoring code to maintain clean design

Tactical programming leads to technical debt. While tactical approaches may seem faster in the short term, they often result in:

• Accumulation of quick fixes and hacks
• Increasing difficulty in making changes over time
• Higher long-term development costs

By adopting a strategic mindset, developers can create systems that are easier to maintain and evolve, ultimately saving time and effort in the long run.

3. Modules should be deep, not shallow

The best modules are those that provide powerful functionality yet have simple interfaces.

Depth creates abstraction. Deep modules hide significant implementation complexity behind simple interfaces. This approach:

• Reduces cognitive load for users of the module
• Allows for easier modification of the implementation
• Promotes information hiding and encapsulation

Shallow modules add complexity. Modules with complex interfaces relative to their functionality are considered shallow. These modules:

• Increase the overall system complexity
• Expose unnecessary implementation details
• Make the system harder to understand and modify

To create deep modules, focus on designing simple, intuitive interfaces that abstract away the underlying complexity. Strive to maximize the ratio of functionality to interface complexity.

4. Good interfaces are the key to managing complexity

The interface to a module contains two kinds of information: formal and informal.

Well-designed interfaces simplify systems. Good interfaces provide a clear abstraction of a module's functionality without exposing unnecessary details. They should:

• Be simple and intuitive to use
• Hide implementation complexities
• Provide both formal (e.g., method signatures) and informal (e.g., high-level behavior descriptions) information

Interfaces should evolve thoughtfully. When modifying existing code:

• Consider the impact on the module's interface
• Avoid exposing implementation details
• Strive to maintain or improve the abstraction provided by the interface

By focusing on creating and maintaining good interfaces, developers can manage complexity and make their systems more modular and easier to understand.

5. Comments are crucial for creating abstractions

Comments provide the only way to fully capture abstractions, and good abstractions are fundamental to good system design.

Comments complete abstractions. While code can express implementation details, comments are essential for capturing:

• High-level design decisions
• Rationale behind choices
• Expectations and constraints
• Abstractions that aren't obvious from the code alone

Write comments first. By writing comments before implementing code:

• You clarify your thinking about the design
• You can evaluate and refine abstractions early
• You ensure documentation is always up-to-date

Focus on what and why, not how. Good comments should:

• Describe things that aren't obvious from the code
• Explain the purpose and high-level behavior of code
• Avoid merely repeating what the code does

By prioritizing clear, informative comments, developers can create better abstractions and improve the overall design of their systems.

6. Consistent naming and formatting enhance readability

Good names are a form of documentation: they make code easier to understand.

Consistency reduces cognitive load. By establishing and following conventions for naming and formatting, developers can:

• Make code more predictable and easier to read
• Reduce the mental effort required to understand code
• Highlight inconsistencies that may indicate bugs or design issues

Choose names carefully. Good names should:

• Be precise and unambiguous
• Create a clear image of the entity being named
• Be used consistently throughout the codebase

Formatting matters. Consistent formatting helps by:

• Making the structure of code more apparent
• Grouping related elements visually
• Emphasizing important information

By paying attention to naming and formatting, developers can significantly improve the readability and maintainability of their code.

7. Continuous refinement is essential for maintaining clean design

If you want a clean software structure, which will allow you to work efficiently over the long-term, then you must take some extra time up front to create that structure.

Design is an ongoing process. Clean software design requires:

• Regular refactoring to improve existing code
• Continual evaluation of design decisions
• Willingness to make changes as the system evolves

Invest in improvement. To maintain a clean design:

• Allocate time for cleanup and refactoring
• Address design issues promptly, before they compound
• View each code change as an opportunity to improve the overall design

Balance perfection and progress. While striving for clean design:

• Recognize that some compromises may be necessary
• Focus on making incremental improvements
• Prioritize changes that provide the most significant benefits

By treating design as a continuous process of refinement, developers can keep their systems clean and manageable as they grow and evolve.

8. Error handling should be simplified, not proliferated

The best way to eliminate exception handling complexity is to define your APIs so that there are no exceptions to handle: define errors out of existence.

Reduce exception cases. To simplify error handling:

• Design APIs to minimize exceptional conditions
• Use default behaviors to handle common edge cases
• Consider whether exceptions are truly necessary

Aggregate error handling. When exceptions are unavoidable:

• Handle multiple exceptions in a single place when possible
• Use exception hierarchies to simplify handling of related errors
• Avoid catching exceptions you can't meaningfully handle

Make normal cases easy. Focus on making the common, error-free path through your code as simple and obvious as possible. This approach:

• Reduces the cognitive load on developers
• Minimizes the chances of introducing bugs
• Makes the code easier to understand and maintain

By simplifying error handling, developers can create more robust and easier-to-understand systems.

9. General-purpose code is usually better than special-purpose solutions

Even if you use a class in a special-purpose way, it's less work to build it in a general-purpose way.

Generality promotes reusability. General-purpose code:

• Can be applied to a wider range of problems
• Is often simpler and more abstract
• Tends to have cleaner interfaces

Avoid premature specialization. When designing new functionality:

• Start with a somewhat general-purpose approach
• Resist the urge to optimize for specific use cases too early
• Allow the design to evolve based on actual usage patterns

Balance generality and simplicity. While striving for general-purpose solutions:

• Avoid over-engineering or adding unnecessary complexity
• Ensure the general-purpose design is still easy to use for common cases
• Be willing to create specialized solutions when truly necessary

By favoring general-purpose designs, developers can create more flexible and maintainable systems that are better equipped to handle future requirements.

10. Write code for readability, not for ease of writing

Software should be designed for ease of reading, not ease of writing.

Prioritize long-term maintainability. When writing code:

• Focus on making it easy to understand for future readers
• Avoid shortcuts or clever tricks that obscure the code's purpose
• Invest time in creating clear abstractions and documentation

Make code obvious. Strive to write code that:

• Can be understood quickly with minimal mental effort
• Uses clear and consistent naming conventions
• Has a logical and easy-to-follow structure

Refactor for clarity. Regularly review and improve existing code:

• Look for opportunities to simplify complex sections
• Break down long methods into smaller, more focused pieces
• Eliminate duplication and inconsistencies

By prioritizing readability over ease of writing, developers can create systems that are easier to maintain, debug, and extend over time. This approach may require more effort initially but pays off in reduced long-term complexity and improved team productivity.

Last updated: July 21, 2024