Ousterhout's Software Design Principles

Quick checks for applying John Ousterhout's "A Philosophy of Software Design" principles. Managing complexity is the primary challenge in software engineering.

Core Formula

Module Value = Functionality - Interface Complexity

The best modules are deep: simple interfaces hiding powerful implementations.

1. Deep vs Shallow Module Check

Classic Deep Module Example

Unix I/O: Four simple operations (open, read, write, close) hide massive complexity (buffering, disk management, filesystem, drivers, caching).

Assessment Questions

Functionality Assessment:

• How much does this module actually do?
• What complexity does it hide from callers?
• Could I significantly change implementation without breaking callers?

Interface Assessment:

• How many public methods/functions?
• How many parameters do they require?
• How much must callers know to use it correctly?

The Feel Test:

• Interface complexity ≈ implementation complexity? → Shallow (eliminate or deepen)
• Simple interface, powerful implementation? → Deep (good!)
• Just wrapping another module? → Shallow (probably unnecessary abstraction)

Warning Signs

❌ Shallow Module Indicators:

• Wrapper class exposing most wrapped methods
• Pass-through functions adding no semantic value
• Abstraction hiding little complexity
• Thin layer that could be eliminated
• Interface almost as complex as implementation

✅ Deep Module Indicators:

• Simple interface (few methods, clear parameters)
• Powerful functionality (does a lot)
• Hides design decisions
• Changing internals doesn't break callers
• Worth the abstraction cost

2. Information Leakage Check

Core Test

"If I change implementation details, do callers break?"

If YES → You have information leakage.

Common Leakage Patterns

Database Schema Leakage:

❌ BAD: Return raw database rows/arrays
   - Callers depend on column order
   - Schema changes break calling code
   - Implementation (SQL) leaks through interface

✅ GOOD: Return domain objects
   - Hide schema behind types
   - Callers use named properties
   - Schema refactoring transparent to callers

Internal Data Structure Leakage:

❌ BAD: Expose internal buffer/cache structure
   - Callers know about your storage choices
   - Can't change data structures freely
   - Implementation details in public API

✅ GOOD: Provide abstract query interface
   - Hide how data is stored
   - Expose "what" not "how"
   - Free to optimize internals

Implementation Detail Leakage:

❌ BAD: Configuration forcing caller knowledge
   - Expose internal IDs, indices, ordering
   - Require callers to know implementation
   - Changes ripple to all callers

✅ GOOD: Domain-centric interface
   - Use domain language
   - Hide technical implementation
   - Stable despite internal changes

Encapsulation Check

Ask yourself:

• Can I refactor internals without breaking callers?
• Do callers know about my implementation choices?
• Does my interface reveal "how" or just "what"?
• Would changing my data structure break callers?

Principle: Each module should hide design decisions from others.

3. Red Flag Scanner

Critical Red Flags (Avoid)

Generic Names:

• Manager, Util, Helper → Vague, non-specific
• Service, Handler, Processor → Sometimes justified, often lazy
• Base, Abstract → Prefix anti-patterns

Ask: Does name reveal what it does? Or just that it does something?

Temporal Decomposition:

• step1(), step2(), step3()
• phase1(), phase2()
• initialize(), process(), finalize()

Problem: Organized by WHEN executed, not WHAT they do.

Better: Organize by functionality. Name by purpose, not sequence.

Warning Signs

Pass-Through Methods:

• Method just delegates to another class
• Adds no transformation or logic
• No semantic value
• Just indirection for indirection's sake

Configuration Overload:

• Too many parameters/options
• Every implementation detail exposed
• Interface complexity ≈ implementation
• Callers must understand internals

Context-Aware Exceptions

Some generic names have domain justification:

• EventManager in event-driven system → Acceptable
• But EventBus or EventDispatcher still better (more specific)

Principle: Prefer specific domain terms over generic technical terms.

Quick Scan Questions

When reviewing a module:

1. Does name reveal what it does (not just that it exists)?
2. Is it organized by execution order (temporal anti-pattern)?
3. Does it just wrap another module without adding value?
4. Could I give it a more specific, intention-revealing name?
5. Does it have too many configuration options?

Application Workflow

When designing:

1. Check module depth (value = functionality - interface complexity)
2. Ensure information hiding (callers isolated from implementation)
3. Avoid red flag names (be specific, avoid generic terms)

When reviewing:

1. Scan for Manager/Util/Helper (red flags)
2. Test: "If implementation changes, do callers break?" (leakage)
3. Assess: Simple interface + powerful implementation? (depth)

When refactoring:

1. Identify shallow modules → Consider eliminating or deepening
2. Find information leakage → Add encapsulation
3. Replace generic names → Use domain-specific terms

Key Principles Summary

1. Deep Modules: Simple interfaces hiding powerful implementations
2. Information Hiding: Each module hides design decisions from others
3. Avoid Complexity: Fight generic names, temporal decomposition, pass-through methods

Remember: Complexity is anything that makes software hard to understand or modify. Fight it with every decision.