Harry Lo

Posted on Jan 24

Clean Architecture vs Vertical Slice: Pragmatism Over Dogma in Modern Software Design

#architecture #softwareengineering #cleancode #newyearnewyou

Why This Article Is Part of My Portfolio Journey

This piece is part of my effort to build a public engineering portfolio focused on sustainable architecture, engineering judgment, and long-term system design.

Rather than showcasing isolated code samples, I’m documenting how I think about system design, trade-offs, and real-world engineering decisions — because architecture is as much about reasoning as it is about implementation.

This article represents the type of engineering thinking I aim to bring into the systems I build.

Pragmatism over Dogma: Engineering Judgment in Modern Software Architecture

This article isn’t about choosing Clean Architecture or Vertical Slice as the “correct” approach.

It’s about understanding how engineering evolves — from structure and theory, through judgment and balance, toward pragmatic delivery.

Clean Architecture and Vertical Slice represent two ends of that spectrum.

The real skill isn’t choosing sides — it’s knowing how to balance them responsibly in real systems.

Software Architecture Patterns: Clean Architecture vs Vertical Slice Architecture – A Comprehensive Analysis

Author: Harry Lo

Date: January 12, 2026

Document Purpose: Research and analysis comparing architectural approaches for modern software development, with a focus on extensibility, maintainability, and avoiding technical debt.

Copyright Notice: © 2026 Harry Lo. This work is licensed under Creative Commons Attribution 4.0 International License (CC BY 4.0). You are free to share and adapt this material with proper attribution.

Architectural Comparison Overview
- 1.1 Clean Architecture
- 1.2 Vertical Slice Architecture
- 1.3 Key Differences
- 1.4 Decision Matrix
Extensibility and Feature Addition
- 2.1 The "Just Add a Slice" Advantage
- 2.2 Comparison with Clean Architecture
- 2.3 Real-World Scenarios
- 2.4 Managing Slice Dependencies
Documentation and Knowledge Management
- 3.1 Co-located Documentation Benefits
- 3.2 Documentation Templates
- 3.3 Living Documentation Practice
- 3.4 Onboarding Advantages
Helper Functions and Technical Debt
Open-Closed Principle in Practice
- 5.1 OCP Applied to Helper Functions
- 5.2 Violations and Warning Signs
- 5.3 Extension Over Modification
- 5.4 Decision Framework
Hybrid Approach
Recommendations and Conclusions
- 7.1 Key Principles
- 7.2 Practical Guidelines
- 7.3 The Core Insight
- 7.4 Final Thoughts
References
- 8.1 Books and Publications
- 8.2 Online Articles and Resources
- 8.3 Microsoft Documentation
- 8.4 Design Principles
- 8.5 Video Resources
- 8.6 Community Discussions
About the Author

1. Architectural Comparison Overview

1.1 Clean Architecture

Definition: A layered architecture emphasizing separation of concerns with core business logic at the center, surrounded by concentric layers for application operations, interfaces, and external systems.

Strengths

Long-term Maintainability: Core business logic isolated from external dependencies (frameworks, databases, UI)
High Testability: Business rules can be tested independently without infrastructure
Technology Independence: Easy to swap databases, frameworks, or external services
Clear Separation of Concerns: Defined layers with strict dependency rules (dependencies point inward)
Complex Business Logic: Ideal for rich domain models and complex rules
Team Scalability: Clear boundaries help new developers understand structure

Weaknesses

High Initial Overhead: Requires significant upfront design and setup
Over-engineering Risk: Can be overkill for simple applications
Boilerplate Code: More interfaces, abstractions, and mapping between layers
Steep Learning Curve: Team needs to understand SOLID principles, dependency inversion
Potential Performance Overhead: Multiple layers can introduce latency

Best Suited For

Large, complex enterprise applications
Long-term projects (3+ years lifespan)
Systems with complex business rules
Projects requiring multiple integrations
Teams with experienced architects

1.2 Vertical Slice Architecture

Definition: Organizes code around specific features or use cases, treating each as a self-contained "slice" spanning all necessary layers from UI to database.

Strengths

Rapid Development: Features can be built end-to-end quickly
Feature-based Organization: All code for a feature lives together (UI → DB)
Team Parallelization: Multiple teams can work on different slices with minimal conflicts
Reduced Cognitive Load: Developers only need to understand one slice at a time
Agile-friendly: Aligns perfectly with user stories and sprint planning
Early Risk Detection: Complete features reveal integration issues sooner
Lower Initial Complexity: Easier to start compared to Clean Architecture

Weaknesses

Code Duplication: Validation, error handling, and other logic may be repeated across slices
Cross-cutting Concerns: Handling logging, security, caching consistently requires discipline
Potential Inconsistency: Different slices might implement similar features differently
Learning Curve: Shift in mindset from horizontal layers to vertical features
Scaling Complexity: Managing dependencies between slices becomes challenging as they increase

Best Suited For

Agile/Scrum teams with frequent releases
Microservices or modular monoliths
Projects with independent features
Startups and MVP development
Teams that value speed over long-term abstractions

1.3 Key Differences

Aspect	Clean Architecture	Vertical Slice Architecture
Organization	By technical responsibilities (layers)	By business features (slices)
Dependencies	Strict inward dependency rules	Feature-specific dependencies
Development Approach	Centralized business logic	Distributed across features
Project Size	Large, complex	Small to medium
Timeline	Long-term (years)	Short to medium
Team Experience	Senior developers	Mixed experience
Business Complexity	High domain complexity	Feature-focused
Change Frequency	Stable requirements	Frequent pivots
Development Speed	Slower initial, faster later	Fast throughout
Maintainability	Layer reuse, separation	Feature independence

1.4 Decision Matrix

Choose Clean Architecture when:

Building large-scale applications where long-term maintainability is paramount
Complex business rules require strict separation
Technology stack may change over time
Multiple teams need clear architectural boundaries
Enterprise requirements demand high flexibility

Choose Vertical Slice when:

Rapid development and deployment are priorities
Features are relatively independent
Agile methodologies are core to workflow
Team prefers feature-complete iterations
Startup or MVP environment

Consider Hybrid when:

Want feature-based organization with engineering rigor
Need some shared domain logic
Cross-cutting concerns require centralization
Long-term project but with agile delivery

2. Extensibility and Feature Addition

2.1 The "Just Add a Slice" Advantage

One of Vertical Slice Architecture's strongest benefits is the ability to add new features by simply creating a new slice without touching existing code.

Example Structure

Features/
  ├── UserRegistration/        ← Existing
  ├── OrderProcessing/          ← Existing
  ├── PaymentHandling/          ← Existing
  └── ProductReviews/           ← NEW! Just add this slice
      ├── AddReview.cs
      ├── GetReviews.cs
      ├── DeleteReview.cs
      └── Models/

Why This Is Powerful

1. Zero Impact on Existing Code

You don't touch other slices
No risk of breaking existing features
Each slice is independent

2. Parallel Development

Team A works on "Product Reviews"
Team B works on "Loyalty Program"
Zero merge conflicts

3. Faster Onboarding

New developers can study one complete example slice
Then copy the pattern for new features
Complete flow visible in one location

4. Progressive Complexity

Start simple: new slice = new feature
Grow organically: add more slices as needed
No upfront architectural decisions required

5. Easy Feature Flags & A/B Testing

// Enable/disable entire features easily
if (featureFlags.IsEnabled("ProductReviews"))
{
    services.AddProductReviewsSlice();
}

2.2 Comparison with Clean Architecture

In Clean Architecture, adding a new feature requires touching multiple layers:

Domain/
  └── Entities/
      └── Review.cs              ← Add here

Application/
  └── UseCases/
      ├── AddReview/
      │   ├── AddReviewUseCase.cs  ← Add here
      │   └── IReviewRepository.cs ← Add here
      └── GetReviews/
          └── GetReviewsUseCase.cs   ← Add here

Infrastructure/
  └── Repositories/
      └── ReviewRepository.cs      ← Add here

Presentation/
  └── Controllers/
      └── ReviewsController.cs     ← Add here

Challenges:

Navigate between 4-5 different folders
Each layer needs awareness of the new feature
More places for things to go wrong
Harder to see the complete feature at once

2.3 Real-World Scenarios

Scenario: E-commerce Platform Evolution

Adding features over 6 months:

With Vertical Slice:

Features/
  ├── ProductReviews/      ← Sprint 1: Just add this
  ├── Wishlist/            ← Sprint 2: Just add this
  ├── GiftCards/           ← Sprint 3: Just add this
  ├── LoyaltyPoints/       ← Sprint 4: Just add this
  ├── ProductComparison/   ← Sprint 5: Just add this
  └── RecentlyViewed/      ← Sprint 6: Just add this

Each sprint = one new folder. Clean. Isolated. Simple.

With Clean Architecture:
Every sprint requires updating:

Domain layer (entities, value objects)
Application layer (use cases, interfaces)
Infrastructure layer (repositories, services)
Presentation layer (controllers, DTOs)

Netflix Example

The Netflix API famously uses a similar approach:

Each team owns complete feature slices
Teams deploy independently
Minimal coordination needed
Can add hundreds of features without architectural paralysis

2.4 Managing Slice Dependencies

Concern: Code Duplication

Problem: Each slice might duplicate validation logic, error handling, etc.

Solutions:

// Shared behaviors via MediatR pipelines
public class ValidationBehavior<TRequest, TResponse> 
    : IPipelineBehavior<TRequest, TResponse>
{
    // All slices automatically get validation!
}

// Shared abstractions
Common/
  ├── Behaviors/
  │   ├── ValidationBehavior.cs
  │   ├── LoggingBehavior.cs
  │   └── TransactionBehavior.cs
  └── Abstractions/
      └── IRepository<T>.cs

Concern: Slices Need to Communicate

Problem: Product Reviews needs Product data

Solutions:

// Option 1: Direct dependency (acceptable for queries)
Features/ProductReviews/
  └── GetReviewsQuery.cs
      // Can query Products directly

// Option 2: Events (for decoupling)
Features/OrderProcessing/
  └── OrderCompleted.cs → raises event

Features/LoyaltyPoints/
  └── OrderCompletedHandler.cs → listens for event

Concern: Shared Domain Logic

Problem: Multiple slices need same business rules

Solution:

Core/
  └── Domain/
      ├── Product.cs
      ├── Order.cs
      └── Rules/          ← Shared business rules
          └── PricingRules.cs

Features/
  ├── OrderProcessing/   ← Uses PricingRules
  └── QuoteGeneration/   ← Uses PricingRules

3. Documentation and Knowledge Management

3.1 Co-located Documentation Benefits

Documentation becomes significantly easier with Vertical Slice Architecture because docs can live right next to the code they describe.

Co-located Structure

Features/
  ├── ProductReviews/
  │   ├── README.md                    ← Feature-specific docs!
  │   ├── AddReview.cs
  │   ├── GetReviews.cs
  │   └── DeleteReview.cs
  ├── OrderProcessing/
  │   ├── README.md                    ← All order docs here!
  │   ├── CreateOrder.cs
  │   └── CancelOrder.cs
  └── PaymentHandling/
      ├── README.md                    ← Payment docs here!
      └── ProcessPayment.cs

Benefits:

Docs live right next to the code they describe
When you work on a feature, docs are immediately visible
No hunting through multiple folders or wikis

Self-Documenting Structure

Each slice tells a complete story in one place. Everything you need to know about "Product Reviews" is in ONE location.

3.2 Documentation Templates

Create a standard template for all slices to ensure consistency:

_FEATURE_TEMPLATE/README.md

# [Feature Name]

## Overview
[Brief description]

## User Stories
- As a [user], I want to [action], so that [benefit]

## Use Cases / Commands / Queries
- [ ] Command/Query name - Description

## API Endpoints
- [ ] METHOD /path - Description

## Business Rules
- [ ] Rule description

## Dependencies
- **Requires:** [Other features needed]
- **Publishes:** [Events this feature raises]
- **Consumes:** [Events this feature listens to]

## Database Changes
- [ ] Tables/Collections affected

## Configuration
- [ ] Settings required

## Testing
- [ ] Test scenarios

## Deployment Notes
- [ ] Special considerations

Every new slice copies this template = consistent documentation across all features!

3.3 Living Documentation Practice

Documentation updates become part of feature work:

Pull Request Structure:

Features/GiftCards/
  ├── README.md          ← Updated as part of PR
  ├── PurchaseGiftCard.cs
  ├── RedeemGiftCard.cs
  └── CheckBalance.cs

Code Review Checklist:

✅ Code implements feature
✅ Tests pass
✅ README.md is updated

Because docs are co-located, developers naturally update them!

3.4 Onboarding Advantages

Comparison: Understanding "Product Reviews"

Vertical Slice Approach:

"Open Features/ProductReviews/ folder and read the README"

Done in 5 minutes! ✅

Clean Architecture Approach:

"Read these docs:
1. Domain/Entities.md (Review entity)
2. Application/UseCases.md (Review use cases)
3. Infrastructure/Repositories.md (Review storage)
4. API/Endpoints.md (Review API)
5. Domain/Events.md for ReviewSubmitted event
6. Application/EventHandlers.md for handlers"

Takes 30+ minutes 😓

Documentation Comparison Table

Aspect	Clean Architecture	Vertical Slice
Doc location	Scattered across layers	Co-located with feature
Finding docs	Search multiple folders	One folder = one feature
Completeness	Often incomplete	Easier to keep complete
Onboarding time	30-60 minutes per feature	10-20 minutes per feature
Update likelihood	Often forgotten	Updated with PRs
Template reuse	Hard to standardize	Easy template per slice
Feature overview	Requires reading multiple docs	Single README

4. Helper Functions and Technical Debt

4.1 The Helper Function Decay Pattern

A common anti-pattern occurs when developers try to make a helper function serve multiple purposes by repeatedly modifying it.

Evolution of Decay

// Stage 1: Initial extraction (seems good!)
public static string FormatUserName(User user)
{
    return $"{user.FirstName} {user.LastName}";
}

// Stage 2: One place needs something slightly different
public static string FormatUserName(User user, bool includeTitle = false)
{
    if (includeTitle)
        return $"{user.Title} {user.FirstName} {user.LastName}";
    return $"{user.FirstName} {user.LastName}";
}

// Stage 3: Another place needs another variation
public static string FormatUserName(User user, bool includeTitle = false, bool lastNameFirst = false)
{
    if (lastNameFirst && includeTitle)
        return $"{user.Title} {user.LastName}, {user.FirstName}";
    if (lastNameFirst)
        return $"{user.LastName}, {user.FirstName}";
    if (includeTitle)
        return $"{user.Title} {user.FirstName} {user.LastName}";
    return $"{user.FirstName} {user.LastName}";
}

// Stage 4: TECHNICAL DEATH 💀
public static string FormatUserName(
    User user, 
    bool includeTitle = false, 
    bool lastNameFirst = false,
    bool includeMiddleName = false,
    bool abbreviateMiddle = false,
    bool uppercase = false,
    NameFormat format = NameFormat.Standard)
{
    // 50+ lines of conditional logic
    // Nobody dares to change it anymore
    // Everyone is afraid it will break something
    // New developers avoid touching it
}

4.2 Why This Leads to Technical Death

1. High Cognitive Load

Developers must understand ALL variations to make ANY change
The function serves too many masters
Testing becomes exponentially complex

2. Shotgun Surgery

A bug in one use case might require touching the shared helper
That change could break OTHER use cases
Ripple effects across the codebase

3. Fear-Driven Development

"Don't touch it, it works"
Developers add MORE parameters instead of refactoring
The problem compounds over time

4. Loss of Intent

The original purpose is obscured by all the variations
Code becomes about "what" not "why"
Business logic is hidden in boolean flags

The Root Cause: Misapplied DRY Principle

The Mistake:

"We have similar code in 3 places, let's extract a helper function!"

The Reality:

Code that LOOKS similar might serve DIFFERENT purposes
Different contexts have different reasons to change
"Duplication is far cheaper than the wrong abstraction" — Sandi Metz

4.3 When Duplication Is Better Than Abstraction

The "Rule of Three"

First occurrence: Write the code inline
Second occurrence: Still write it inline (just 2 places)
Third occurrence: NOW consider extracting IF:
- ✅ The logic is truly identical
- ✅ It changes for the SAME reasons
- ✅ It serves the SAME purpose
- ✅ It has the SAME business context

Example: When NOT to Extract

BAD Extraction (similar code, different purposes):

// DON'T extract this - different purposes!
// Invoice: needs title for legal reasons
FormatUserName(user, includeTitle: true);

// Welcome email: needs friendly name for UX
FormatUserName(user, includeTitle: false);

// These LOOK similar but will evolve differently!

GOOD Extraction (truly identical, same purpose):

// DO extract this - same purpose
public static string ToUpperSnakeCase(string input)
{
    return string.Join("_", input.Split(' '))
                 .ToUpper();
}

// Used consistently for environment variable names
var dbVar = ToUpperSnakeCase("database url");
var apiVar = ToUpperSnakeCase("api key");

How Vertical Slice Prevents This

Features/
├── CustomerInvoice/
│   └── FormatCustomerName.cs
│       // Formats: "Title FirstName LastName" for invoices
│       // This is for LEGAL/BILLING purposes
│
├── EmailNotifications/
│   └── FormatFriendlyName.cs
│       // Formats: "FirstName" for friendly emails
│       // This is for USER EXPERIENCE
│
└── AdminUserList/
    └── FormatDisplayName.cs
        // Formats: "LastName, FirstName" for sorting
        // This is for ADMINISTRATIVE purposes

Benefits:

✅ Each slice has its OWN formatting logic
✅ Changes in one don't affect others
✅ Intent is clear from context
✅ If they diverge over time, no problem!
✅ No boolean flags or complex conditionals

4.4 Better Alternatives

Option 1: Multiple Specific Functions

// Clear purpose, no flags
public static string FormatForInvoice(User user) 
    => $"{user.Title} {user.FirstName} {user.LastName}";

public static string FormatForEmail(User user) 
    => user.FirstName;

public static string FormatForAdminList(User user) 
    => $"{user.LastName}, {user.FirstName}";

Option 2: Strategy Pattern

public interface INameFormatter
{
    string Format(User user);
}

public class InvoiceNameFormatter : INameFormatter
{
    public string Format(User user) 
        => $"{user.Title} {user.FirstName} {user.LastName}";
}

public class EmailNameFormatter : INameFormatter
{
    public string Format(User user) => user.FirstName;
}

Option 3: Extension Methods (Context-Specific)

// In Features/Invoice/
public static class UserExtensions
{
    public static string ToInvoiceName(this User user)
        => $"{user.Title} {user.FirstName} {user.LastName}";
}

// In Features/Email/
public static class UserExtensions
{
    public static string ToFriendlyName(this User user)
        => user.FirstName;
}

Option 4: Keep It Inline (Often the Best!)

// In Features/Invoice/GenerateInvoice.cs
var customerName = $"{user.Title} {user.FirstName} {user.LastName}";

// In Features/Email/SendWelcome.cs
var greeting = $"Hi {user.FirstName}!";

// Just 1 line each - no need to extract!

Red Flags: When Your Helper Function Is Dying

🚩 More than 3 boolean parameters

FormatUserName(user, true, false, true, false) // What does this mean??

🚩 Parameter names with "Or" or "Also"

FormatUserName(user, includeTitleOrPrefix: true)

🚩 Comments explaining parameter combinations

// Use includeTitle=true and lastNameFirst=false for invoices
// Use includeTitle=false and lastNameFirst=true for admin

🚩 Nested conditionals based on parameters

🚩 The function name is generic

ProcessData()
HandleStuff()
DoTheThing()

5. Open-Closed Principle in Practice

5.1 OCP Applied to Helper Functions

Open-Closed Principle:

"Software entities should be open for extension, but closed for modification" — Bertrand Meyer

In plain English:

You should be able to ADD new behavior (open for extension)
WITHOUT changing existing code (closed for modification)

The Bad Pattern (Violates OCP)

// Initial version
public static string FormatUserName(User user)
{
    return $"{user.FirstName} {user.LastName}";
}

// ❌ Modification #1 - VIOLATES OCP
public static string FormatUserName(User user, bool includeTitle = false)
{
    if (includeTitle)
        return $"{user.Title} {user.FirstName} {user.LastName}";
    return $"{user.FirstName} {user.LastName}";
}

// ❌ Modification #2 - VIOLATES OCP AGAIN
public static string FormatUserName(User user, bool includeTitle = false, bool lastNameFirst = false)
{
    // More conditional logic...
}

Why this violates OCP:

Every new requirement MODIFIES the existing function
Existing callers are at risk (regression bugs)
The function is NOT closed for modification
You're changing working code repeatedly

The Good Pattern (Follows OCP)

Multiple Specific Functions:

// Original function - NEVER modified again ✅
public static string FormatUserName(User user)
{
    return $"{user.FirstName} {user.LastName}";
}

// New requirement? ADD new function, don't modify existing ✅
public static string FormatUserNameWithTitle(User user)
{
    return $"{user.Title} {user.FirstName} {user.LastName}";
}

// Another requirement? ADD another function ✅
public static string FormatUserNameLastFirst(User user)
{
    return $"{user.LastName}, {user.FirstName}";
}

// Extension through ADDITION, not MODIFICATION ✅

Strategy Pattern (Classic OCP):

// Interface defines the contract - rarely changes ✅
public interface IUserNameFormatter
{
    string Format(User user);
}

// Original formatter - NEVER modified ✅
public class StandardNameFormatter : IUserNameFormatter
{
    public string Format(User user) 
        => $"{user.FirstName} {user.LastName}";
}

// New requirement? ADD new class (extension) ✅
public class TitledNameFormatter : IUserNameFormatter
{
    public string Format(User user) 
        => $"{user.Title} {user.FirstName} {user.LastName}";
}

5.2 Violations and Warning Signs

OCP as a Decision Tool

"Am I MODIFYING existing code, or ADDING new code?"

Scenario	Violates OCP?	Technical Debt Risk
Adding boolean parameter to helper	✅ YES	HIGH
Adding conditional logic to helper	✅ YES	HIGH
Creating new specific function	❌ NO	LOW
Creating new strategy class	❌ NO	LOW
Inline code in feature slice	❌ NO	NONE

Red Flags for OCP Violations

🚩 Growing parameter list

FormatUserName(user, flag1, flag2, flag3, flag4...)
// Each new param = OCP violation

🚩 Increasing cyclomatic complexity

if (flag1)
    if (flag2)
        if (flag3)
            // OCP violated repeatedly

🚩 Fear of changing the function

// "What if I break something?"
// This fear means OCP was already violated

🚩 Function serves multiple "masters"

// Used by: Invoice, Email, Admin, Report, Dashboard
// Each master has different needs
// Function can't be "closed" because needs conflict

5.3 Extension Over Modification

Real-World Example: Email Formatting Evolution

The Wrong Way (Violates OCP):

// Week 1: Initial implementation
public static string FormatEmail(string template, User user)
{
    return template.Replace("{NAME}", user.FirstName);
}

// Week 3: "We need HTML emails too!"
// ❌ MODIFYING existing function
public static string FormatEmail(string template, User user, bool isHtml = false)
{
    var name = user.FirstName;
    if (isHtml)
        name = $"<strong>{name}</strong>";
    return template.Replace("{NAME}", name);
}

// Week 8: Now it's unmaintainable
public static string FormatEmail(
    string template, 
    User user, 
    bool isHtml = false, 
    bool includeLastName = false,
    string locale = "en-US")
{
    // 50 lines of conditional logic
    // OCP completely violated
}

The Right Way (Follows OCP):

// Base abstraction - CLOSED for modification ✅
public interface IEmailFormatter
{
    string Format(string template, User user);
}

// Week 1: Text email formatter
public class TextEmailFormatter : IEmailFormatter
{
    public string Format(string template, User user)
        => template.Replace("{NAME}", user.FirstName);
}

// Week 3: HTML email formatter - ADDED, not modified ✅
public class HtmlEmailFormatter : IEmailFormatter
{
    public string Format(string template, User user)
        => template.Replace("{NAME}", $"<strong>{user.FirstName}</strong>");
}

// Week 8: Localized formatter - ADDED, not modified ✅
public class LocalizedEmailFormatter : IEmailFormatter
{
    private readonly string _locale;

    public string Format(string template, User user)
    {
        var name = _locale switch
        {
            "ja-JP" => $"{user.LastName} {user.FirstName}",
            "en-US" => $"{user.FirstName} {user.LastName}",
            _ => user.FirstName
        };
        return template.Replace("{NAME}", name);
    }
}

What changed over 8 weeks?

❌ Text email formatter? NEVER modified
❌ HTML email formatter? NEVER modified
❌ Original interface? NEVER modified
✅ System capabilities? Expanded through extension

5.4 Decision Framework

When Considering Modifying a Helper Function

Is the function used in multiple places?
├─ NO → ✅ Safe to modify (low coupling)
└─ YES → Is this a bug fix or refactor (same behavior)?
    ├─ YES → ✅ Probably safe to modify
    └─ NO → Are you adding conditional logic/parameters?
        ├─ YES → ❌ STOP! Violates OCP
        │         → Consider: new function or strategy pattern
        └─ NO → Are all callers okay with the change?
            ├─ YES → ⚠️ Modify with caution
            └─ NO → ❌ STOP! Violates OCP
                    → Definitely use extension approach

When Modification Is Actually OK

OCP isn't absolute. Modification is acceptable when:

1. Fixing a Bug

// ✅ OK to modify - it was WRONG before
public static string FormatUserName(User user)
{
    // Was: return $"{user.FirstName} {user.LastName}";
    return $"{user.FirstName?.Trim()} {user.LastName?.Trim()}";
}

2. Refactoring Internal Implementation

// ✅ OK to modify - external behavior unchanged
public static string FormatUserName(User user)
{
    // Optimized implementation, same result
    return string.Join(" ", user.FirstName, user.LastName);
}

3. The Function Has Only ONE Caller

// ✅ OK to modify - no other dependencies
// Features/Invoice/InvoiceHelpers.cs
private static string FormatForInvoice(User user)
{
    // Only used by Invoice feature, safe to evolve
}

4. Breaking Change Is Intentional & Communicated

// ✅ OK to modify - with proper versioning
[Obsolete("Use FormatUserNameV2 instead")]
public static string FormatUserName(User user) { }

public static string FormatUserNameV2(User user) { }

6. Hybrid Approach

Many modern teams combine the best of both architectures:

Structure

Features/
  ├── UserRegistration/
  │   ├── Commands/
  │   ├── Validators/
  │   ├── Handlers/
  │   └── Models/
  ├── OrderProcessing/
  └── PaymentHandling/
Common/
  ├── Behaviors/ (cross-cutting concerns)
  └── Infrastructure/

Benefits

Use Vertical Slices for feature organization (folder structure)
Apply Clean Architecture principles within each slice (separation of concerns, dependency inversion)
Centralize cross-cutting concerns using middleware/pipelines (MediatR pattern in .NET)
Share common domain logic in a core layer

OCP + Vertical Slice = Natural Fit

Vertical Slice Architecture naturally encourages OCP:

Features/
├── InvoiceGeneration/             ← Closed for modification
├── WelcomeEmail/                  ← Closed for modification
├── AdminDashboard/                ← Closed for modification
└── CustomerReport/                ← NEW! Extension via addition

This is OCP at the feature level!

7. Recommendations and Conclusions

Key Principles

"Duplication is far cheaper than the wrong abstraction" — Sandi Metz
- Don't rush to extract helper functions
- Different contexts may have different reasons to change
- Duplication across feature boundaries is acceptable and healthy
"Optimize for change, not for avoiding duplication"
- Make it easy to understand and modify
- Loose coupling is more valuable than DRY
- Consider future evolution, not just current state
Open-Closed Principle is your guide
- If you can't extend without modifying, your abstraction is wrong
- Repeated modification of a helper function is a red flag
- Extension through addition prevents technical debt
Vertical Slice naturally prevents common anti-patterns
- Feature isolation prevents over-sharing
- Co-location improves understanding
- Independent evolution reduces coupling

Practical Guidelines

For Helper Functions

Before extracting, ask:

❓ Does this code change for the SAME reason?
❓ Is it truly identical, or just similar?
❓ Will future requirements diverge these use cases?
❓ Is the extraction making the code EASIER to understand?

If you answered "No" or "Maybe" to any → DON'T extract yet.

When duplication is better:

Code serves different business purposes
Different contexts have different evolution paths
Extracting would introduce coupling
The code is simple enough to keep inline

For Architecture Choice

Choose Clean Architecture when:

Building enterprise systems with complex business rules
Long-term maintainability (3+ years) is critical
Technology stack may change
Team has senior architects

Choose Vertical Slice when:

Rapid feature delivery is priority
Features are relatively independent
Agile/iterative development
Team values pragmatism over purity

Consider Hybrid when:

Want feature organization with engineering discipline
Need some shared domain logic
Long-term project with agile delivery

For Documentation

Vertical Slice advantages:

Co-locate docs with code
Use templates for consistency
Update docs as part of feature PRs
One folder = complete feature understanding

The Core Insight

Helper Function Decay → OCP Violation → Technical Death

The pattern is clear:

Helper function created (seems reusable)
New requirement → function MODIFIED (violates OCP)
Another requirement → MODIFIED again (repeated OCP violation)
Function becomes complex, fragile, scary (technical death)

OCP violation is the root cause of technical death in helper functions.

Final Thoughts

Architecture is about trade-offs, not absolutes
Choose patterns that fit your team and project
Be willing to duplicate code when it prevents coupling
Extend through addition, not modification
Keep features independent and well-documented
Trust the Open-Closed Principle as your guide

Mature engineering isn’t defined by the frameworks we use, but by the decisions we make. Clean Architecture and Vertical Slice are just tools. What matters is the judgment to apply them wisely and the discipline to avoid turning principles into dogma.

"Make it work, make it right, make it fast" — Kent Beck

Start simple, refactor when patterns emerge, and always prefer the simplest solution that could work.

References

Books and Publications

Martin, Robert C. (2017). Clean Architecture: A Craftsman's Guide
to Software Structure and Design. Prentice Hall.
Meyer, Bertrand (1988). Object-Oriented Software Construction.
Prentice Hall.
Metz, Sandi (2016). The Wrong Abstraction. Sandi Metz Blog.\
Available at:
https://sandimetz.com/blog/2016/1/20/the-wrong-abstraction
Beck, Kent (2004). Extreme Programming Explained: Embrace Change
(2nd ed.). Addison-Wesley Professional.
Hunt, Andrew & Thomas, David (1999). The Pragmatic Programmer: From
Journeyman to Master. Addison-Wesley.

Architecture & Design Articles

Bogard, Jimmy (2018). Vertical Slice Architecture.\
Available at:
https://www.jimmybogard.com/vertical-slice-architecture/
Jovanović, Milan (2023). Vertical Slice Architecture in Modern
Applications.\
Available at:
https://www.milanjovanovic.tech/blog/vertical-slice-architecture
Ozkaya, Mehmet (2023). Vertical Slice Architecture and Comparison
with Clean Architecture. Medium.\
Available at:
https://mehmetozkaya.medium.com/vertical-slice-architecture-and-comparison-with-clean-architecture-76f813e3dab6
Ngom, Issa (2024). Clean Architecture vs Vertical Slice. LinkedIn
Pulse.\
Available at:
https://fr.linkedin.com/pulse/clean-architecture-vs-vertical-slice-issa-ngom-76bhe
Asmak9 (2025). Vertical Slice Architecture in ASP.NET Core.\
Available at:
https://www.asmak9.com/2025/07/vertical-slice-architecture-in-aspnet.html
Onishi, Yuki (2024). Benefits and Drawbacks of Adopting Clean
Architecture. Dev.to.\
Available at:
https://dev.to/yukionishi1129/benefits-and-drawbacks-of-adopting-clean-architecture-2pd1
Fowler, Martin (2018). Modular Monoliths.\
Available at: https://martinfowler.com/bliki/ModularMonolith.html

Official Documentation & Principles

Microsoft Learn (2025). Extension Methods (C# Programming Guide).\
Available at:
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/extension-methods
Martin, Robert C. (n.d.). The Open-Closed Principle. The
Principles of OOD.\
Available at:
https://butunclebob.com/ArticleS.UncleBob.PrinciplesOfOod

Community Resources

Monday.com (2024). Vertical Slice Architecture.\ Available at: https://monday.com/blog/rnd/

About the Author

Harry Lo is a passionate software developer focused on improving software development environments and engineering practices. Through experience, he has observed that even good principles can lead to negative outcomes when misapplied or overused. His work is driven by a desire to find the right balance between structure and pragmatism in building sustainable, developer-friendly systems.

For questions, feedback, or discussions about this article, please feel free to reach out.

End of Document

Why This Article Is Part of My Portfolio Journey

Software Architecture Patterns: Clean Architecture vs Vertical Slice Architecture – A Comprehensive Analysis

Table of Contents

1. Architectural Comparison Overview

1.1 Clean Architecture

Strengths

Weaknesses

Best Suited For

1.2 Vertical Slice Architecture

Strengths

Weaknesses

Best Suited For

1.3 Key Differences

1.4 Decision Matrix

2. Extensibility and Feature Addition

2.1 The "Just Add a Slice" Advantage

Example Structure

Why This Is Powerful

2.2 Comparison with Clean Architecture

2.3 Real-World Scenarios

Scenario: E-commerce Platform Evolution

Netflix Example

2.4 Managing Slice Dependencies

Concern: Code Duplication

Concern: Slices Need to Communicate

Concern: Shared Domain Logic

3. Documentation and Knowledge Management

3.1 Co-located Documentation Benefits

Co-located Structure

Self-Documenting Structure

3.2 Documentation Templates

3.3 Living Documentation Practice

3.4 Onboarding Advantages

Documentation Comparison Table

4. Helper Functions and Technical Debt

4.1 The Helper Function Decay Pattern

Evolution of Decay

4.2 Why This Leads to Technical Death

1. High Cognitive Load

2. Shotgun Surgery

3. Fear-Driven Development

4. Loss of Intent

The Root Cause: Misapplied DRY Principle

4.3 When Duplication Is Better Than Abstraction

The "Rule of Three"

Example: When NOT to Extract

How Vertical Slice Prevents This

4.4 Better Alternatives

Option 1: Multiple Specific Functions

Option 2: Strategy Pattern

Option 3: Extension Methods (Context-Specific)

Option 4: Keep It Inline (Often the Best!)

Red Flags: When Your Helper Function Is Dying

5. Open-Closed Principle in Practice

5.1 OCP Applied to Helper Functions

The Bad Pattern (Violates OCP)

The Good Pattern (Follows OCP)

5.2 Violations and Warning Signs

OCP as a Decision Tool

Red Flags for OCP Violations

5.3 Extension Over Modification

Real-World Example: Email Formatting Evolution

5.4 Decision Framework

When Considering Modifying a Helper Function

When Modification Is Actually OK

6. Hybrid Approach

Structure

Benefits

OCP + Vertical Slice = Natural Fit

7. Recommendations and Conclusions

Key Principles

Practical Guidelines

For Helper Functions

For Architecture Choice

For Documentation

The Core Insight

Final Thoughts

References

Books and Publications

Architecture & Design Articles