Modular Monolith: Architecture that combines the benefits of Monolith and Microservices

Sun, 27 Jul 2025 00:00:00 +0000

What is Modular Monolith and Why It’s an Interesting Alternative

Modular Monolith is a software architecture that combines the benefits of Monolith and Microservices by organizing code into modules with clear bounded contexts while maintaining single deployment unit.

Problems with Traditional Monolith

Traditional Monolith often suffers from:

Tightly Coupled Code: All parts are tightly bound, making changes difficult
Single Point of Failure: If one part fails, the entire system fails
Technology Lock-in: Must use the same technology stack throughout
Difficult to Scale: Cannot scale specific parts independently

Problems with Microservices

While Microservices solve many monolith problems, they introduce additional complexity:

Distributed System Complexity: Complexity of distributed systems
Network Latency: Delays from network communication
Data Consistency: Data consistency challenges
Operational Overhead: Requires strong DevOps teams
Development Complexity: Complexity in development and debugging

Modular Monolith: A Balanced Solution

Modular Monolith offers a balanced approach:

✅ Benefits:

Modularity: Clear module separation by business domain
Loose Coupling: Modules communicate through defined interfaces
Single Deployment: Easy to deploy and maintain
Easier Testing: Easier to test than distributed systems
Gradual Migration: Can be extracted to microservices in the future

⚠️ Considerations:

Technology Constraint: Still limited by primary technology
Shared Database: May have data coupling issues
Resource Scaling: Cannot scale individual modules separately

Architecture of Modular Monolith E-commerce Application

I created a Modular Monolith E-commerce Application using C# and ASP.NET Core to demonstrate the application of these principles in real system development.

🏗️ Architecture Overview

ECommerceApp/
├── src/
│ ├── ECommerceApp/ # Main Web API Application
│ │ ├── Controllers/ # API Controllers
│ │ ├── Program.cs # Application entry point
│ │ └── ECommerceApp.csproj
│ ├── ECommerceApp.Shared/ # Shared Kernel
│ │ ├── Events/ # Domain Events
│ │ ├── Kernel/ # Base entities, interfaces
│ │ └── ECommerceApp.Shared.csproj
│ └── Modules/
│ ├── Orders/ # Orders Module
│ │ ├── Application/ # Use cases, commands, queries
│ │ ├── Domain/ # Domain entities, events
│ │ ├── Infrastructure/ # Data access, repositories
│ │ ├── OrdersModule.cs # Module registration
│ │ └── ECommerceApp.Modules.Orders.csproj
│ ├── Products/ # Products Module
│ │ ├── Application/
│ │ ├── Domain/
│ │ ├── Infrastructure/
│ │ ├── ProductsModule.cs
│ │ └── ECommerceApp.Modules.Products.csproj
│ └── Customers/ # Customers Module
│ ├── Domain/
│ ├── Infrastructure/
│ ├── CustomersModule.cs
│ └── ECommerceApp.Modules.Customers.csproj

🎯 Key Principles of Modular Monolith

1. Modules Each module represents a distinct business capability such as Orders, Products, Customers

2. Encapsulation Modules hide internal details and expose only necessary functionality through interfaces

3. Shared Kernel Common functionality such as base entities, common utilities, domain events

4. Communication Modules communicate through interfaces or domain events, avoiding direct dependencies

5. Single Deployment All modules are deployed together as a single unit

Getting Started

Prerequisites

.NET 8.0 SDK
Visual Studio Code or Visual Studio

Quick Start

1. Clone Repository:

git clone https://github.com/suriyasonp/modular-monolith-ecommerce.git
cd modular-monolith-ecommerce

2. Run Application:

# Method 1: Using script (Recommended)
./run.sh

# Method 2: Manual run
dotnet build
dotnet run --project src/ECommerceApp/ECommerceApp.csproj

3. Access API:

Swagger UI: http://localhost:5000/swagger/index.html
API Base URL: http://localhost:5000/api
Sample Products: Data will be automatically seeded on startup

Testing the Application

Quick Test Script:

./test-api.sh

Key API Endpoints

Products API

GET /api/products - Get all products
GET /api/products/{id} - Get product by ID
POST /api/products - Create new product
PUT /api/products/{id}/price - Update product price
PUT /api/products/{id}/stock - Update product stock

Customers API

GET /api/customers - Get all customers
GET /api/customers/{id} - Get customer by ID
POST /api/customers - Create new customer

Orders API

GET /api/orders - Get all orders
GET /api/orders/{id} - Get order by ID
GET /api/orders/customer/{customerId} - Get orders by customer
POST /api/orders - Create new order
POST /api/orders/{id}/confirm - Confirm order
POST /api/orders/{id}/ship - Ship order
POST /api/orders/{id}/deliver - Deliver order

Benefits and Drawbacks of Modular Monolith

✅ Benefits

1. Deployment Simplicity

Deploy as single unit, reducing infrastructure complexity
No need to worry about service discovery or load balancing
Easy rollbacks

2. Performance

No network latency between modules
Communication is in-process calls
Cross-module transactions are straightforward

3. Development and Debugging

Easier to debug than distributed systems
Better IDE support
Simpler end-to-end testing

4. Operational Costs

Lower infrastructure requirements
No need for large DevOps teams
Simpler monitoring and logging

5. Future Flexibility

Can be extracted to microservices when necessary
Well-designed modules are easily extractable

⚠️ Drawbacks

1. Technology Constraints

Must use the same primary technology
Difficult to use different tech stacks

2. Scaling

Cannot scale individual modules separately
Resource utilization may not be optimal

3. Database Coupling

Shared database may create coupling issues
Schema migrations need careful consideration

4. Team Independence

Teams still need to coordinate deployments
Shared codebase may be problematic for large teams

When to Use Modular Monolith

🎯 Suitable for

1. Small to Medium Teams (2-20 people)

Limited resources for infrastructure complexity
Need high development velocity

2. New Applications

Uncertain about domain boundaries
Need rapid prototyping and iteration

3. Unclear Requirements

Business requirements change frequently
Need flexibility for adjustments

4. Limited DevOps Capability

Team not ready for distributed systems
Infrastructure automation not mature

🚫 Not Suitable for

1. Large Organizations with Many Teams

Teams need high independence
Different release cycles

2. Very Different Requirements

Different scalability requirements per module
Need different technologies

3. High Compliance and Security Constraints

Need isolation between components
Strict regulatory requirements

Migration Path Planning

Phase 1: Start with Modular Monolith

Traditional Monolith → Modular Monolith

Activities:

Identify domain boundaries
Create modules by business capabilities
Implement interfaces for inter-module communication
Add domain events infrastructure

Phase 2: Improve Module Independence

Tightly Coupled Modules → Loosely Coupled Modules

Activities:

Separate databases per module
Use event-driven communication
Implement distributed tracing
Add per-module monitoring

Phase 3: Selective Microservices Extraction

Modular Monolith → Hybrid Architecture

Activities:

Identify modules suitable for extraction
Extract modules with different scaling needs
Implement API gateways
Setup service mesh

Conclusion

Modular Monolith is a suitable architecture for many types of organizations, especially teams that want the benefits of modularity but are not ready for the complexity of microservices.

🔑 Key Takeaways

1. Start Simple

Use Modular Monolith as starting point
Focus on domain modeling and clean boundaries
Improve gradually

2. Design for Change

Use interfaces for inter-module communication
Implement domain events infrastructure
Prepare monitoring and observability

3. Plan Migration Path

Identify modules that may need extraction in the future
Design data access patterns that support separation
Build culture of modular thinking

4. Focus on Business Value

Don’t let technical complexity overshadow business goals
Use architecture appropriate for team and context
Remember: Architecture is means, not end

🚀 Next Steps

Study the Code: Visit the repository to understand implementation details
Try Running: Execute ./test-api.sh to see all functionality
Add Features: Try creating new modules following established patterns
Apply to Production: Adapt for production environment

Modular Monolith is not a silver bullet, but it’s an effective tool for building maintainable, scalable, and evolvable systems in the long term.

References

Modular Monolith E-commerce Repository - Implementation Example
Domain-Driven Design - Eric Evans
Modular Monoliths - Kamil Grzybek
Building Microservices - Sam Newman
Clean Architecture - Robert C. Martin
ASP.NET Core Documentation - Microsoft Docs
Monolith vs Microservices vs Modular Monoliths - Monolith vs Microservices vs Modular Monoliths

E-Commerce on TECHFOR by Suriya Sonphu

Modular Monolith: Architecture that combines the benefits of Monolith and Microservices

What is Modular Monolith and Why It’s an Interesting Alternative

Problems with Traditional Monolith

Problems with Microservices

Modular Monolith: A Balanced Solution

Architecture of Modular Monolith E-commerce Application

🏗️ Architecture Overview

🎯 Key Principles of Modular Monolith

Getting Started

Prerequisites

Quick Start

Testing the Application

Key API Endpoints

Products API

Customers API

Orders API

Benefits and Drawbacks of Modular Monolith

✅ Benefits

1. Deployment Simplicity

2. Performance

3. Development and Debugging

4. Operational Costs

5. Future Flexibility

⚠️ Drawbacks

1. Technology Constraints

2. Scaling

3. Database Coupling

4. Team Independence

When to Use Modular Monolith

🎯 Suitable for

1. Small to Medium Teams (2-20 people)

2. New Applications

3. Unclear Requirements

4. Limited DevOps Capability

🚫 Not Suitable for

1. Large Organizations with Many Teams

2. Very Different Requirements

3. High Compliance and Security Constraints

Migration Path Planning

Phase 1: Start with Modular Monolith

Phase 2: Improve Module Independence

Phase 3: Selective Microservices Extraction

Conclusion

🔑 Key Takeaways

🚀 Next Steps

References