What Are The Differences Between Modular Monolith Vs Microservices in .NET: When To Choose What?

Choosing between a modular monolith and microservices isn’t just a technical decision, it’s a strategic one. Each architecture serves different stages of a product’s lifecycle, different team sizes, and different scalability needs. If you are building a new .NET app from scratch, or planning to scale it, you should know the trade-offs and strengths of both the methods. This can help you avoid massive rewrite expenses down the road:

What is a Modular Monolith

A modular monolith is an architectural choice where your application is built as a single deployment unit, but it’s structured internally as independent, loosely-coupled modules with well-defined boundaries. 

Think of it as a monolith with a clean architecture, it has modules separated logically, but they run in the same process.

What are Microservices

Microservices architecture involves breaking the application into independent, small services. Each service:

• Owns its data
• Has its own deployment pipeline
• Communicates with others via APIs (usually HTTP or messaging)

Think of each microservice as a mini-application with complete independence.

Ecommerce System Example with Code Snippets

Modular Monolith Example

You build an app with the following internal modules:

• UserModule
• ProductModule
• OrderModule
• PaymentModule

They share the same:

• Database
• Codebase
• Deployment pipeline

Each module communicates internally via services/interfaces, but everything is packaged as a single deployable unit (e.g., a .NET Web API).

// ProductModule service
public class ProductService
{ public Product GetById(int id) { ... }
}

Microservices Example

You break the app into:

• UserService (runs on port 5001)
• ProductService (runs on port 5002)
• OrderService (runs on port 5003)
• PaymentService (runs on port 5004)

Each service:

• Has its own database
• Is deployed independently
• Communicates over HTTP (e.g., REST)

Example API Call:

GET http://product-service/api/products/1

Advantages of Monolith Architecture

• Easier to Develop: simpler architecture for small/medium teams
• Easy Debugging: All logic runs in one process, easy to trace bugs
• Easier Testing: End-to-end testing is simpler
• Faster Development: Less infrastructure overhead
• Cost Effective: No need for cloud-native orchestration (e.g., Kubernetes)

Advantages of Microservices Architecture

• Independent Deployment: Services can be deployed, versioned separately
• Scalable: Each service can scale horizontally as needed
• Team Autonomy: Teams can own and deliver microservices independently
• Technology Flexibility: Each service can use a different stack
• Fault Isolation: Failure in one service doesn’t crash the entire app

Disadvantages of Monolith Architecture

• Single Point of Failure: If one module crashes, entire app may go down
• Harder to Scale Selectively: Can’t scale a single module independently
• Tight Coupling Risk: Without discipline, modules may become tightly coupled
• Deployment Size: Always deploy everything, even for small changes

Disadvantages of Microservices Architecture

• Complexity Overhead: Requires handling service discovery, communication, retries, etc.
• DevOps Maturity Needed: CI/CD, monitoring, logging, and infrastructure automation are a must
• Communication Overhead: Services need to communicate over the network (can cause latency)
• Debugging Difficulty: Distributed tracing is complex
• Data Consistency: Maintaining ACID across services is hard

When to Choose Monolith vs Microservices in .NET

Start with a modular monolith to reduce complexity. Move to microservices only when your app outgrows the monolith and you need independent deployment, team scaling, or horizontal scaling of specific parts.

Final Words

Modular monoliths are great when you’re starting out or working with a small team. They keep things simple and easier to manage. As your app grows in scale and complexity, microservices offer the flexibility and independence teams need. If you’re at that stage, it may be time to hire .NET developers who can guide the transition and build scalable services.