Modular Monoliths in .NET – Scalable Without Microservices

Microservices aren’t the only way to achieve scale. A well-designed modular monolith lets you ship features quickly, keep runtime simple, and still split into services later—if you ever need to.

Real-Life Analogy: The Department Store

Imagine a large department store. Inside one building you have Clothing, Electronics, and Homeware—each with its own staff, stockroom, and tills. Customers feel it’s one seamless shop, but departments operate semi-independently. If Electronics grows too big, you can spin it off into its own storefront without rebuilding the others.

Why Not Jump Straight to Microservices?

• Operational Overhead —multiple deployables, Dockerfiles, pipelines, monitoring stacks.
• Distributed Failures —network calls, retries, eventual consistency.
• Cost —extra infrastructure for each tiny service.

Unless you have distinct load profiles or organisational reasons, a single deployable is faster and cheaper—if it stays modular.

Defining Modules

• Autonomous Domain —owns its data and business rules.
• Internal Cohesion —high coupling within the module.
• External Contracts —other modules can interact only through clear interfaces or messages.

Folder & Project Layout

src/
 ├─ Modules/
 │    ├─ Ordering/
 │    │    ├─ Ordering.Application/
 │    │    ├─ Ordering.Domain/
 │    │    └─ Ordering.Infrastructure/
 │    └─ Inventory/
 │         ├─ Inventory.Application/
 │         ├─ Inventory.Domain/
 │         └─ Inventory.Infrastructure/
 └─ WebApi/                # ASP.NET Core host with only references to *.Application projects

Inter-Module Communication

Inside one process, MediatR or a simple in-memory message bus keeps modules ignorant of each other’s internals:

// Cross-module notification
public record ProductReserved(Guid ProductId, int Quantity) : INotification;

public class InventoryHandler : INotificationHandler<ProductReserved>
{
    public Task Handle(ProductReserved evt, CancellationToken ct)
    {
        // adjust stock levels...
        return Task.CompletedTask;
    }
}

Database Strategy

• Schema-per-Module —same physical DB, separate schemas; easiest to start.
• DB-per-Module —logical isolation, still one connection string.
• Event-carry State —emit integration events for true eventual consistency.

Evolution Path to Microservices

1. Identify a module with unique scaling or deployment needs.
2. Extract its data to its own physical database.
3. Wrap the module behind an HTTP/gRPC façade; other modules switch to network calls.
4. Move the code into a separate repo and deploy independently.

Common Pitfalls

• Leaky Dependencies: a module reaches into another’s data models “just this once.” Don’t.
• Shared Infrastructure Code: keep cross-cutting concerns (logging, auth) in a shared package, not as random static classes referenced everywhere.
• Premature Splitting: extract only when metrics or team structure demand it.

Final Thoughts

A modular monolith offers 80 % of the independence of microservices at a fraction of the complexity. Keep your boundaries strict, your contracts lean, and your modules cohesive; scalability will follow—without the pager-nightmares of distributed debugging.