Sep 29, 2025 - 13 MIN READ

Designing Scalable Backend Architectures: From Monolith to Microservices

Strategic approach to backend architecture evolution, including domain-driven design, service boundaries, communication patterns, and scaling considerations.

Your Name

Every senior developer eventually faces the same architectural decision: when should we refactor our monolithic application into microservices? I've experienced this transition multiple times, and each time has taught me valuable lessons about architecture, scalability, and organizational complexity.

Understanding Architectural Evolution

Architecture isn't determined by a single decision. It evolves as your application grows, your team expands, and business requirements change.

Phase 1: Monolithic Architecture Done Right

Before jumping to microservices, a well-structured monolith can scale remarkably. The key is internal structure.

Domain-Driven Design Foundation

I reorganized the monolith using Domain-Driven Design (DDD) principles:

src/
├── Core/
│   ├── Domain/
│   │   ├── Users/
│   │   │   ├── User.cs
│   │   │   └── UserId.cs
│   │   ├── Orders/
│   │   │   ├── Order.cs
│   │   │   └── OrderId.cs
│   ├── Application/
│   └── Infrastructure/
└── Presentation/
    └── Api/

Event-Driven Communication

Within the monolith, domains communicate through domain events:

public class OrderPlacedEvent : DomainEvent
{
    public OrderId OrderId { get; set; }
    public UserId CreatedByUserId { get; set; }
    public decimal Amount { get; set; }
}

public class OrderPlacedEventHandler : IEventHandler<OrderPlacedEvent>
{
    private readonly IPaymentService _paymentService;
    
    public async Task HandleAsync(OrderPlacedEvent @event)
    {
        await _paymentService.ProcessPaymentAsync(
            @event.OrderId,
            @event.Amount);
    }
}

Phase 2: Strategic Service Decomposition

Eventually, even a well-structured monolith reaches limits.

Identifying Service Boundaries

I use three criteria to identify where to split services:

Scalability needs — Different load patterns
Team structure — Teams should own distinct services
Deployment independence — Different deployment schedules

Building Your First Microservice

I extracted the Payment service first—it was clearly bounded and independently scalable.

public class PaymentService
{
    public async Task StartListeningAsync()
    {
        var channel = _connection.CreateModel();
        
        channel.ExchangeDeclare(
            exchange: "domain_events",
            type: "topic");
        
        var consumer = new AsyncEventingBasicConsumer(channel);
        consumer.Received += async (model, ea) =>
        {
            var message = Encoding.UTF8.GetString(ea.Body.ToArray());
            var @event = JsonSerializer.Deserialize<OrderPlacedEvent>(message);
            
            await ProcessPaymentAsync(@event);
        };
        
        channel.BasicConsume("payment-queue", autoAck: true, consumer: consumer);
    }
}

Conclusion

The path from monolith to microservices isn't always linear, and that's okay. What matters is intentional decision-making at each step.

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