Introduction to Event-Driven Architecture

I’ve spent the last year migrating systems from request-response to event-driven patterns. The shift in thinking is significant, but the benefits are real. Here’s my introduction to event-driven architecture (EDA) and why you should care.

What is Event-Driven Architecture?

Instead of services directly calling each other (request-response), they communicate through events. When something happens, a service publishes an event. Other services that care about that event subscribe and react.

It’s like the difference between asking someone to do something versus announcing what happened and letting interested parties respond.

The Core Concepts

Events

An event is a fact that something happened. It’s immutable—you can’t change the past. Good event names are past tense: OrderPlaced, PaymentReceived, UserRegistered.

public class OrderPlaced
{
    public Guid OrderId { get; }
    public Guid CustomerId { get; }
    public decimal TotalAmount { get; }
    public DateTime OccurredAt { get; }
    
    public OrderPlaced(Guid orderId, Guid customerId, decimal totalAmount)
    {
        OrderId = orderId;
        CustomerId = customerId;
        TotalAmount = totalAmount;
        OccurredAt = DateTime.UtcNow;
    }
}

Producers and Consumers

Producers publish events. Consumers subscribe to events they care about. The beauty is that producers don’t know (or care) who’s listening.

Message Broker

Events flow through a message broker—RabbitMQ, Kafka, Azure Service Bus, etc. The broker handles routing, persistence, and delivery guarantees.

Benefits

• Loose coupling: Services don’t depend on each other directly
• Scalability: Add consumers without changing producers
• Resilience: If a consumer is down, events queue up
• Audit trail: Events provide a history of what happened

Challenges

• Eventual consistency: Data isn’t immediately consistent across services
• Debugging: Tracing issues across async boundaries is harder
• Complexity: More moving parts to manage
• Ordering: Events might arrive out of order

A Simple Example

Consider an e-commerce order flow:

Order Service publishes: OrderPlaced
    └── Inventory Service (subscribes) → reserves stock
    └── Payment Service (subscribes) → charges customer
    └── Notification Service (subscribes) → sends confirmation email

The Order Service doesn’t call these services directly. It just announces that an order was placed. Each downstream service decides what to do.

When to Use It

EDA shines when:

• You have multiple services that need to react to the same event
• You need to decouple bounded contexts
• Systems need to scale independently
• You want audit/replay capabilities

It’s overkill for simple CRUD apps or tightly coupled workflows.

Next Steps

In future posts, I’ll cover specific patterns like Event Sourcing, CQRS, and the Outbox Pattern. For now, think about your current architecture—where would events make sense?

References

• What do you mean by Event-Driven? – Martin Fowler
• Event-driven architecture style – Azure Architecture