Domain Events in DDD and Domain vs Integration Events in Microservices Architecture

4 min readMar 25, 2024

We’ll learn the concept of Domain Events in Domain-Driven Design (DDD) and distinguish between Domain and Integration Events. We’ll also discuss how to use them effectively in a real-world context.

Eshop Microservices Domain and Integration Events

I have just published course — .NET 8 Microservices: C# 12, DDD, CQRS, Vertical/Clean Architecture.

The Essence of Domain Events in DDD 🎉

Domain Events represent something that happened in the past and the other parts of the same service boundary -same domain- need to react to these changes. A Domain Event is a business event that occurs within the domain model. It often represents a side effect or a result of a domain operation. It’s a technique that is used to achieve consistency between aggregates in the same domain.

Domain Events are like the heartbeat of our domain, signaling significant business occurrences. They’re the whispers of change within our domain, telling us stories of user actions, system updates, or business rule applications.

Implementing Domain Events: A How-To Guide 🛠️

Domain Events are used to encapsulate the event details and dispatch them to interested parties. Typically, they are used to communicate changes within the domain to external handlers which may perform actions based on these events.

To leverage Domain Events, encapsulate event details, and dispatch them to the keen ears waiting to act upon these signals. It’s about capturing moments of change and ensuring the right parts of our system can respond and adapt. 🎬

A Real-World Scenario: E-Shop Orders 🛒

Imagine our e-shop, bustling with activity. When a customer transforms into a purchaser, it’s not just a transaction; it’s an orchestration of events! From inventory adjustments to sending out that joyous email confirmation, an OrderPlaced Domain Event sets the symphony in motion. 🎻

https://dotnet.microsoft.com/learn/dotnet/architecture-guides

When an order is placed, an OrderPlaced event might be triggered. These events are crucial in capturing the intention and outcome of domain actions, and they can be used to trigger side effects or notify other parts of the system about changes within the domain. That moment when a customer’s order lights up our system with an OrderPlaced event. 📦💡

Domain vs. Integration Events: Understanding the Distinction 🔍

At the core, they are the same thing: “A representation of something that happened in the past”. However, their purposes, use-cases, and implementation details are different:

Domain Events

Domain Events published and consumed within a single domain.
So you publish and subscribe to the event in the same application instance.
They are strictly within the boundary of the microservice/domain context.
They typically indicate something that has happened within the aggregate.
Domain Events happen in-process and synchronously, sent using an in-memory message bus. Example: OrderPlacedEvent.

Integration Events

They are used to communicate state changes or events between different bounded contexts or microservices.
They are more about the overall system’s reaction to certain domain events.
Integration Events should be asynchronously, sent with a message broker over a queue.
Integration events are consumed by other subsystems.
Example: After handling OrderPlacedEvent, an OrderPlacedIntegrationEvent might be published to a message broker like RabbitMQ, which could then be consumed by other microservices.

Illustrating the Concept: From Order Creation to Fulfillment 📈

Domain Event: An OrderCreatedEvent within our Ordering Microservice sparks internal actions, updating customer records in a self-contained narrative.
Integration Event: Once the OrderCreatedEvent is handled, an OrderCreatedIntegrationEvent is published. As the domain tale concludes, an OrderCreatedIntegrationEvent takes flight, reaching distant microservices like Shipping and Inventory, prompting them to spring into action. The Shipping and Inventory Microservices subscribes to this event. When it receives the event, it begins the process to ship the items.

Wrapping Up: The Power of Domain and Integration Events 🌐

In summary, Domain Events are a powerful part of DDD, allowing for a clear and concise way to capture significant occurrences in the domain.
They facilitate decoupling of components within a system, making the system more maintainable and adaptable to changes. Understanding the difference between Domain and Integration Events is crucial in designing a system with clear boundaries and responsibilities. 🎊📚