When to Use a Domain Event
Overview
This document summarizes when to use a domain event.
Introduction
A Domain Event is an event representing a past occurrence in the domain that can have a meaningful impact on other parts of the system.
Domain events help resolve strong coupling issues — they allow you to inform related objects of state changes while maintaining low dependency and strong independence.
Below is a simple example expressed in mermaid:
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When to Use Domain Events
Domain events offer many advantages: reduced coupling, improved scalability, asynchronous processing, and performance optimization. However, overusing them can lead to performance degradation and unintended behaviors. Therefore, you should establish clear criteria to prevent misuse.
Consider using domain events when:
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The state change of the domain model is a meaningful fact
- Used to announce the completion of specific business actions.
- Examples: Order created, Payment completed, Sign-up succeeded.
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A reaction is needed across multiple bounded contexts (or modules)
- One event can trigger multiple actions without tightly coupling the modules.
- With strong coupling, you’d need to propagate changes to each dependency manually.
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Eventual consistency or asynchronous processing is acceptable
- If transaction order is not critical, you can leverage asynchronous and eventual consistency benefits.
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You need auditing or history management based on events
- Enables tracking of who did what and when.
- Plays a core role in event sourcing and event storage.
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You want to separate side effects
- Focus the core logic on essential work and delegate side effects through events.
- Keeps the core logic clean while loosely coupling secondary processes.
Final Thoughts
A domain event is a powerful design pattern. When applied correctly, it can become a strong asset to your system. However, if misused, it can harm performance and maintainability. Use domain events thoughtfully, guided by the principles outlined in this document.