15 Nov 2023
Domain events have several important characteristics:
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Immutable: Domain events represent past occurrences and should not be modified once they are created. This immutability ensures that the history of the domain remains consistent and that event handlers can rely on the event data being accurate.
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Describes a Change of State: A domain event encapsulates a significant change in the state of the domain model. It captures the "what" and "when" of the change, not the "how." This decoupling allows event handlers to react to the event without being concerned with the specific implementation details that triggered it.
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Timestamped: Domain events should include a timestamp indicating the time when the event occurred. This timestamp provides a chronological order for events and facilitates auditing and debugging.
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Identifiable: Each domain event should have a unique identifier that distinguishes it from other events. This identifier is crucial for tracking events, replaying event histories, and ensuring event handlers process events correctly.
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Self-Contained: Domain events should contain all the necessary information to be understood and processed by interested parties. They should not rely on external data sources or context to be meaningful.
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Domain-Focused: Domain events should be expressed in terms of the domain model's concepts and terminology. They should avoid technical jargon or implementation details that are not relevant to the domain.
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Asynchronous: Domain events are typically published asynchronously, allowing event handlers to consume them at their own pace. This asynchronous nature decouples event producers from event consumers and promotes loose coupling in the system.
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Pub-Sub Pattern: Domain events are often used in conjunction with the publish-subscribe pattern, enabling interested parties to subscribe to specific event types and receive notifications when those events occur. This pattern facilitates decoupling and promotes event-driven architectures.
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Audit Trail: Domain events provide an audit trail of significant changes within the domain model. They can be used to track historical events, identify causes of changes, and ensure compliance with regulatory requirements.
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CQRS Enabler: Domain events are a key component of command query responsibility segregation (CQRS), an architectural pattern that separates read and write operations. They facilitate the publication of events following command execution, enabling event handlers to update read models asynchronously.
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Ubiquitous Language: Domain events should be named and described using the ubiquitous language of the domain, ensuring that they are understandable to domain experts and developers alike. This common language promotes shared understanding and reduces communication overhead.
