In Event Storming, a collaborative domain modeling technique, the concept of a Domain Event plays a central role. It represents something significant that happens in the business domain, often marking a state change that is important to the business or system. Here’s a more detailed breakdown of Domain Events within the context of Event Storming:
What is a Domain Event?
- A Domain Event is a notable occurrence in the business domain that is meaningful to business experts and stakeholders.
- It captures something that has happened in the past, typically denoted in the past tense (e.g., Order Placed, Payment Received).
- Domain Events are at the core of how a system reacts and responds to changes over time.
Representation in Event Storming
- Domain Events are visually represented as orange sticky notes during an Event Storming session.
- They are placed on a timeline, where the flow of time runs from left to right, helping participants map out the sequence of events that happen within the domain.
- They provide an intuitive way to describe the behavior of the system and how it evolves through various interactions.
Role in Understanding the Domain
- Domain Events serve as the foundation for understanding the business processes.
- By focusing on these key events, you can map how the system interacts with users, other systems, or internal processes.
- Events help stakeholders and developers align their understanding of how the business operates.
Triggering Commands and Reactions
- Often, Domain Events are triggered by commands (explicit actions taken by users or systems, e.g., Place Order, Ship Product).
- A Domain Event might lead to further reactions or trigger subsequent commands, which can lead to more events. This chain of events helps define the flow of business processes.
Key Characteristics
- Immutable: Once a Domain Event occurs, it cannot be changed (because it has already happened).
- Observable: It represents something that has observable importance to the domain or business stakeholders.
- Past Tense: Described in terms of what has happened rather than what is happening.
- Business-focused: It is expressed in the language of the business, not in technical terms.
Examples of Domain Events
- In an e-commerce system:
- Order Placed: Represents a customer placing an order.
- Payment Processed: Indicates that the payment was successfully processed.
- Order Shipped: Signifies that the order was dispatched.
- In a banking system:
- Funds Deposited: Indicates that money was added to an account.
- Account Opened: Reflects the creation of a new account.
Benefits of Using Domain Events in Event Storming
- Common Language: Helps both technical and non-technical stakeholders communicate using a shared vocabulary.
- Focus on Behavior: Rather than concentrating on technical solutions, it brings the focus on the behavior of the system and how it reacts to key events.
- Evolutionary: Captures the changes and progression of the business processes over time, which can then be translated into technical implementations, such as event-driven architectures.
From Domain Event to System Design
- Domain Events often guide system design, especially in event-driven architectures where systems react to state changes by publishing and consuming events.
- These events can become a key part of system documentation and an integral component of software design, reflecting the business's actual processes.
In Event Storming, understanding and capturing Domain Events is crucial for mapping out a business process and identifying how the system interacts with various actors and other systems. It is a key concept for bridging the gap between business understanding and software design.
