What is Event Storming?
Event Storming is a collaborative workshop technique used to explore complex domains by mapping out domain events in a visual, timeline-like format. It helps domain experts, developers, and stakeholders build a shared understanding of the system.
- Invented by Alberto Brandolini
- Focuses on DDD Domain Event
🧩 Core Concepts (Post-it Colors):
| Type | Meaning | Example |
|---|---|---|
| 🟠 Orange | Domain Event (past tense) | OrderPlaced, PaymentReceived |
| 🟢 Green | Command (intention to do something) | PlaceOrder, CancelBooking |
| 🔵 Blue | Aggregate/Entity | Order, User, Product |
| 🟡 Yellow | Policy/Rule | If payment fails, cancel order |
| 🟣 Purple | External System/User | Customer, CRM System |
| 🟥 Red | Problem/Hotspot | “Unclear business rule here” |
✅ How to Conduct a Successful Event Storming Session
1. Set the Stage
- Invite a cross-functional group: domain experts, developers, product owners.
- Book a large physical or virtual whiteboard space (e.g., Miro, MURAL).
- Use sticky notes (physical or digital) and color code them.
2. Start with Domain Events (🟠)
- Ask: “What are the key things that happen in the system or sub-system?”
- Place events in chronological order.
- Example: UserRegistered → OrderPlaced → PaymentReceived.
3. Add Commands (🟢)
- For each event, ask: “What action caused this to happen?”
- Example: PlaceOrder leads to OrderPlaced.
4. Identify Aggregates (🔵)
- Ask: “What entity holds the logic or ensures consistency?”
- Example: Order aggregate handles the PlaceOrder command.
5. Insert Policies (🟡)
- Ask: “What should automatically happen after this event?”
- Example: After OrderPlaced, send confirmation email.
6. Add External Actors (🟣)
- Who triggers commands or receives events?
- Example: Customer initiates PlaceOrder.
7. Mark Hotspots (🟥)
- Capture unclear, risky, or contentious areas.
- Useful for follow-up discussions or deeper dives.
8. Group and Refactor
- Spot bounded contexts naturally forming.
- Cluster events/commands by business subdomains.