Event Storming

Event Storming

This skill provides knowledge and facilitation guidance for Event Storming workshops — a collaborative domain discovery technique invented by Alberto Brandolini that uses color-coded "sticky notes" to explore complex business domains.

When to Use

This skill should be used when:

• Starting a new project that requires domain understanding
• Exploring complex business processes end-to-end
• Identifying aggregate and transactional boundaries
• Building shared understanding between developers and domain experts
• Discovering pain points and opportunities in existing processes

Core Concepts

Element	Color	Format	Example
Domain Event	🟧 Orange	Past tense fact	Order Placed
Command	📘 Blue	Imperative action	Place Order
Actor	👤 Yellow	Person/system	Customer
Aggregate	📙 Yellow rect	Entity boundary	Order
Policy	💜 Lilac	When-then rule	When OrderPlaced then NotifyWarehouse
Read Model	🟢 Green	Information view	Order Dashboard
External System	🟩 Green	3rd party	Payment Gateway
Hotspot	🔴 Red/Pink	Question/conflict	Who owns pricing?

Depth Levels

Choose the appropriate depth before starting:

Level	Scope	Detail
Big Picture	Entire value stream end-to-end	Events and hotspots only
Process Level	One bounded context or process	Commands, aggregates, policies
Design Level	Implementation-ready	Invariants, state machines, preconditions

Session Flow

Phase 1: Set the Stage (2-3 exchanges)

Establish scope, context, and depth level. Open with psychological safety — quantity over quality in early phases.

Phase 2: Chaotic Exploration — Domain Events (5-10 exchanges)

Discover domain events as past-tense facts. Focus on happy paths first, then exceptions and compensations. Work forward from triggers or backward from outcomes.

Phase 3: Enforce Timeline (3-5 exchanges)

Organize discovered events into a left-to-right timeline. Identify strict sequences vs. parallel processes.

Phase 4: Commands, Actors & External Systems (5-7 exchanges)

Add commands (what triggers events), actors (who triggers commands), and external systems. Format: Actor performs Command → results in Event.

Phase 5: Read Models & Views (3-5 exchanges)

Identify information actors need to see before performing commands. Format: View shows Data to help Actor decide Command.

Phase 6: Aggregates & Boundaries (3-5 exchanges)

Identify aggregates that handle commands and enforce invariants. Cluster related events and commands into entity boundaries.

Phase 7: Bounded Contexts (3-5 exchanges)

Identify where language or ownership changes. Look for Upstream/Downstream, Shared Kernel, and Anti-Corruption Layer patterns.

Phase 8: Policies & Reactions (3-5 exchanges)

Add automated reactions to events. Format: When [Event] then [Action]. Include compliance, notifications, and side effects.

Phase 9: Hotspots & Questions (2-3 exchanges)

Mark areas of confusion, conflict, or opportunity. Capture unanswered questions.

Phase 10: Synthesis (2-3 exchanges)

Summarize discoveries, validate with participants, identify follow-up sessions.

Output Artifact

Generate a markdown document using templates/event-storming-board.md containing:

1. Session context and scope
2. Event timeline (happy path + exceptions)
3. Commands & actors
4. Read models
5. Aggregates with responsibilities and invariants
6. Bounded contexts and relationships
7. Policies and business rules
8. External systems
9. Hotspots and open questions
10. Ubiquitous language glossary

Facilitation Principles

• Keep energy high — exploration, not documentation
• Embrace chaos early, organize later
• Domain language matters — use the words experts use
• Events are facts, not actions — past tense is crucial
• When stuck, ask "and then what happens?" or work backwards from the end
• Conflicts reveal complexity — capture BOTH views, mark as hotspot
• For Big Picture sessions, stay on events and hotspots; skip aggregate details
• For Design Level sessions, dig into invariants, state transitions, and preconditions

References

File	Content
references/facilitator-prompts.md	Detailed facilitator prompts for each session phase
templates/event-storming-board.md	Output template for the event storming board artifact