Designing Event Models

Designing Event Models

Workflow: Design an Event Model

1. Clarify scope. Ask what system or feature to model. Identify actors, key business processes, and external systems.
2. Brainstorm events. List all domain events. Group into swimlanes by business capability.
3. Build the timeline. Order events chronologically within each swimlane.
4. Apply the four patterns to each step:
   • State Change: Screen → Command → Event
   • State View: Event(s) → Read Model → Screen
   • Automation: Event → Read Model → ⚙️ → Command → Event
   • Translation: External Event → ⚙️ → Command → Event
5. Define slices. Each pattern application = one slice. Name it after the business action or view.
6. Write GWTs (Given/When/Then scenarios) for each slice. Minimum: one happy path + one error/edge case. Use concrete example data ("Jane Doe", "$4.50"), never abstract placeholders.
7. Run the Information Completeness Check. For every field in every read model, trace it to a source event. For every command field, trace it to user input or a prior event. Flag gaps.
8. Output the model as YAML using the format below.

Workflow: Review an Event Model

1. Check completeness. Every read model field must trace to a source event. Flag orphan data.
2. Check anti-patterns:
   • Events in present tense (should be past tense)
   • God slice (>10 GWTs in one slice — decompose)
   • Horizontal over-orchestration (UI firing sequential commands — use automations)
   • Logic on the board (if/else branching — belongs in code, not model)
   • Technology references (no "Kafka topic" or "PostgreSQL table" — model is tech-agnostic)
   • Missing screens (every command and read model should connect to a screen)
3. Check GWT quality. Concrete data? Both happy and error paths? Named scenarios?
4. Propose fixes with before/after.

YAML Output Format

event_model:
  name: "Shopping Cart"
  swimlanes:
    - name: "Cart"
      slices:
        - name: "Add Item"
          pattern: state_change
          screen: "Product Page — [Add to Cart] button"
          command:
            name: AddItem
            fields: {itemId: "abc-123", name: "Latte", price: 4.50}
          event:
            name: ItemAdded
            fields: {itemId: "abc-123", name: "Latte", price: 4.50, addedAt: "2024-07-01T10:00:00Z"}
          gwts:
            - name: "Happy path — add first item"
              given: []
              when: {command: AddItem, fields: {itemId: "abc-123", name: "Latte", price: 4.50}}
              then: {event: ItemAdded, fields: {itemId: "abc-123", name: "Latte", price: 4.50}}
            - name: "Error — max 3 items"
              given:
                - {event: ItemAdded, fields: {itemId: "a"}}
                - {event: ItemAdded, fields: {itemId: "b"}}
                - {event: ItemAdded, fields: {itemId: "c"}}
              when: {command: AddItem, fields: {itemId: "d"}}
              then: {error: "Maximum 3 items per cart"}

        - name: "Cart Items"
          pattern: state_view
          events_consumed: [ItemAdded, ItemRemoved]
          read_model:
            name: CartItems
            fields: {items: [{itemId: "abc-123", name: "Latte", price: 4.50}], totalPrice: 4.50}
          screen: "Cart Page — item list with totals"
          gwts:
            - name: "Shows remaining items after removal"
              given:
                - {event: ItemAdded, fields: {itemId: "a", name: "Latte", price: 4.50}}
                - {event: ItemAdded, fields: {itemId: "b", name: "Espresso", price: 3.00}}
                - {event: ItemRemoved, fields: {itemId: "a"}}
              then: {read_model: CartItems, fields: {items: [{itemId: "b", name: "Espresso", price: 3.00}], totalPrice: 3.00}}

        - name: "Send Confirmation Email"
          pattern: automation
          trigger_event: CartSubmitted
          read_model: {name: SubmittedCartData, fields: {email: "jane@co.com", items: ["..."]}}
          command: {name: SendConfirmationEmail, fields: {to: "jane@co.com", body: "..."}}
          result_event: {name: ConfirmationEmailSent, fields: {to: "jane@co.com", sentAt: "..."}}

        - name: "Inventory Sync"
          pattern: translation
          external_event: {name: InventoryChanged, source: "Warehouse API", fields: {sku: "abc-123", qty: 42}}
          command: {name: UpdateInventory, fields: {productId: "abc-123", quantity: 42}}
          event: {name: InventoryUpdated, fields: {productId: "abc-123", quantity: 42}}

Key Rules

• Events are past tense. "Item Added", never "Add Item." Commands are imperative: "Add Item", "Submit Cart."
• Always use concrete example data in every field — names, prices, IDs, timestamps. Never abstract placeholders.
• One slice = one pattern application. If a slice has >10 GWTs, decompose it.
• The model is technology-agnostic. No databases, frameworks, or protocols.
• Information Completeness Check is mandatory. Every read model field must trace to an event. No exceptions.

Edge Cases

• System has no UI (API only): Replace screen mockups with API endpoint descriptions (e.g., "POST /cart/items — request body: {name, price}"). The pattern structure stays the same.
• CRUD-heavy areas with no business rules: Model as simple state change slices with minimal GWTs. Note that if there are truly no rules, Event Modeling may be overkill for that area.
• External system integration: Use the Translation pattern. Always model the translator as a separate slice even if the mapping seems trivial — it makes system boundaries explicit.

Reference Material

Detailed guidance on all Event Modeling concepts:

• Foundations — what Event Modeling is, origins, building blocks (events, commands, read models, screens): references/01-foundations.md
• Four patterns & completeness check — state change, state view, automation, translation, and the information completeness invariant: references/02-patterns-and-completeness.md
• Specifications, slices & workshops — GWT scenarios, slice structure, and how to run modeling workshops: references/03-specifications-slices-workshops.md
• Anti-patterns & best practices — constant cost curve, event modeling vs event storming, common mistakes, playbook: references/04-anti-patterns-and-best-practices.md
• DDD/CQRS/ES context & tooling — relationship to related methodologies, tooling, when to use Event Modeling: references/05-context-and-tooling.md