Event-Driven Communication

Services publish domain events when important business facts happen.

events

Detailed Description

Events describe facts that already happened. Subscribers decide what to do with those facts.

Use stable event contracts and versioning because events become shared integration APIs.

Visual Diagram

Domain Events
  OrderService emits:
    → OrderPlaced    { orderId, items, userId }
    → OrderCancelled { orderId, reason }
    → OrderShipped   { orderId, trackingId }

  Subscribers react:
    EmailSvc    listens → OrderPlaced
    AnalyticsSvc listens → all events
    InventorySvc listens → OrderPlaced, OrderCancelled

Tradeoffs

Pros

Very low coupling, easy to add subscribers

Cons

Harder to trace, duplicate events, eventual consistency

Examples: Kafka events, EventBridge, Node EventEmitter

Domain Events

OrderService emits:

-> OrderPlaced { orderId, items, userId }

-> OrderCancelled { orderId, reason }

-> OrderShipped { orderId, trackingId }

Subscribers react:

EmailSvc listens -> OrderPlaced

AnalyticsSvc listens -> all events

InventorySvc listens -> OrderPlaced, OrderCancelled