Event-Driven Architecture

Introduction

Event-Driven Architecture (EDA) replaces synchronous calls between services with events on a log. Instead of order service calls billing service calls notification service, the order service emits OrderPlaced and others react.

Syntax reference

Synchronous vs event-driven:

# Synchronous (tight coupling)
Order ──► Billing ──► Inventory ──► Notification
  ⨯ any failure breaks the chain
  ⨯ each new consumer = code change in Order

# Event-driven (loose coupling)
Order ──► Kafka [order.placed] ──► Billing
                              └──► Inventory
                              └──► Notification
                              └──► Analytics
  ✓ add consumers without touching producer
  ✓ retries, dead-letter queues, replay from log

Best practices

• Use the Outbox pattern: write to DB + outbox table in one TX, ship outbox rows to Kafka.
• Version every event; consumers must tolerate added fields.
• Always have a dead-letter queue for poison messages.