Microservices - Event Driven Architecture (EDA)

 Here is a clear, practical, and interview-ready explanation of Microservices with Event-Driven Architecture (EDA) 👇

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🌐 Microservices – Event-Driven Architecture (EDA)

🔹 What is Event-Driven Architecture?

In EDA, microservices communicate by publishing and consuming events instead of calling each other directly.

👉 An event = “Something important happened”

Example:
OrderPlaced, PaymentCompleted, InventoryReserved

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🤔 Why EDA with Microservices?

Problems with REST-only microservices ❌

• Tight coupling

• Service dependencies

• Cascade failures

• Poor scalability

EDA Benefits ✅

• Loose coupling

• High scalability

• Better fault tolerance

• Async communication

• Easy extensibility

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🧩 Core Components of EDA

1️⃣ Event Producer

• Service that publishes an event

Order Service → OrderPlacedEvent

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2️⃣ Event Broker

Message system:

• Kafka

• RabbitMQ

• AWS SNS/SQS

• Azure Event Hub

Acts as:

• Buffer

• Router

• Durable storage

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3️⃣ Event Consumer

• Services that react to events

Inventory Service
Payment Service
Notification Service

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🔄 Event Flow (Example)

Order Service
   |
   | OrderPlacedEvent
   v
Kafka Topic
   |
   |------------------|
Inventory Service   Payment Service

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🛒 Real-World Example: E-Commerce

Scenario: Place Order

1. User places order

2. Order Service saves order

3. Publishes OrderPlacedEvent

4. Inventory Service reserves stock

5. Payment Service charges payment

6. Notification Service sends email

👉 No service calls another directly

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🧠 Event Types

🔹 Domain Events (Most Common ⭐)

• Represent business facts

• Immutable

Example:

{
  "eventType": "OrderPlaced",
  "orderId": "123",
  "amount": 5000
}

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🔹 Integration Events

• For cross-service communication

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⚠️ Key Design Challenges (Very Important)

1️⃣ Eventual Consistency

• No immediate consistency

• Data sync happens over time

👉 Use Saga Pattern

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2️⃣ Idempotency

• Same event processed multiple times

• Consumer must handle duplicates

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3️⃣ Message Ordering

• Important for financial flows

• Use partitioning (Kafka)

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4️⃣ Schema Evolution

• Backward compatible changes

• Use schema registry (Avro / Protobuf)

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🔁 Saga Pattern (Brief)

🔹 Choreography-based Saga (Common ⭐)

• Services react to events

• No central controller

OrderPlaced → Payment → Inventory → OrderConfirmed

🔹 Orchestration-based Saga

• Central Saga manager controls flow

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🧪 Failure Handling

• Dead Letter Queue (DLQ)

• Retry mechanism

• Circuit breaker (for sync parts)

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🔐 Best Practices ⭐

• One event = one business fact

• Don’t share databases

• Use async wherever possible

• Keep events immutable

• Version events carefully

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🎯 Interview One-Liners

Q: Why Event-Driven Microservices?
👉 To achieve loose coupling, scalability, and fault tolerance.

Q: Difference between REST and EDA?
👉 REST is synchronous; EDA is asynchronous and decoupled.

Q: What is eventual consistency?
👉 Data becomes consistent over time via events.

Q: Kafka vs RabbitMQ?
👉 Kafka = high-throughput event streaming
RabbitMQ = message-oriented queue

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🛠️ Tech Stack Example

• Spring Boot

• Kafka

• Schema Registry

• Docker

• Kubernetes

• OpenTelemetry

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If you want, I can:

• Draw EDA architecture diagram

• Show Spring Boot + Kafka example

• Explain Saga with code

• Share real interview Q&A PDF

Just tell me 👍