Microservices - DDD (Domain Driven Design)

 Here is a clear and interview-ready explanation of Microservices with DDD (Domain-Driven Design), with examples 👇

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🌐 Microservices + DDD (Domain-Driven Design)

🔹 Why DDD with Microservices?

Microservices work best when each service owns a clear business responsibility.
DDD helps you:

• Split services by business capability, not by technical layers

• Reduce tight coupling

• Align developers with business language

👉 DDD answers: “What should be a microservice?”

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🧩 Core DDD Concepts (Must Know)

1️⃣ Domain

The business problem you are solving
Example: E-commerce Domain

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2️⃣ Subdomain

Parts of the domain

• Core Subdomain → Competitive advantage

• Supporting Subdomain → Helps core

• Generic Subdomain → Common (Auth, Email)

E-commerce Example:

• Core → Order Management

• Supporting → Inventory

• Generic → Payment, Notification

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3️⃣ Bounded Context (Very Important ⭐)

A logical boundary where a domain model is valid.

👉 Rule:
✔ One Bounded Context = One Microservice

Example:

• Order Service → Order model

• Payment Service → Payment model

Even if both have status, meaning is different.

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4️⃣ Ubiquitous Language

Common language used by:

• Developers

• Business

• QA

Example:

• “Order is CONFIRMED”

• “Payment is AUTHORIZED”

No technical confusion.

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🏗️ DDD Building Blocks (Inside a Microservice)

🟢 Entity

• Has identity

• Mutable

class Order {
    OrderId id;
    OrderStatus status;
}

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🔵 Value Object

• No identity

• Immutable

class Money {
    BigDecimal amount;
    String currency;
}

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🟣 Aggregate

• Cluster of objects

• One Aggregate Root

Order (Root)
 ├── OrderItem
 └── Address

👉 Only Aggregate Root is accessed from outside

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🟠 Repository

Handles persistence (DB logic)

interface OrderRepository {
    Order findById(OrderId id);
    void save(Order order);
}

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🔴 Domain Service

Business logic that doesn’t fit entity

class OrderPricingService {
    Money calculateTotal(Order order);
}

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🔄 Microservices Communication (DDD Friendly)

🔹 Synchronous

• REST / gRPC

• Use only when required

🔹 Asynchronous (Recommended ⭐)

• Kafka / RabbitMQ

• Domain Events

OrderPlacedEvent → Inventory → Payment

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📦 Example: E-Commerce Microservices with DDD

Customer Service (Customer Context)
Order Service (Order Context)
Payment Service (Payment Context)
Inventory Service (Inventory Context)

Each service:

• Owns its database

• Has its own domain model

• Communicates via events

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🧠 Anti-Patterns to Avoid ❌

• Shared database between services

• Anemic domain model

• Splitting services by technical layers

• Chatty synchronous calls

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🎯 Interview One-Liner Answers

Q: Why use DDD in microservices?
👉 To align services with business capabilities and reduce coupling.

Q: What is Bounded Context?
👉 A boundary where a specific domain model applies, usually mapped to one microservice.

Q: Can two services share entities?
👉 No. They communicate via APIs or events, not shared models.

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🚀 Real-World Usage

Used by:

• Amazon

• Netflix

• Uber

• Spotify

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If you want, I can:

• Draw DDD + Microservices architecture diagram

• Give Spring Boot DDD project structure

• Provide event-driven DDD example with Kafka

• Share interview Q&A PDF

Just tell me 👍