🏗️ Microservices Design Principles

The 12-Factor App methodology, Domain-Driven Design boundaries, and key principles that separate production-grade microservices from toy projects.

🎯 The 12-Factor App

The industry standard for building cloud-native microservices:

📐 Domain-Driven Design (DDD) Boundaries

Bounded Contexts

Each microservice owns ONE bounded context:

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flowchart LR
    subgraph Order["🛒 Order Context"]
        OE[/"Order Entity<br/>id, items, total, status"/]
        OS{{"OrderService"}}
    end

    subgraph Payment["💳 Payment Context"]
        PE[/"Payment Entity<br/>id, orderId, amount, method"/]
        PS{{"PaymentService"}}
    end

    subgraph Shipping["📦 Shipping Context"]
        SE[/"Shipment Entity<br/>id, orderId, address, tracking"/]
        SS{{"ShippingService"}}
    end

    Order -->|"OrderPlaced event"| Payment
    Payment -->|"PaymentCompleted event"| Shipping

    style Order fill:#E3F2FD,stroke:#1565C0,color:#000
    style Payment fill:#E8F5E9,stroke:#2E7D32,color:#000
    style Shipping fill:#FFF3E0,stroke:#E65100,color:#000

Decomposition Strategies

🔑 Key Design Principles

1. Single Responsibility

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flowchart LR
    subgraph Bad["❌ Kitchen-Sink Service"]
        M["Monolithic Service<br/>Orders + Payments + Emails + Reports"]
    end

    subgraph Good["✅ Focused Services"]
        O["Order Service<br/>CRUD orders"]
        P["Payment Service<br/>Process payments"]
        N["Notification Service<br/>Send emails/push"]
    end

    style M fill:#FFCDD2,stroke:#C62828,color:#000
    style O fill:#E8F5E9,stroke:#2E7D32,color:#000
    style P fill:#E8F5E9,stroke:#2E7D32,color:#000
    style N fill:#E8F5E9,stroke:#2E7D32,color:#000

2. Database per Service

Each service owns its data. Never share databases:

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flowchart LR
    subgraph Bad["❌ Shared Database"]
        S1[["Service A"]] --> DB[("🗄️ Shared DB")]
        S2[["Service B"]] --> DB
        S3[["Service C"]] --> DB
    end

    subgraph Good["✅ Database per Service"]
        A[["Service A"]] --> DA[("🗄️ DB A")]
        B[["Service B"]] --> DB2[("🗄️ DB B")]
        C[["Service C"]] --> DC[("🗄️ DB C")]
    end

    style DB fill:#FFCDD2,stroke:#C62828,color:#000
    style DA fill:#E8F5E9,stroke:#2E7D32,color:#000
    style DB2 fill:#E8F5E9,stroke:#2E7D32,color:#000
    style DC fill:#E8F5E9,stroke:#2E7D32,color:#000

3. Smart Endpoints, Dumb Pipes

• Services contain all business logic (smart endpoints)
• Communication channels (HTTP, Kafka) are simple transport (dumb pipes)
• No business logic in the message broker or API gateway

4. Design for Failure

Assume everything will fail:

// Every external call should have:
// 1. Timeout
// 2. Retry with backoff
// 3. Circuit breaker
// 4. Fallback

@CircuitBreaker(name = "payment", fallbackMethod = "paymentFallback")
@Retry(name = "payment", fallbackMethod = "paymentFallback")
@TimeLimiter(name = "payment")
public CompletableFuture<PaymentResult> processPayment(Order order) {
    return CompletableFuture.supplyAsync(() -> paymentClient.charge(order));
}

public CompletableFuture<PaymentResult> paymentFallback(Order order, Throwable t) {
    return CompletableFuture.completedFuture(PaymentResult.pending("Queued for retry"));
}

5. Evolutionary Design

Start with a monolith, extract services when needed:

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flowchart LR
    M["🏗️ Modular Monolith<br/>(start here)"] -->|"Extract when needed"| MS["🔲 Microservices<br/>(when scale/team demands it)"]

    style M fill:#E8F5E9,stroke:#2E7D32,color:#000
    style MS fill:#E3F2FD,stroke:#1565C0,color:#000

📊 Service Size Guidelines

🎯 Interview Questions