Microservices Architecture

// Monolith — single deployable unit
// ┌─────────────────────────────────┐
// │           Monolith               │
// │  ┌──────┐ ┌──────┐ ┌──────┐    │
// │  │Users │ │Orders│ │Pay   │    │
// │  │Module│ │Module│ │Module│    │
// │  └──┬───┘ └──┬───┘ └──┬───┘    │
// │     └────────┴────────┘         │
// │         Shared Database         │
// └─────────────────────────────────┘

// Microservices — independent services
// ┌──────────┐  ┌──────────┐  ┌──────────┐
// │ User Svc  │  │ Order Svc │  │ Pay Svc   │
// │ (Java)    │  │ (Java)    │  │ (Go)      │
// └─────┬────┘  └─────┬────┘  └─────┬────┘
//       │             │             │
//   Users DB      Orders DB     Payments DB
//  (PostgreSQL)  (PostgreSQL)    (DynamoDB)

// Communication:
// Sync: REST / gRPC (request-response)
// Async: Kafka / SQS (event-driven)

// Key microservices principles:

// 1. Single Responsibility — one business capability per service
// UserService: registration, profile, authentication
// OrderService: order creation, status, history
// PaymentService: charge, refund, billing

// 2. Database per service — no shared tables
// UserService → users_db (PostgreSQL)
// OrderService → orders_db (PostgreSQL)
// PaymentService → payments_db (DynamoDB)

// 3. API contract — services communicate via defined interfaces
@RestController
@RequestMapping("/api/v1/users")
public class UserController {
    @GetMapping("/{id}")
    public UserDTO getUser(@PathVariable String id) { ... }
}

// 4. Independent deployment
// Deploy OrderService v2.3 without touching UserService
// Feature flags for gradual rollout

// 5. Decentralised governance
// Teams choose their own tech stack, frameworks, deployment cadence

// Challenge → Solution mapping
//
// Distributed transactions  → Saga pattern (choreography or orchestration)
// Cross-service queries     → API composition or CQRS read model
// Service-to-service calls  → Circuit breaker + retry + timeout
// Debugging distributed flows → Distributed tracing (Jaeger, Zipkin)
// Authentication             → API Gateway + JWT token propagation
// Configuration management   → Spring Cloud Config / Consul
// Service discovery          → Kubernetes DNS / Eureka / Consul
// Monitoring                 → Prometheus + Grafana + ELK stack

// When NOT to use microservices:
// ❌ Small team (< 5 developers)
// ❌ Simple domain (CRUD app)
// ❌ Startup MVP (need speed)
// ❌ No DevOps maturity (can't manage 20+ deployments)
// Start with a monolith, extract services as complexity grows