Microservices Architecture Guide

What are Microservices?

Imagine a large company where one person does everything - accounting, sales, customer support, and product development. If that person gets sick, the entire company stops. Now imagine breaking that into separate departments, each with its own team. If one department has issues, others continue working.

Microservices work the same way. Instead of one big application (monolith), you break it into small, independent services that communicate with each other.

MONOLITH vs MICROSERVICES

Monolith Application:
┌─────────────────────────────────────┐
│         Single Application          │
│  ┌─────┐ ┌─────┐ ┌─────┐ ┌─────┐   │
│  │Users│ │Order│ │Pay  │ │Ship │   │
│  └─────┘ └─────┘ └─────┘ └─────┘   │
│         One Database                │
└─────────────────────────────────────┘

Microservices:
┌─────────┐  ┌─────────┐  ┌─────────┐
│  User   │  │  Order  │  │ Payment │
│ Service │  │ Service │  │ Service │
│   DB    │  │   DB    │  │   DB    │
└────┬────┘  └────┬────┘  └────┬────┘
     │            │            │
     └────────────┼────────────┘
                  │
           API Gateway

Why Microservices?

Independent Deployment: Update one service without affecting others
Technology Freedom: Each service can use different languages/databases
Scalability: Scale only the services that need it
Team Independence: Different teams own different services
Fault Isolation: One service failing doesn't crash everything

When to Use Microservices?

USE MICROSERVICES WHEN:
✓ Large team (10+ developers)
✓ Complex domain with clear boundaries
✓ Need independent scaling of components
✓ Different parts have different technology needs
✓ Rapid, independent deployments required

STICK WITH MONOLITH WHEN:
✓ Small team (less than 5 developers)
✓ Simple application
✓ Just starting a project (start monolith, split later)
✓ Limited DevOps expertise
✓ Tight deadlines for MVP

Pro Tip: Don't start with microservices! Build a monolith first, understand your domain, then split when you need to.

Key Components of Microservices

1. API Gateway

The "front door" for all client requests. It routes requests to appropriate services.

// Spring Cloud Gateway Configuration
@Configuration
public class GatewayConfig {

    @Bean
    public RouteLocator customRoutes(RouteLocatorBuilder builder) {
        return builder.routes()
            .route("user-service", r -> r
                .path("/api/users/**")
                .uri("lb://USER-SERVICE"))
            .route("order-service", r -> r
                .path("/api/orders/**")
                .uri("lb://ORDER-SERVICE"))
            .build();
    }
}

2. Service Discovery (Eureka)

Services register themselves and discover other services dynamically - like a phone book for services.

// Eureka Server
@SpringBootApplication
@EnableEurekaServer
public class DiscoveryServerApplication {
    public static void main(String[] args) {
        SpringApplication.run(DiscoveryServerApplication.class, args);
    }
}

// Eureka Client (in each microservice)
@SpringBootApplication
@EnableEurekaClient
public class UserServiceApplication {
    public static void main(String[] args) {
        SpringApplication.run(UserServiceApplication.class, args);
    }
}

3. Config Server

Centralized configuration management for all services.

# application.yml for Config Server
spring:
  cloud:
    config:
      server:
        git:
          uri: https://github.com/your-org/config-repo
          default-label: main

Communication Between Services

Synchronous (REST/HTTP)

// Using RestTemplate (simple)
@Service
public class OrderService {

    @Autowired
    private RestTemplate restTemplate;

    public User getUser(Long userId) {
        return restTemplate.getForObject(
            "http://USER-SERVICE/api/users/" + userId,
            User.class
        );
    }
}

// Using Feign Client (recommended)
@FeignClient(name = "USER-SERVICE")
public interface UserClient {

    @GetMapping("/api/users/{id}")
    User getUserById(@PathVariable Long id);
}

Asynchronous (Message Queues)

// Publishing an event (RabbitMQ)
@Service
public class OrderService {

    @Autowired
    private RabbitTemplate rabbitTemplate;

    public void createOrder(Order order) {
        // Save order
        orderRepository.save(order);

        // Publish event for other services
        rabbitTemplate.convertAndSend(
            "order-exchange",
            "order.created",
            new OrderCreatedEvent(order.getId())
        );
    }
}

// Consuming the event
@Service
public class NotificationService {

    @RabbitListener(queues = "notification-queue")
    public void handleOrderCreated(OrderCreatedEvent event) {
        // Send notification to customer
        sendEmail(event.getOrderId());
    }
}

Handling Failures: Circuit Breaker

When a service is down, don't keep trying - fail fast and provide fallback.

// Using Resilience4j Circuit Breaker
@Service
public class OrderService {

    @CircuitBreaker(name = "userService", fallbackMethod = "getUserFallback")
    public User getUser(Long userId) {
        return userClient.getUserById(userId);
    }

    // Fallback when user service is down
    public User getUserFallback(Long userId, Exception ex) {
        return new User(userId, "Unknown", "Service temporarily unavailable");
    }
}

Best Practices

One service, one responsibility: Each service does one thing well
Own your data: Each service has its own database
Design for failure: Assume services will fail, handle it gracefully
Use async communication: Prefer message queues over REST for non-critical operations
Implement health checks: Let the system know when a service is healthy
Centralize logging: Use ELK stack or similar for distributed logging
Use distributed tracing: Track requests across services (Zipkin, Jaeger)

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