Spring Boot Microservices Basics

Microservices is an architecture where one large application is split into small, independent services. Each service runs on its own, handles one business capability, and communicates with other services over HTTP or messaging.

Monolith vs Microservices

Monolith (single app):             Microservices:
──────────────────────────────     ────────────────────────────────────────
┌──────────────────────────┐       ┌──────────┐  ┌──────────┐  ┌────────┐
│   Single Spring Boot App │       │  User    │  │  Order   │  │Payment │
│  ┌────────────────────┐  │       │  Service │  │  Service │  │Service │
│  │  Users   Orders    │  │  →    └──────────┘  └──────────┘  └────────┘
│  │  Payment Inventory │  │            │              │             │
│  └────────────────────┘  │            └──────────────┴─────────────┘
└──────────────────────────┘                    HTTP / Message Bus

When Monolith Makes Sense

• Small teams (1–5 developers)
• Early-stage product with frequent pivots
• Simple domain logic

When Microservices Make Sense

• Different teams own different parts of the system
• Parts need to scale independently (payment service handles 10x more load)
• Parts use different technologies or deployment cycles

Communication Between Services

Synchronous (HTTP REST)

Service A calls Service B and waits for the response:

Order Service needs user details:

OrderService ──GET /users/5──▶ UserService ──▶ returns User JSON
     ◀────────────────────────────────────── User JSON response
     │
continues processing

// In Order Service — calling User Service
@Service
public class OrderService {

    private final RestTemplate restTemplate;

    public OrderService(RestTemplate restTemplate) {
        this.restTemplate = restTemplate;
    }

    public User getUserDetails(Long userId) {
        return restTemplate.getForObject(
            "http://user-service/api/users/" + userId,
            User.class
        );
    }
}

Asynchronous (Message Queue)

Service A publishes an event. Service B processes it later — no waiting:

Order Service ──"OrderPlaced" event──▶ Message Queue (RabbitMQ/Kafka)
                                              │
                                              ▼ (async)
                                    Notification Service reads event
                                    and sends email to customer

Service Discovery with Eureka

With many services, hardcoding URLs is impractical. Eureka is a service registry — each service registers itself, and others look up addresses by name.

┌─────────────────────────────────────┐
│        Eureka Server (Registry)     │
│  user-service    → 192.168.1.5:8081 │
│  order-service   → 192.168.1.6:8082 │
│  payment-service → 192.168.1.7:8083 │
└─────────────────────────────────────┘
         ▲                   ▲
  Register on startup   Look up service address
         │                   │
  User Service         Order Service

Eureka Server Setup

@SpringBootApplication
@EnableEurekaServer
public class DiscoveryServer { ... }

Eureka Client (each microservice)

# application.properties
spring.application.name=user-service
eureka.client.service-url.defaultZone=http://localhost:8761/eureka/

@SpringBootApplication
@EnableDiscoveryClient
public class UserServiceApp { ... }

API Gateway

An API Gateway is a single entry point for all client requests. It routes each request to the correct service:

  Mobile App / Browser
          │
          ▼
    API Gateway (:80)
    ┌─────────────────────────────────────────────────┐
    │  /api/users/**   ──▶  user-service:8081         │
    │  /api/orders/**  ──▶  order-service:8082        │
    │  /api/pay/**     ──▶  payment-service:8083      │
    └─────────────────────────────────────────────────┘

Spring Cloud Gateway is the recommended API gateway for Spring Boot microservices.

Key Spring Cloud Components

Component                  Purpose
─────────────────────────  ─────────────────────────────────────────────
Spring Cloud Eureka        Service registry and discovery
Spring Cloud Gateway       API gateway and routing
Spring Cloud Config        Centralized configuration management
Spring Cloud LoadBalancer  Client-side load balancing
Spring Cloud OpenFeign     Declarative HTTP client between services
Resilience4j               Circuit breaker, retry, rate limiting

Summary

• Microservices split one large app into small, independently deployable services
• Services communicate via REST (synchronous) or message queues (asynchronous)
• Eureka provides service registration and discovery — no hardcoded URLs
• An API Gateway is the single entry point that routes requests to correct services
• Spring Cloud extends Spring Boot with tools built specifically for microservices