1️⃣ What is Load Balancing?

• Distributes incoming requests across multiple instances of a service to:
  • Improve performance
  • Increase availability
• Two main types:
  1. Client-side load balancing → the caller decides which service instance to call (e.g., Ribbon)
  2. Server-side load balancing → a proxy/load balancer (like Nginx or AWS ELB) routes requests

2️⃣ Netflix Ribbon

• Ribbon is a client-side load balancer in Spring Cloud.
• Your microservice queries service registry (Eureka) or manual list of servers, then chooses an instance to call.

Example 1: Ribbon with Eureka

pom.xml

<dependency>
  <groupId>org.springframework.cloud</groupId>
  <artifactId>spring-cloud-starter-netflix-ribbon</artifactId>
</dependency>

Product Service calling Pricing Service

@Bean
@LoadBalanced  // Ribbon intercepts this WebClient / RestTemplate
public RestTemplate restTemplate() {
    return new RestTemplate();
}

@Autowired
private RestTemplate restTemplate;

public String getPrice(String productId) {
    // Ribbon chooses one of the pricing-service instances from Eureka
    return restTemplate.getForObject("<http://pricing-service/price/>" + productId, String.class);
}

Visual Flow:

[Product Service] ---(Ribbon chooses instance)--> [Pricing Service: 8081 or 8082]
      ^                                             ^
      |<-------------Service Registry (Eureka)-----|

Example 2: Ribbon Without Eureka

• You can configure Ribbon with a static list of servers.

application.yml

pricing-service:
  ribbon:
    listOfServers: localhost:8081,localhost:8082