How Load Balancing Works with Service Discovery in Spring

Load balancing is essential when you have multiple instances of the same service running in your system. Service discovery helps your application find these instances dynamically, so you do not need to hardcode their addresses. Together, load balancing and service discovery ensure that requests are distributed efficiently, improving reliability and scalability.

When a client sends a request, Spring uses service discovery to get a list of available service instances. A load balancer then picks one instance from this list to handle the request. This process is automatic and transparent, so you do not need to manage instance addresses or distribution logic manually.

Why Load Balancing is Needed:

• Prevents any single service instance from being overloaded;
• Improves system resilience by routing around failed instances;
• Enables your system to scale horizontally by simply adding more service instances.

Request Distribution Concept

With load balancing, each request is sent to a different instance based on a specific strategy. The most common strategy is round-robin, where requests are distributed evenly in a circular order. Other strategies include random selection or weighted distribution, but round-robin is the default in Spring Cloud.

Example: Using Spring Cloud LoadBalancer

Suppose you have a client that needs to call a user-service. Instead of specifying the address, you refer to the service by its name. Spring Cloud LoadBalancer works with your service discovery tool (like Eureka or Consul) to resolve the address automatically and distribute the load.

Here is a simple example using RestTemplate and Spring Cloud LoadBalancer:

// Mark RestTemplate as load-balanced
define a RestTemplate bean with @LoadBalanced
@Bean
@LoadBalanced
public RestTemplate restTemplate() {
    return new RestTemplate();
}

// Use service name instead of a hardcoded address
String url = "http://user-service/api/users/1";
User user = restTemplate.getForObject(url, User.class);

When you call restTemplate.getForObject, Spring automatically:

• Queries the service registry for available user-service instances;
• Chooses one instance using the load balancing strategy;
• Sends the request to the selected instance.

This approach allows your system to adapt to changes in the number or location of service instances without any code changes.