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Explanation

Load balancing distributes incoming requests across multiple instances of the same microservice. This helps improve performance, availability, and scalability.

Code:

Client
   ↓
API Gateway
   ↓
┌───────────────┐
│ Load Balancer │
└───────┬───────┘
        │
   ┌────┼────┐
   ↓    ↓    ↓
User  User  User
:8081 :8082 :8083
``

Explanation

Without load balancing, all requests may go to a single service instance. If that instance becomes overloaded or unavailable, the application can be affected.

Code:

Client
  ↓
User Service :8081
  ↑
All Requests

Problem:
Single instance can become overloaded.

Explanation

With Spring Cloud LoadBalancer, requests can be distributed across multiple instances of a service. Eureka can provide the available service instances.

Code:

                 Eureka
                    │
        ┌───────────┼───────────┐
        ▼           ▼           ▼
   USER-SERVICE USER-SERVICE USER-SERVICE
      :8081        :8082        :8083
        \           │           /
         └────── Load Balancer ──────┘
                    ↑
                    │
                 Gateway

Explanation

Spring Cloud Gateway can use the lb:// prefix to route requests through Spring Cloud LoadBalancer. The service name is resolved through service discovery such as Eureka.

Code:

spring:
  cloud:
    gateway:
      routes:
        - id: user-service
          uri: lb://USER-SERVICE
          predicates:
            - Path=/api/users/**

Explanation

When multiple instances of USER-SERVICE are registered, the load balancer selects an available instance for each request. This allows the service to scale horizontally.

Code:

Request 1 → USER-SERVICE :8081
Request 2 → USER-SERVICE :8082
Request 3 → USER-SERVICE :8083
Request 4 → USER-SERVICE :8081

Multiple instances
        ↓
Better scalability

Explanation

Load balancing improves availability because requests can be sent to other healthy instances when one instance becomes unavailable. Service discovery and load balancing work together in a typical Spring Cloud architecture.

Code:

React
  ↓
API Gateway
  ↓
Load Balancer
  ↓
Eureka
  ↓
┌────────┬────────┬────────┐
│ :8081  │ :8082  │ :8083  │
│  UP ✓  │ DOWN ✗ │  UP ✓  │
└────────┴────────┴────────┘

Requests → Healthy instances