Custom Filters in Spring Cloud

Filters in Spring Cloud Gateway allow you to modify requests and responses as they pass through the gateway. You can create custom filters for use cases like logging, authentication, or request transformation.

Creating a Custom Gateway Filter

Below is an example of a custom filter that logs request details:

@Component
public class LoggingFilter implements GatewayFilter {

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        System.out.println("Request Path: " + exchange.getRequest().getPath());
        return chain.filter(exchange);
    }
}

Register the filter in your application configuration:

spring:
  cloud:
    gateway:
      routes:
        - id: custom-filter-route
          uri: http://example.com
          filters:
            - name: LoggingFilter

Load Balancing with Spring Cloud LoadBalancer

Spring Cloud LoadBalancer provides client-side load balancing for distributing requests across multiple instances of a service.

Setting Up Load Balancing

Add the necessary dependency:

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

Configure the service instances in your application properties:

spring:
  cloud:
    loadbalancer:
      hint:
        service-name:
          instances: 
            - http://localhost:8081
            - http://localhost:8082

Use the load balancer to make requests:

@RestController
public class ClientController {

    private final WebClient.Builder webClientBuilder;

    public ClientController(WebClient.Builder webClientBuilder) {
        this.webClientBuilder = webClientBuilder;
    }

    @GetMapping("/call-service")
    public String callService() {
        return webClientBuilder.build()
                .get()
                .uri("http://service-name/api/data")
                .retrieve()
                .bodyToMono(String.class)
                .block();
    }
}

Best Practices

• Use filters judiciously: Avoid overloading filters with complex logic; keep them focused on specific tasks.
• Monitor load balancer performance: Use observability tools like Prometheus to track load distribution.
• Optimize retry logic: Implement retries and timeouts in load balancers to handle transient failures.

Conclusion

Spring Cloud's custom filters and load balancing capabilities empower developers to optimize request handling and workload distribution in microservices. By leveraging these features effectively, you can enhance the performance, reliability, and scalability of your applications.