Filters in an API Gateway are powerful tools that help you control and manage how requests and responses move through your system. When a client sends a request to your API, filters can inspect, modify, or even block that request before it reaches your backend services. Similarly, filters can change the response before it goes back to the client.

You use filters in an API Gateway for tasks such as adding authentication, logging requests, handling errors, or rewriting URLs. This helps keep your backend services simpler and more secure, since the gateway handles these cross-cutting concerns. Filters make your API Gateway flexible, allowing you to add new features or rules without changing your backend code.

How Filters Work in a Spring Boot API Gateway

Filters in a Spring Boot API Gateway act like checkpoints that process requests and responses as they pass through the gateway. When a client sends a request, filters can inspect, modify, or even block the request before it reaches your backend services. Similarly, filters can change the response before it is sent back to the client.

Common Uses for Filters

• Add or check authentication information;
• Log details about requests and responses;
• Modify request headers or response headers;
• Handle errors or return custom error messages;
• Limit how many requests a client can make.

Basic Example

Suppose you want to log every request that comes through your API Gateway. You can set up a filter that prints the request path:

@Component
public class LoggingFilter implements GatewayFilter, Ordered {
    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        System.out.println("Request path: " + exchange.getRequest().getPath());
        return chain.filter(exchange);
    }

    @Override
    public int getOrder() {
        return 0;
    }
}

This filter prints the path of each incoming request. You can also use filters to add a custom header, check for a valid token, or handle errors in a central place. Filters help you manage cross-cutting concerns without changing your backend services.