API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) · Lesson

Pre/Post Processing with Filters

Apply filters for actions before (pre) a request reaches the service and after (post) the service responds.

Pre/Post Processing with Filters is a free API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Intro to Pre/Post Filters

Welcome! In this lesson, we'll explore how Spring Cloud Gateway filters can process requests before they reach a backend service (pre-processing) and after the service responds (post-processing).

This fine-grained control is crucial for handling complex API gateway tasks.

The Filter Chain Revisited

Recall that filters operate in a chain. Each GatewayFilter or GlobalFilter can perform actions:

Before calling chain.filter(exchange): This is pre-processing.
After chain.filter(exchange) returns a Mono<Void>: This is post-processing.

The reactive nature of Spring Cloud Gateway, using Mono, makes this sequential execution powerful.

Pre-Processing: Before the Service

Pre-processing filters execute before the gateway forwards the request to your actual backend service. Think of them as interceptors for incoming requests.

Common pre-processing tasks include:

Authenticating and authorizing requests
Adding or modifying request headers
Rewriting the URL path or query parameters
Implementing rate limiting

Pre-Filter Example: Add Header

Let's look at a simple pre-processing example: adding a custom header to the request before it reaches the service. Spring Cloud Gateway provides built-in filters like AddRequestHeader for this.

This configuration adds an X-Request-Source header with the value gateway to all requests matching the path /api/pre/**.

spring:
  cloud:
    gateway:
      routes:
        - id: pre_header_route
          uri: http://localhost:8080/service/echo
          predicates:
            - Path=/api/pre/**
          filters:
            - AddRequestHeader=X-Request-Source, gateway

Runnable Pre-Filter Code

Try running this full Spring Boot application. It sets up a gateway that adds a request header and routes to a simple echo endpoint within the same application. Check the backend's console or response to see the added header!

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.gateway.route.RouteLocator;
import org.springframework.cloud.gateway.route.builder.RouteLocatorBuilder;
import org.springframework.context.annotation.Bean;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestHeader;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication
@RestController
public class GatewayApp {

  public static void main(String[] args) {
    SpringApplication.run(GatewayApp.class, args);
  }

  // Dummy backend service to echo headers
  @GetMapping("/service/echo")
  public String echoHeaders(@RequestHeader(required = false) java.util.Map<String, String> headers) {
    return "Backend received headers: " + headers.get("x-request-source");
  }

  @Bean
  public RouteLocator customRouteLocator(RouteLocatorBuilder builder) {
    return builder.routes()
      .route("pre_header_route", r -> r.path("/api/pre/**")
        .filters(f -> f.addRequestHeader("X-Request-Source", "gateway"))
        .uri("http://localhost:8080/service/echo")) // Route to its own dummy backend
      .build();
  }
}

Post-Processing: After the Service

Post-processing filters execute after the backend service has responded but before the gateway sends that response back to the client.

This allows you to inspect or modify the backend's response.

Typical post-processing tasks include:

Modifying response headers or body
Logging response details (status, latency)
Adding metrics for response processing
Implementing response caching

Custom Post-Processing Filter

For post-processing, you often need to create custom filters. These filters use the Mono returned by chain.filter(exchange) to add logic that executes upon successful completion.

The .then(Mono.fromRunnable(() -> { ... })) pattern is common for post-processing actions.

Runnable Post-Filter Code

Here's a full Spring Boot application with a custom GlobalFilter that adds a X-Post-Processed header to every response. Notice how the filter logic is chained using .then(Mono.fromRunnable(...)).

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.gateway.filter.GatewayFilterChain;
import org.springframework.cloud.gateway.filter.GlobalFilter;
import org.springframework.cloud.gateway.route.RouteLocator;
import org.springframework.cloud.gateway.route.builder.RouteLocatorBuilder;
import org.springframework.context.annotation.Bean;
import org.springframework.core.Ordered;
import org.springframework.stereotype.Component;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.server.ServerWebExchange;
import reactor.core.publisher.Mono;

@SpringBootApplication
@RestController
public class GatewayApp {

  public static void main(String[] args) {
    SpringApplication.run(GatewayApp.class, args);
  }

  // Dummy backend service
  @GetMapping("/service/data")
  public String getData() {
    return "Backend response data!";
  }

  @Bean
  public RouteLocator customRouteLocator(RouteLocatorBuilder builder) {
    return builder.routes()
      .route("post_filter_route", r -> r.path("/api/post/**")
        .uri("http://localhost:8080/service/data")) // Route to itself
      .build();
  }
}

@Component
class CustomPostFilter implements GlobalFilter, Ordered {

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        return chain.filter(exchange).then(Mono.fromRunnable(() -> {
            // This code runs AFTER the downstream service responds
            exchange.getResponse().getHeaders().add("X-Post-Processed", "true");
            System.out.println("Post-processing: Added X-Post-Processed header.");
        }));
    }

    @Override
    public int getOrder() {
        return -1; // Execute relatively late in the filter chain
    }
}

Pre vs. Post: Key Differences

Understanding when to use pre vs. post processing is key:

Pre-filters: Act on the request before it leaves the gateway. Ideal for security, routing logic, request transformation.
Post-filters: Act on the response after it returns from the backend. Ideal for response transformation, logging response details, metrics collection.

Choose the right stage to efficiently manage your API traffic.

Filter Timing Challenge

Consider a Spring Cloud Gateway setup. Which of these tasks are typically performed by filters before forwarding a request to a backend service?

Recap: Pre/Post Processing

Great job! You've learned about the powerful concept of pre-processing and post-processing with Spring Cloud Gateway filters.

Pre-filters modify requests before they hit the backend.
Post-filters modify responses after the backend has replied.
The reactive Mono chain enables precise control over filter execution timing.

This understanding is vital for building robust and feature-rich API gateways!

Frequently asked questions

Is the “Pre/Post Processing with Filters” lesson free?

Yes — the full text of “Pre/Post Processing with Filters” is free to read here on the web, and the API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) course, upgrade to CoddyKit PRO.

What will I learn in “Pre/Post Processing with Filters”?

Apply filters for actions before (pre) a request reaches the service and after (post) the service responds. You practise API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway)?

No prior experience is required. API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Pre/Post Processing with Filters” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) lesson?

Yes. Every API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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