404 error in spring cloud gateway for every alternate request

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06-10-2024

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Spring Cloud Gateway: Why Every Other Request Gets a 404

The Problem: A 404 Every Other Time?

Imagine you're building a microservices architecture with Spring Cloud Gateway as your API gateway. You've meticulously configured routes, but for some strange reason, every other request results in a 404 error. This frustrating inconsistency can leave you scratching your head and wondering what's gone wrong.

Scenario: The Code and the Mystery

Let's assume you have a basic Spring Cloud Gateway setup with a simple route definition:

@Bean
public RouteLocator customRouteLocator(RouteLocatorBuilder builder) {
  return builder.routes()
    .route(r -> r.path("/api/**").uri("lb://my-service"))
    .build();
}

This code defines a route that proxies requests to the /api path to a service named my-service. However, you're observing a frustrating pattern:

• Request 1: Successful, reaching your service.
• Request 2: Returns a 404 Not Found.
• Request 3: Successful.
• Request 4: 404 Not Found.

What's going on here?

The Root of the Problem: Asynchronous Behaviors

The culprit behind this issue is often the asynchronous nature of Spring Cloud Gateway's internal workings. When a request hits the gateway, it's handled by a reactive pipeline. This pipeline can involve multiple asynchronous operations:

1. Filtering: Pre- and post-filters might execute before and after the request is routed.
2. Routing: Determining the appropriate route based on the path, headers, etc.
3. Load Balancing: Selecting the correct instance of your service for the request.
4. Proxy: Forwarding the request to the target service.

The Problem: The Gateway's reactive components might work too quickly, leading to a race condition. A new request can arrive at the gateway before the previous request's response is fully processed. This can result in the new request being handled before the route or service is properly configured, leading to the 404 error.

The Solution: Synchronization is Key

To prevent this race condition, we need to ensure that the gateway properly handles one request at a time. Here's how:

1. Increase Gateway Thread Pool Size: Consider increasing the size of the gateway's thread pool by configuring server.reactor.netty.tcp.maxConnections and server.reactor.netty.tcp.maxPendingConnectionAcquires in your application's application.properties file. This will improve its capacity to handle concurrent requests.

2. Synchronous Execution: For more complex situations, consider using Mono.block() within your route definitions. While this isn't the most elegant solution, it provides a more synchronous approach. It's important to use Mono.block() sparingly, as it can impact performance and should be used only when necessary.

3. Optimize Your Application: Ensure your backend service (my-service in our example) is optimized to handle requests efficiently. Slow responses from your service can contribute to the asynchronous race condition.

Additional Tips:

• Logging: Enable detailed logging in Spring Cloud Gateway (adjust the logging level) to pinpoint the exact error.
• Debugging: Step through the gateway code using a debugger to analyze the execution flow.
• Monitor: Utilize monitoring tools to track response times and error rates for a better understanding of your system's performance.

Conclusion:

404 errors in Spring Cloud Gateway can often be attributed to asynchronous processing. By understanding the root cause and taking steps to synchronize the flow of requests, you can overcome this hurdle and achieve a more stable and predictable API gateway. Remember to prioritize efficient code, proper configuration, and continuous monitoring to ensure a seamless microservice experience.