Скачать или смотреть Resolving the 502 Bad Gateway Error from Your AWS Load Balancer

Discover why your AWS Load Balancer returns a `502 Bad Gateway` error on the first request and succeeds on subsequent requests. Here’s how to troubleshoot and resolve the issue.
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This video is based on the question https://stackoverflow.com/q/77047782/ asked by the user 'Ali Siddiqui' ( https://stackoverflow.com/u/22506171/ ) and on the answer https://stackoverflow.com/a/77047819/ provided by the user 'alegria' ( https://stackoverflow.com/u/7156098/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the 502 Bad Gateway Error in AWS Load Balancer

Dealing with the 502 Bad Gateway error can be frustrating, especially when it occurs sporadically, such as only on the first request. In this post, we’ll explore why this error occurs when using an Application Load Balancer (ALB) with a Spring Boot application on an Amazon EC2 instance, and provide a methodical solution to get your application running smoothly.

The Problem at a Glance

If your architecture consists of:

A ReactJS frontend, hosted on a S3 bucket and accessible via CloudFront.

A Spring Boot backend, running on an EC2 instance.

You might encounter the issue where the first API request sends a 502 Bad Gateway response, while the second request succeeds with a 200 OK. This pattern continues alternating between these two responses.

Why This Happens

When you send a request through an AWS Application Load Balancer, it might route your request to one of the registered targets (your EC2 instances). If one of the EC2 instances is unhealthy or fails to respond correctly to the ALB's health checks, you would ultimately receive a 502 Bad Gateway error on the first request, as it's likely being routed to the unhealthy instance.

Troubleshooting Steps

To effectively troubleshoot and resolve this issue, follow these steps:

1. Bypass the Load Balancer

Direct Invocation: Start by sending requests directly to your EC2 instance’s IP address or hostname, completely bypassing the Load Balancer.

Use tools like Postman or curl to test the backend directly.

This will help you determine if the issue lies within your application or if it’s related to the Load Balancer itself.

2. Check EC2 Instance Status

Health Check: Ensure both your target instances are operational and responding to requests.

Logging into the EC2 instance and checking the application logs can help identify if there's an initialization error or if the instance is not running as expected.

3. Review Application Configuration

Increase Connection Timeout: Since you've added the following properties:

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Ensure they are correctly configured and being utilized by your Spring Boot application.

Warm-Up Time: Sometimes, applications require "warm-up" time after startup. Ensure your application has fully initialized before the Load Balancer routes traffic to it.

4. Adjust Load Balancer Settings

Health Checks: Confirm that ALB health checks are configured properly.

The health check path and response codes should be aligned with what your application sends during initialization.

Load Balancer Target Group: Check the target group settings in the AWS management console, ensuring your instances are healthy.

5. Monitor Logs and Metrics

CloudWatch Metrics: Monitor AWS CloudWatch for any anomalies or metrics that indicate issues with EC2 instances or Load Balancer performance.

Application Logs: Review application logs to see if there are any errors recorded at the time of the 502 response.

Conclusion

Experiencing a 502 Bad Gateway error can disrupt the user experience for your application. By systematically bypassing the Load Balancer and investigating your EC2 instances, you can effectively isolate and resolve the issue.

The key takeaway is to ensure both the Load Balancer and backend instances are healthy, configured correctly, and capable of handling the incoming requests consistently. Following these troubleshooting steps should help ensure a smoother and more reliable application performance in your AWS environment.