Question about Application Load Balancer and 504 Error Codes

Question

Good afternoon, dear all!

Currently, I have a question regarding the following scenario:

When making several requests that are being directed by an Application Load Balancer, I have noticed that some of them are returning the 504 error code, even though everything functions normally on other occasions. Initially, we had 40 subdomains within the load balancer, but I removed 13 of them and transferred those rules to another Application Load Balancer. Since then, the requests have normalized.

Could this be related to the decision time that the Load Balancer needs to choose one of the subdomain rules and also to the request load per second?

Thank you,

Riku_Kobayashi · Answer

Hello.

> also to the request load per second?

This occurs not only on ALB but also when there is no response on the target side.  
Therefore, you need to check the ALB logs and target web server logs to determine how far the request was received.  
So, as you know, this can occur if the timeout value on the ALB side is exceeded when executing a process with a high load.

The following document is about troubleshooting when a 504 error occurs in ALB.  
https://repost.aws/knowledge-center/504-error-alb

Question about Application Load Balancer and 504 Error Codes

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