AWS Load Balancer 502 Bad Gateway - node.js

Question

I have microservices written in node/express hosted on EC2 with an application load balancer.
Some users are getting a 502 even before the request reaches the server.
I register every log inside each instance, and I don't have the logs of those requests, I have the request immediately before the 502, and the requests right after the 502, that's why I am assuming that the request never reaches the servers. Most users solve this by refreshing the page or using an anonymous tab, which makes the connection to a different machine (we have 6).
I can tell from the load balancer logs that the load balancer responds almost immediately to the request with 502. I guess that this could be a TCP RST.
I had a similar problem a long time ago, and I had to add keepAliveTimeout and headersTimeout to the node configuration. Here are my settings (still using the LB default of the 60s):
server.keepAliveTimeout = 65000;
server.headersTimeout = 80000;
The metrics, especially memory and CPU usage of all instances are fine.
These 502 errors started after an update we made where we introduced several packages, for instance, axios. At first, I thought it could be axios, because the keep-alive is not enabled by default. But it didn't work. Other than the axios, we just use the request.
Any tips on how should I debug/fix this issue?

Answer 1

HTTP 502 errors are usually caused by a problem with the load balancer. Which would explain why the requests are never reaching your server, presumably because the load balancer can't reach the server for some or other reason.
This link has some hints regarding how to get logs from a classic load balancer. However, since you didn't specify, you might be using an application load balancer, in which case this link might be more useful.

Answer 2

From the ALB access logs I knew that either the ALB couldn't connect the target or the connection was being immediately terminated by the target.
And the most difficult part was figure out how to replicate the 502 error.
It looks like the node version I was using has a request header size limit of 8kb. If any request exceeded that limit, the target would reject the connection, and the ALB would return a 502 error.
Solution:
I solved the issue by adding --max-http-header-size=size to the node start command line, where size is a value greater than 8kb.

Answer 3

A few common reasons for an AWS Load Balancer 502 Bad Gateway:
Be sure to have your public subnets (that your ALB is targeting) are set to auto-assign a public IP (so that instances deployed are auto-assigned a public IP).
Security group for your alb allows http and/or https traffic from the IPs that you are connecting from.

Answer 4

I was also Having the same problem from 1 or 2 months something like that and I didn't found the solution. And I was also having AWS Premium support but they were also not able to find the solution. I was getting 502 Error randomly loke may be 10 times per day. Finally after reading the docs from AWS
The target receives the request and starts to process it, but closes the connection to the load balancer too early. This usually occurs when the duration of the keep-alive timeout for the target is shorter than the idle timeout value of the load balancer.
https://aws.amazon.com/premiumsupport/knowledge-center/elb-alb-troubleshoot-502-errors/
SOLUTION:
I was running "Apache" webserver in EC2 so Increased "KEEPALIVETIMEOUT=65". This did the trick. For me.