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I am running an Ubuntu server (DigitalOcean droplet) with 2 services. React (Create React App) frontend in PORT 3000 and Node.js backend/API in PORT 8765 with PM2(pm2.keymetrics.io).

Screenshot of PM2 running 2 services

The ports are open (3000 and 8765). I checked.

lsof command showing the open ports

Problem: When the frontend app (in my browser) tries to access the backend it returns the '503 Service Unavailable' status code.

Screeshot of the browser developer tools showing 503

Question: What could be the reason? Can you suggest any steps to try?

Note: It worked fine during the past few weeks. (I used to pull the new changes from the Bitbucket) But today I got this issue.

What I have tried so far

  • Restart the server. Same error.
  • Change the Backend port from 9000 to 8765
  • Run the same code in a fresh AWS EC2 instance. There it works fine.
  • Force to listen on IPv4 (based on @adel comment) Screenshot using IPv4
Praveen Dias
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  • based on the lsof command you are using ipv6, use app.listen(8765, "0.0.0.0") to force node.js to use ipv4 – adel Apr 10 '22 at 13:48
  • @adel now app listens ipv4, but still the problem exists – Praveen Dias Apr 10 '22 at 17:05
  • If you are not using ipv6 try disabling it on your EC2 instance then restart all your services – adel Apr 11 '22 at 09:48
  • @adel The problem happens on my clients' server (which is a DigitalOcean droplet). I personally tried the same code on an EC2 instance for testing (On EC2 it works fine). I updated the question. I should try this option too. – Praveen Dias Apr 11 '22 at 14:52
  • are you sure that you disabled firewall or added the need rules. – adel Apr 13 '22 at 07:10
  • Yes, I did. But the problem was something else. I explained it as an answer. – Praveen Dias Apr 16 '22 at 04:01

1 Answers1

2

I called the backend (using the frontend) multiple times and observed the running processes with the Ubuntu 'top' utility. See the below screenshot.

enter image description here

I noted that the process id for the backend node.js application process is 12225. And this process is repeatedly called even though '503 Service Unavailable' is returned.

It turned out that the backend application code returns this status code programmatically (with a try-catch block, whenever the error occurs)

try{
 ...
}
catch (e) {
 res.send(503, ...)
}

I struggled this with for a few days. Hope this will help someone.

(with this finding the new problem is why the particular error happens on a particular server only. It works fine on EC2 instance. I think it is a different question.)

Praveen Dias
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