How do I debug a broken deployment on a DigitalOcean Droplet when I have no idea what went wrong?

Question

I deployed a new version of my app to my Droplet and now it is not working. The site is returning a 502 Bad Gateway. I am not very experienced with Linux and I am not sure where to even start looking.

I have SSH access to the Droplet but when I look at the logs I am not sure what I am reading. What is the standard process for diagnosing a broken deployment on a Linux server?

Bobby · Answer

Hi there, 502 Bad Gateway means Nginx is running but cannot reach your app. The app process is either crashed, not running, or listening on the wrong port. Here is how to diagnose it step by step. First check if your app is actually running: pm2 status # or if you are using systemd systemctl status your-app If it is stopped or errored, check the logs: pm2 logs your-app --lines 50 # or journalctl -u your-app --since "30 minutes ago" Check if something is actually listening on the port your Nginx config expects: ss -tlnp | grep 3000 If nothing is listening there, your app is not running. Start it and the 502 should clear. If the app is running but Nginx still returns 502, check the Nginx error log: tail -50 /var/log/nginx/error.log That will usually tell you exactly what is failing, connection refused, upstream timeout, and so on. Also verify your Nginx config is valid after any recent changes: sudo nginx -t sudo systemctl reload nginx If you are still getting comfortable with Linux commands in general, this interactive terminal simulator lets you practice in a safe environment without touching your real server: https://devops-daily.com/games/linux-terminal DigitalOcean also has a monitoring agent worth installing if you have not already, it gives you CPU, memory, and disk graphs over time which helps correlate what happened around the time of a broken deploy: https://docs.digitalocean.com/products/monitoring/

alexdo · Answer

Hi there, If you go through all of those and the app’s own logs show nothing useful, or the process just stops with no error message at all, it is worth checking whether the kernel killed it rather than the app crashing on its own. Run: dmesg | grep -i kill If you see a line mentioning your app process and “Out of memory”, that is the OOM killer, and it explains why there is nothing informative in the app’s own logs. The process did not get a chance to log an error, it was just terminated. This is a common one right after a deploy specifically, since a build step, a new dependency, or just a slightly larger version of the app can push memory usage over what a smaller Droplet has available. If that is what happened, the standard fix on a Droplet is to add a swap file, since Droplets do not ship with swap configured by default. DigitalOcean’s guide on adding swap space on Ubuntu walks through creating and enabling a swap file with sizing guidance based on your Droplet’s RAM, roughly twice your RAM if you are under 2 GB, or matching it one to one between 2 and 8 GB. It will not fix a genuine memory leak, but it gives the system headroom to survive a deploy or a temporary spike instead of getting killed outright. Also worth a quick df -h while you are in there, since a full disk during a deploy can produce similarly confusing silent failures that will not show up in the app’s logs either. Regards

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How do I debug a broken deployment on a DigitalOcean Droplet when I have no idea what went wrong?

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