502 Bad Gateway with nginx | Google App Engine | Node JS

Question

I am hosting the web app on Google Cloud Platform with App Engine and I am using ExpressJS and MongoDB, which is hosted on mLab.

Everything worked well until 1/1/201

Answer 1

Google App Engine uses an nginx front to load balance all requests for node.js apps. With nginx acting as a forward proxy, this error usually happens when the request the user is making in the browser is reaching nginx (you see the unstyled 502 bad gateway error page) but the nginx server is not able to correctly forward the request to your node app. There could be many issues why this is happening but here are some common ones:

1. By default, App Engine assumes your node app is running on 8080. nginx itself will run on 80 and forward the request to 8080. Check if your app's port number is 8080.

2. You app may have a hostname defined like a domain something.appspot.com or an IP 127.18.21.21 or the like. Remove any hostnames from your server.listen or config.json or vhost wherever. App Engine will take care of domains, IPs etc so you dont have to.

3. Your app may be crashing before its sending a response to nginx. Check the logs of both nginx AND your node app.

To check logs / find out what is going on use this guide https://cloud.google.com/appengine/docs/flexible/nodejs/debugging-an-instance#connecting_to_the_instance to SSH directly inside the VM behind app engine. There will be one docker process with nginx where you can see the nginx error log and one docker image with your node app to check your node app's error message.

I'm just wondering, based on the activity in this question and the timestamps, why hasn't Google updated its documentation to cover this issue!!! ???