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 1/1/2017. I had vm:true before and now was forced to change the env to flex. Now I am getting 502 bad gateway error with nginx. App engine doesn't allow us to change the nginx config file.

I had tried the suggestion from this post: Google App Engine 502 (Bad Gateway) with NodeJS but still doesn't work.

For some reason, I have another app with exactly the same setting on app engine and it works perfectly.

Any suggestion will be greatly appreciated. Thank you.

Solution 1

app should always listen to port 8080, google forwards all request from 80 to 8080 https://cloud.google.com/appengine/docs/flexible/custom-runtimes/build#listen_to_port_8080

Solution 2

check out the logs for any deployment errors

$ gcloud app logs read

I have came across a similar issue with the code provided by this tutorial (https://cloud.google.com/nodejs/getting-started/authenticate-users)

And found there was a missing dependency. I fixed the missing dependency and the app is deployed and working fine.

Details into the issue: https://github.com/GoogleCloudPlatform/nodejs-getting-started/issues/106

Solution 3

I had the same problem with Express. What solved it for me was to not provide an IP address for the app.

So my old code would be:

var ip = "127.0.0.1";
var port = "8080";
var server = http.createServer(app);
server.listen(port, ip);

This would result in a 502 in app engine.

Removing the ip was the solution for me.

server.listen(port);

Solution 4

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!!! ???

Solution 5

Set the host to 0.0.0.0

Port 8080 is set by default by the engine. In fact, you are not able to define the environment var PORT as it is reserved.

Run the next command (as mentioned by @sravan )

gcloud app logs read tail

and make sure it looks like this,

[Sun May 27 2018 10:32:44 GMT+0000 (UTC)] serving app on 0.0.0.0:8080

Cheers

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Comments

[MarsAndBack]

Another thing to look at is whether SSL is setup on gcloud. I'm developing on localhost with a self-signed certificate to enable HTTPS development (to use Service Workers). After deploying to gcloud, check your appengine settings for SSL.

[pr-]

Led me to the my problem as well. Ended up being that I forgot to define a requirements.txt file. My error log showed "ModuleNotFoundError: No module named 'flask'"

[Aseem]

I was using psycopg2 in my django code which wasnt mentioned in my requirements.txt file. Found it out when I read logs.

[Christophe Chenel]

For a better debug experience in the cloud : cloud.google.com/appengine/docs/flexible/nodejs/…

[wandesky]

Even though my ports were correctly set up, the command gcloud app logs read proved to be a live saver for me.