I am hosting my Rails app on Rackspace with nginx webserver.
When calling any Rails API, I see this message in /var/log/nginx/error.log: *49 connect() failed (111: Connection refused) while connecting to upstream, client: 10.189.254.5, server: , request: "POST /api/v1/users/sign_in HTTP/1.1", upstream: "http://127.0.0.1:3001/api/v1/users/sign_in", host: "anthemapp.com"
I spent several hours with 5-6 different Rackspace tech people (they didn't know how to resolve this). This all started when I took the server into rescue mode and followed the steps here: https://community.rackspace.com/products/f/25/t/69. Once I came out of rescue mode and rebooted the server, I started receiving the error I am writing about. Tnx!
Nginx is a reverse proxy server - its role on your server is to accept HTTP requests and proxy them to another process on the same host. The "upstream" that the error message is talking about is referring to the bit in nginx's configuration (part of which is the /etc/nginx/sites-available/default file) that tells it where to send incoming requests. The error message you're seeing indicates that nginx received a request, but couldn't send it to the other process it was supposed to.
When your server rebooted, the nginx process started back up, but your Rails process -- the one that's meant to be listening on port 3001 -- did not!
How you restart the Rails process depends on the way that you started it before and the way your server is configured. It may be as simple as cd'ing into your Rails application's directory on the server and running:
rails server -b 127.0.0.1 -p 3001 -e production -d
...but, to prevent problems like this from happening in the future (and to improve the performance of your Rails app!), it would be better to use some kind of production-ready Rails application server. I would recommend using Phusion Passenger because it's the most turn-key solution -- their user's guide for nginx describes installation and configuration -- but there are plenty of alternatives. There's a great writeup of what your options are, what they all mean, and how they relate on the top answer of this StackOverflow question.