Reverse proxy and 502

nginx stands between you and the application. This step shows where the 502 comes from.

nginx does not keep the shop's pages. It works as a reverse proxy: it takes the client's request, sends its own request to the application and passes the application's answer back to the client. The application behind a proxy is called the upstream. The request's path splits into two legs — client → nginx and nginx → application — each with a connection of its own.

The location block inside server says where to send the requests:

server {
    listen 80;
    server_name api.example.test;
    location / {
        proxy_pass http://127.0.0.1:3000;
    }
}

location / covers the paths that start with /, so any path. proxy_pass is the application's address: nginx will connect to 127.0.0.1, port 3000.

A 502 Bad Gateway answer means the first leg worked: nginx got the request and answered it itself. It got no answer from the application — the second leg broke. nginx writes why to error.log, in a line with the words while connecting to upstream.