EventSource / Server-Sent Events through Nginx

Your Nginx config is correct, you just miss few lines.

Here is a "magic trio" making EventSource working through Nginx:

proxy_set_header Connection '';
proxy_http_version 1.1;
chunked_transfer_encoding off;

Place them into location section and it should work.

You may also need to add

proxy_buffering off;
proxy_cache off;

That's not an official way of doing it.

I ended up with this by "trial and errors" + "googling" :)

Another option is to include in your response a 'X-Accel-Buffering' header with value 'no'. Nginx treats it specially, see http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_buffering

Don't write this from scratch yourself. Nginx is a wonderful evented server and has modules that will handle SSE for you without any performance degradation of your upstream server.

Check out https://github.com/wandenberg/nginx-push-stream-module

The way it works is the subscriber (browser using SSE) connects to Nginx, and the connection stops there. The publisher (your server behind Nginx) will send a POST to Nginx at a corresponding route and in that moment Nginx will immediately forward to the waiting EventSource listener in the browser.

This method is much more scalable than having your ruby webserver handle these "long-polling" SSE connections.