If you have a public url for a file, then you can just point your users to that url and use sendfile=on. Nginx will then use sendfile.

But consider the usecase where you have a bigger download hidden behind some kind of authentication. You'd do that authentication in your normal backend and return the X-Accel-Redirect header in your response. Nginx will then take over and send the data using sendfile.

You are correct! There is a simple sendfile in the http core module.

http://nginx.org/en/docs/http/ngx_http_core_module.html#send...

I linked to the "harder to grok" piece because that's what would give you programmatic control by setting an X header. That's as opposed to calling sendfile() yourself from your app. Helpful if you're trying to do something more than just serve up static files...like maybe in a page cache, where you also want dynamic control over other headers, etc.

The simpler sendfile=on would just be instructing nginx to use it for static files.