To give some examples what I mean:
- Apache can be a FastCGI server (mod_fcgid) and proxy (mod_proxy_fcgi) - Nginx (and most other Webservers I checked) is only a FastCGI proxy
Wikipedia [1] lists both as "Web servers that implement FastCGI". For me it took some time to recognize the difference whether a server speaks the FastCGI protocol or can also host/spawn FastCgi supporting applications which do not daemonize by themself.
For the (probably) most used FastCGI application PHP this is easy because php-fpm is both a FastCGI server and application.
An example for a (Fast)CGI application which is not also a server is MapServer [2] which is listed by Wikipedia as CGI program and by its own documentation with "FastCGI support" or "FastCGI-enabled".
The fact that it is only a FastCGI application and needs an additional FastCGI server to host it is (in my opinion) not clearly communicated.
A common tutorial to combine MapServer with Nginx is to use fcgiwrap or spawn-fcgi as FastCGI server.
Since a FastCGI application can usually also act as CGI application it is easy to miss that the first option is running the application as CGI and only the second as FastCGI where one instance serves more than one request.
For me the main difference is that a FastCGI server will open a socket and can spawn worker processes (which php-fpm does and MapServer does not) and a FastCGI application (the worker process) will accept and handle connections/requests.
A FastCGI proxy translates requests (usually from HTTP to FastCGI protocol) and redirects them to a FastCGI server. It does not spawn processes like Apaches mod_fcgid does.
Due to the simplicity it should be even possible to use systemd as simple FastCGI server. [3] (I have never tried it)
My recommendation if you ever come across an FastCGI application which cannot open a socket / daemonize by itself use Apache mod_fcgid. Even if you already use another Webserver to handle HTTP. From all options I checked it has by far the most useful options to dynamically limit the number of worker processes and respawn them from time to time.
[1] https://en.wikipedia.org/wiki/FastCGI [2] https://en.wikipedia.org/wiki/MapServer [3] https://nileshgr.com/2016/07/09/systemd-fastcgi-multiple-pro...