fcgi vs mod_fastcgi on apache server

Indeed, mod_fastcgi does not support multiplexing. I suppose this is because the Apache web server handles concurrent processing itself. You've probably dealt with it's various Multi-Processing-Models (MPMs) already...

Apache is highly optimized around the several (request) phases provided. The various modules can hook in where-ever you like, which makes the Apache an excellent server to directly integrate high performance and/or really complex applications (e.g. with custom modules in c, mod_perl and so on) as modules themselves.But both, mod_fastcgi and cgi-fcgi, are IMHO only used to provide response and/or filter handler. Thus; many of the great features (configuration, mapping, post-request logging & cleanup...) provided with Apache are just not used in such a setup.

Thus; if your application is built on top of FGCI, I'd rather not recommend using Apache. Especially for high performance applications under high load; One may prefer a more lightweight but fast HTTP daemon. There are plenty of alternatives like nginx or lighttpd.Usually one would use them as proxies/balancer to the FCGI processes, cache, SSL handler and logging provider. Of course, Apache is also capable of these tasks, but it's somehow like using a helicopter to direct the traffic at the intersection...

Cheers!