ITT YOU MAKE THE CLAIM RUBY CAN DO EVERYTHING

BECAUSE YOU CAN ALWAYS CALL C CODE TO DO IT RIGHT. AM I RITE?

I AM

To oversimplify things a bit, mod_perl/php/python eliminate the overhead of loading and forking a new process each time a request is made. Relative to what the process itself does, this overhead is usually gigantic.

FastCGI does the same thing, except instead of embedding an interpreter in the httpd daemon, it spawns a seperate persistent process that doesn't terminate. The same process sits there, accepting requests and returning data.

Performance-wise, there isn't a big difference between mod_* and FastCGI, but a FastCGI process won't affect the stability of the httpd daemon (particularly useful if you're not using the MPM prefork), and it scales better (distribute processes across multiple machines). The downside is that things like mod_perl give you incredible access to Apache's innards.