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HTTP vs FastCGI quick comparision

HTTP

Pros:
HTTP, easy to debug
Also works directly behind load-balancers (without frontends)
Many PSGI server implementations, preforking Starman, Starlet and non-blocking Twiggy

Cons:
Need to configure X-Forwarded-* headers both on fronend and backend
Keep-alive can be problematic to starve workers (e.g. mod_proxy + mpm_prefork)

FastCGI:

Pros:
Potentially faster protocol because of binary
Sysadmin friendly (errors usually go to the web server's logs)

Cons:
No major server supports persisten FastCGI connections
Needs some tweaks to pass SCRIPT_NAME etc. correctly in nginx/lighttpd
Many broken web server implementations such as mod_fcgid or earlier version of lighttpd
FCGI.pm requires C/XS, FCGI::ProcManager options are confusing (hence fastpass and Net::FastCGI)
Less backend implementations, especially the one with proper preforking and non-blocking options