Explain and implementation with Tcl for CGI / FastCGI / SCGI / WSGI protocols.
CGI = Common Gateway Interface
CGI make it possible to provide dynamic response through separate process.
Web server pass HTTP headers to CGI process through %ENV - environment variables.
FastCGI appeared in mid-1990 to solve the performance issue of CGI that need spawn process for each request.
FastCGI do it by start CGI process once and use a stream to accept and serve multiple request.
SCGI = SimpleCGI = Simple Common Gateway Interface
SCGI is similar as FastCGI to use long-running process to serve mutiple request.
It's designed to be easier to parse. SCGI appeared in 2001.
WSGI = WEb Server Gateway Interface
WSGI appeared to define an interface for Python based web applications.