The Simple Common Gateway Interface (SCGI) is a protocol for applications to interface with

HTTP HTTP (Hypertext Transfer Protocol) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web, wher ...

servers, as an alternative to the CGI protocol. It is similar to

FastCGI FastCGI is a binary protocol for interfacing interactive programs with a web server. It is a variation on the earlier Common Gateway Interface (CGI). FastCGI's main aim is to reduce the overhead related to interfacing between web server and CGI pr ...

but is designed to be easier to parse. Unlike CGI, it permits a long-running service process to continue serving requests, thus avoiding delays in responding to requests due to setup overhead (such as connecting to a database). SCGI is a ''protocol'' which defines ''communication'' between a web server and an application server. This is in contrast to CGI, which is an earlier application (gateway) interface designed to let the application programmer avoid the complexity of sockets and long-running service processes when poor scalability and high overhead are acceptable. The SCGI protocol leverages the fact that the web server has already parsed and validated the HTTP request, and canonically communicates the request to the SCGI server while letting the application programmer avoid parsing ambiguities and protocol edge cases. This avoids the complicated header-parsing and header-combining rules from , saving significant complexity in the SCGI server process.

History

Neil Schemenauer published the original SCGI protocol specification dated October 2001. He developed the first implementations of SCGI and initially published them in April 2002.

Specification

The client connects to a SCGI server over a reliable stream protocol allowing transmission of 8-bit bytes. The client begins by sending a request. When the SCGI server sees the end of the request it sends back a response and closes the connection. The format of the response is not specifically specified by this protocol, although CGI-equivalent HTTP responses are generally used.

Request format

A SCGI request is the

concatenation In formal language theory and computer programming, string concatenation is the operation of joining character strings end-to-end. For example, the concatenation of "snow" and "ball" is "snowball". In certain formalizations of concatenati ...

of netstring-encoded headers and a body. A SCGI response is a normal HTTP response. Each header consists of a

name–value pair A name–value pair, also called an attribute–value pair, key–value pair, or field–value pair, is a fundamental data representation in computing systems and applications. Designers often desire an open-ended data structure that allows fo ...

, where both the name and the value are

null-terminated string In computer programming, a null-terminated string is a character string stored as an array containing the characters and terminated with a ''null character'' (a character with an internal value of zero, called "NUL" in this article, not same a ...

s ( C strings). The value can be an

empty string In formal language theory, the empty string, or empty word, is the unique String (computer science), string of length zero. Formal theory Formally, a string is a finite, ordered sequence of character (symbol), characters such as letters, digits ...

, in which case the terminating null still remains. Neither name nor value can contain any embedded null bytes. These considerations are standard for C strings, but are often confusing for programmers used to other standards for string-handling. All provided headers are concatenated to form a single byte sequence, then netstring-encoded. The raw body, if any, is then appended. Duplicate names are not allowed in the request headers; RFC 2616-compliant header combining must already have taken place. The first request header must have the name "CONTENT_LENGTH" and a value that is the length of the body in decimal. The "CONTENT_LENGTH" request header must always be present, even if its value is "0". There must also always be a request header with the name "SCGI" and a value of "1". Standard CGI

environment variable An environment variable is a user-definable value that can affect the way running processes will behave on a computer. Environment variables are part of the environment in which a process runs. For example, a running process can query the va ...

s should be provided in SCGI headers for compatibility when converting older CGI programs to SCGI. The body (if any) provided in the request follows the headers; its length is specified by the "CONTENT_LENGTH" request header. While the SCGI protocol insulates the service programmer from some HTTP considerations, various details (such as interpreting the octets of the message body as per the Transfer-Encoding header, the CONTENT_LENGTH being the number of octets after the body has been encoded for transmission, etc.) still require knowledge of the HTTP protocol specification.

Example

The web server (a SCGI client) opens a connection and sends the concatenation of the following strings to the service process (a SCGI server): "70:" "CONTENT_LENGTH" <00> "27" <00> "SCGI" <00> "1" <00> "REQUEST_METHOD" <00> "POST" <00> "REQUEST_URI" <00> "/deepthought" <00> "," "What is the answer to life?" The SCGI server sends the following response back to the web server: "Status: 200 OK" <0d 0a> "Content-Type: text/plain" <0d 0a> "" <0d 0a> "42" The SCGI server closes the connection.

Web servers that implement SCGI

''(this list is not complete)'' *

Apache HTTP Server The Apache HTTP Server ( ) is a free and open-source software, free and open-source cross-platform web server, released under the terms of Apache License, Apache License 2.0. It is developed and maintained by a community of developers under the ...

Cherokee The Cherokee (; , or ) people are one of the Indigenous peoples of the Southeastern Woodlands of the United States. Prior to the 18th century, they were concentrated in their homelands, in towns along river valleys of what is now southwestern ...

Lighttpd lighttpd (prescribed pronunciation: "lighty") is an open-source web server optimized for speed-critical environments while remaining standards-compliant, secure and flexible. It was originally written by Jan Kneschke as a proof-of-concept of the ...

* Microsoft

Internet Information Services Microsoft IIS (Internet Information Services, IIS, 2S) is an extensible web server created by Microsoft for use with the Windows NT family. IIS supports HTTP, HTTP/2, HTTP/3, HTTPS, FTP, FTPS, SMTP and NNTP. It has been an integral part o ...

wit
ISAPI SCGI extension
*

nginx (pronounced "engine x" , stylized as NGINX or nginx) is a web server that can also be used as a reverse proxy, load balancer, mail proxy and HTTP cache. The software was created by Russian developer Igor Sysoev and publicly released in 20 ...

* Althttpd

Language bindings for the SCGI API

SCGI can be implemented in any language that supports network sockets and netstrings. The following is a partial list of languages with known SCGI bindings: *

Cobra COBRA or Cobra, often stylized as CoBrA, was a European avant-garde art group active from 1948 to 1951. The name was coined in 1948 by Christian Dotremont from the initials of the members' home countries' capital cities: Copenhagen (Co), Brussels ...

* D, with th
arsd.cgi
library *

Emacs Lisp Emacs Lisp is a Lisp dialect made for Emacs. It is used for implementing most of the editing functionality built into Emacs, the remainder being written in C, as is the Lisp interpreter. Emacs Lisp code is used to modify, extend and customi ...

, with th
url-scgi
library * Go, with th
scgi
package *

Haskell Haskell () is a general-purpose, statically typed, purely functional programming language with type inference and lazy evaluation. Designed for teaching, research, and industrial applications, Haskell pioneered several programming language ...

Java Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...

, with th
SCGI connector
or with th

library * Lisp (programming language), Lisp *

Perl Perl is a high-level, general-purpose, interpreted, dynamic programming language. Though Perl is not officially an acronym, there are various backronyms in use, including "Practical Extraction and Reporting Language". Perl was developed ...

, with th
SCGI
package o
Plack
framework *

PHP PHP is a general-purpose scripting language geared towards web development. It was originally created by Danish-Canadian programmer Rasmus Lerdorf in 1993 and released in 1995. The PHP reference implementation is now produced by the PHP Group. ...

* Python * Racket, with th
scgi
library *

Ruby Ruby is a pinkish-red-to-blood-red-colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sapph ...

Rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH) ...

, with th
tokio-scgi
crate * Scheme *

Tcl TCL or Tcl or TCLs may refer to: Business * TCL Technology, a Chinese consumer electronics and appliance company ** TCL Electronics, a subsidiary of TCL Technology * Texas Collegiate League, a collegiate baseball league * Trade Centre Limited ...

* Nim

Notes

:1.The specification document was placed in the public domain by Neil Schemenauer on 12 January 2006. :2.For HTTP header combining, se
RFC2616 section 4.2

References

External links

*
SCGI protocol specification
{{Web interfaces Network protocols