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

HTTP The Hypertext Transfer Protocol (HTTP) 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 We ...

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 ...

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 RFC 2616, 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 formalisations of concatenat ...

netstring In computer programming, a netstring is a formatting method for byte strings that uses a declarative notation to indicate the size of the string. Netstrings store the byte length of the data that follows, making it easier to unambiguously pass te ...

-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 for ...

, 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 a value of zero, called NUL in this article). Alternative names are C str ...

s ( C strings). The value can be an

empty string In formal language theory, the empty string, or empty word, is the unique string of length zero. Formal theory Formally, a string is a finite, ordered sequence of characters such as letters, digits or spaces. The empty string is the special cas ...

, 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 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 formalisations of concatenat ...

to form a single byte sequence, then

-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 dynamic-named value that can affect the way running processes will behave on a computer. They are part of the environment in which a process runs. For example, a running process can query the value of the TEMP envi ...

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 *

Cherokee The Cherokee (; chr, ᎠᏂᏴᏫᏯᎢ, translit=Aniyvwiyaʔi or Anigiduwagi, or chr, ᏣᎳᎩ, links=no, translit=Tsalagi) are one of the indigenous peoples of the Southeastern Woodlands of the United States. Prior to the 18th century, t ...

Lighttpd lighttpd (pronounced "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 c10k problem � ...

* Microsoft

Internet Information Services Internet Information Services (IIS-pronounced 2S, formerly Internet Information Server) is an extensible web server software created by Microsoft for use with the Windows NT family. IIS supports HTTP, HTTP/2, HTTPS, FTP, FTPS, SMTP and NN ...

wit
ISAPI SCGI extension
*

nginx Nginx (pronounced "engine x" ) 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 Igor Sysoev and publicly released in 2004. Nginx is free and open-source software ...

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 * D, with th
arsd.cgi
library *

Emacs Lisp Emacs Lisp is a dialect of the Lisp programming language used as a scripting language by Emacs (a text editor family most commonly associated with GNU Emacs and XEmacs). It is used for implementing most of the editing functionality built into Em ...

, with th
url-scgi
library *

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 has pioneered a number of programming lan ...

Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mos ...

, with th
SCGI connector
or with th

library * Lisp (programming language), Lisp *

Perl Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offic ...

, with th
SCGI
package o
Plack
framework * PHP *

Python Python may refer to: Snakes * Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia ** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia * Python (mythology), a mythical serpent Computing * Python (pro ...

* Racket, with th
scgi
library *

Ruby A 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 ...

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( ...

, with th
tokio-scgi
crate * Scheme * Tcl * 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