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5 FastCGI Programmer's Guide - Chapter 1, The Fast Common Gateway Interface
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23 1 The Fast Common<BR>
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29 The Fast Common Gateway Interface (FastCGI) is an enhancement to the existing CGI (Common Gateway Interface),
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30 which is a standard for interfacing external applications with Web servers.
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33 <A NAME="8373"></A> FastCGI is a proposed open standard and we expect both free and commercial Web servers to
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34 support it. FastCGI is included in Open Market WebServer and Secure WebServer, versions 2.0 and greater.
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39 Advantages of FastCGI
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43 FastCGI extends and enhances the CGI model in several ways:
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52 FastCGI enables applications to persist between client requests, eliminating application start up overhead
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53 and allowing the application to maintain state between client calls. <A NAME="7995"></A>
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56 FastCGI enables applications to reside on remote systems (rather than having to reside on the same system
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57 as the Web server) <A NAME="7997"></A>
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60 FastCGI enables additional flexibility in application functionality, with explicit support for applications
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61 that do client authentication and filtering of input.
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65 Long-lived Applications
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69 CGI applications are ephemeral and short-lived: each time a client requests a CGI application, the server asks
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70 the operating system to spawn a new CGI process. After the CGI process satisfies the request, the server kills
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71 it. The server spawns and subsequently kills a new process for each client request.
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74 <A NAME="8459"></A> FastCGI applications are long-lived, and can persist between client calls. The server
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75 spawns the FastCGI process once and it continues to run and satisfy client requests until it is explicitly
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76 terminated. You can also ask the Web server to start multiple copies of a FastCGI application, if you expect
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77 that concurrent processing will improve the application's performance.
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80 <A NAME="5761"></A> Long-lived applications have two important advantages over short-lived applications:
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89 A short-lived application pays start up overhead on every request; a long-lived application spreads the
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90 overhead over many requests. For an application that has a heavy start up cost, such as opening a database,
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91 doing initialization on every call can be very inefficient. Reinitializing for every client is also very
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92 inefficient for Perl programs, where the interpreter reads through the entire program before executing any
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93 of it. <A NAME="9204"></A>
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96 A long-lived application can cache information in memory between requests, allowing it to respond more
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97 quickly to later requests.
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100 <A NAME="8733"></A>
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102 FastCGI is not the only way to get a long-lived application on the Web, however. For example, there are many
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103 existing search engines that are implemented as long-lived applications.
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106 <A NAME="8734"></A> In most cases, these applications rely on customized Web servers. In other words, since
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107 most Web servers do not support long-lived applications, a programmer must code this support into a Web
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108 server. This approach requires a tremendous amount of work and also ties the application to a particular
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112 <A NAME="8735"></A> Another way to get a long-lived application is to write code that calls routines from the
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113 Web server's API. This alternative involves a lot of extra coding, ties the application to a particular
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114 Web server, and introduces problems of maintainability, scalability, and security.
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117 <A NAME="8736"></A> We believe that FastCGI is the most general and flexible strategy for building long-lived
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123 Separating Application and Server
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125 <A NAME="8446"></A>
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127 CGI applications must run on the same node as the Web server; FastCGI applications can run on any node that
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128 can be reached from your Web server using TCP/IP protocols. For example, you might want to run the FastCGI
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129 application on a high-speed computer server or database engine, and run the Web server on a different node.
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134 FastCGI "Roles"
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136 <A NAME="8777"></A>
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138 CGI and FastCGI applications are effective ways to allow an application to act as an extension to the Web
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139 server. CGI provides no explicit support for different kinds of applications: under CGI, every application
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140 receives an HTTP request, does something with it, and generates an HTTP response. FastCGI provides explicit
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141 support for several common "roles" that applications can play.
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144 <A NAME="8769"></A> The three roles supported by the WebServer 2.0 are:
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150 <A NAME="8409"></A>
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153 Responder <A NAME="8410"></A>
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156 Filter <A NAME="8411"></A>
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163 Responder Applications
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165 <A NAME="8679"></A>
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167 A <EM>responder</EM> application is the most basic kind of FastCGI application: it receives the information
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168 associated with an HTTP request and generates an HTTP response. Responder is the role most similar to
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169 traditional CGI programming, and most FastCGI applications are responders.
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174 Filter Applications
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176 <A NAME="8681"></A>
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178 A <EM>filter</EM> FastCGI application receives the information associated with an HTTP request, plus an extra
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179 stream of data from a file stored on the Web server, and generates a "filtered" version of the data
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180 stream as an HTTP response.
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183 <A NAME="8421"></A> With filter applications, the system administrator maps a particular MIME-type to a
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184 particular filter FastCGI application. When a client requests a URL with that MIME-type, the Web server
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185 invokes the filter application, which processes the file at the specified URL and sends a response (usually
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186 HTML text) back to the client.
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189 <A NAME="8422"></A> For example, suppose you write a filter FastCGI application that converts SGML text to
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190 HTML, and map the extension .sgml (MIME-type SGML) to your filter FastCGI application. Now, suppose that a
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191 user requests the following URL:
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196 <A NAME="8423">/www.aerjug.com/docs/chap1.sgml
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199 <A NAME="8424"></A>
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201 Given this URL, the Web server passes <CODE>chap1.sgml</CODE> as input to your filter FastCGI application,
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202 which processes <CODE>chap1.sgml</CODE> and returns an HTML version of it to the requesting client.
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207 Authorizer Applications
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209 <A NAME="8426"></A>
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211 An <EM>authorizer</EM> FastCGI application receives the information in an HTTP request header and generates a
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212 decision whether to authorize the request.
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215 <A NAME="8428"></A> To mark a FastCGI application as having the authorizer role, the system administrator
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216 names the application inside the server configuration file, using a directive called
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217 <CODE>AuthorizeRegion</CODE>. (See the Open Market Web Server manual for information on server configuration
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221 <A NAME="8429"></A> When a client requests a URL that meets the <CODE>AuthorizeRegion</CODE> criteria, the Web
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222 server calls your authorizer FastCGI application. If your application grants authorization (by returning a
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223 response code of 200), the Web server resumes execution of commands in the <CODE>AuthorizeRegion</CODE>
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224 section. If your application denies authorization (by returning any other response code), the Web server stops
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225 processing subsequent commands in the <CODE>AuthorizeRegion</CODE> section, and returns the response from your
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226 FastCGI application to the client.
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229 <A NAME="8431"></A> Authorizer applications can return headers containing environment variables. Other CGI or
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230 FastCGI programs accessing this request (including other authorizers) can access these environment variables.
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231 The headers must have the following format:
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236 <A NAME="8432">Variable-<EM>name</EM>: <EM>value</EM>
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239 <A NAME="8433"></A>
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241 For example, the following header
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246 <A NAME="8434">Variable-AUTH_METHOD: database lookup
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249 <A NAME="8435"></A>
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251 causes the environment variable <CODE>AUTH_METHOD</CODE> to be set to <CODE>"database lookup"</CODE>
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252 for this request. Other CGI or FastCGI applications running on this request can access the value of
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253 <CODE>AUTH_METHOD</CODE>.
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256 <A NAME="8437"></A> Authorizer applications cannot successfully read from standard input. Any attempts to read
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257 from standard input result in an immediate EOF.
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260 <A NAME="8438"></A> All data that authorizer applications write to standard error will get written to the
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261 traditional server error logs.
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266 Writing FastCGI Applications
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268 <A NAME="9301"></A>
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270 The work involved in writing a FastCGI application depends in large part on the I/O libraries that you use.
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271 This manual describes how to write FastCGI applications in terms of the Open Market libraries, which are
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272 available for C, Perl, and Tcl. FastCGI is an open standard and you are welcome to build your own libraries
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273 for other languages as well, but this manual focuses on building FastCGI applications in the context of the
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274 Open Market libraries.
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277 <A NAME="9443"></A>
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280 <A NAME="9450"></A> In general, the goal of the libraries is to make the job of writing a FastCGI application
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281 as much like writing a CGI application as possible. For example, you use the same techniques for query string
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282 decoding, HTML output to stdout, use of environment variables, and so on. When you use our libraries, porting
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283 CGI applications to FastCGI is mostly a matter of restructuring the code to take advantage of FastCGI features
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291 <A NAME="9470"></A>
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293 The main task of converting a CGI program into a FastCGI program is separating the initialization code from
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294 the code that needs to run for each request. The structure should look something like this:
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299 <A NAME="9471">Initialization code
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301 <A NAME="9472">Start of response loop
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303 <A NAME="9473"> body of response loop
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305 <A NAME="9474">End of response loop
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308 <A NAME="9475"></A>
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310 The <EM>initialization code</EM> is run exactly once, when the application is initialized. Initialization code
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311 usually performs time-consuming operations such as opening databases or calculating values for tables or
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315 <A NAME="9477"></A> The <EM>response loop</EM> runs continuously, waiting for client requests to arrive. The
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316 loop starts with a call to <CODE>FCGI_Accept</CODE>, a routine in the FastCGI library. The
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317 <CODE>FCGI_Accept</CODE> routine blocks program execution until a client requests the FastCGI application.
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318 When a client request comes in, <CODE>FCGI_Accept</CODE> unblocks, runs one iteration of the response loop
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319 body, and then blocks again waiting for another client request. The loop terminates only when the system
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320 administrator or the Web server kills the FastCGI application.
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325 Initial Environment Variables
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327 <A NAME="9786"></A>
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329 When a FastCGI process starts up, it has not yet accepted a request, and therefore none of the CGI environment
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333 <A NAME="9787"></A> You set the initial environment of a FastCGI process started by the <CODE>AppClass</CODE>
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334 directive using the <CODE>-initial-env</CODE> option. The process would use this environment to configure its
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335 options and locate files or databases.
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338 <A NAME="9829"></A> In FastCGI processes started by the <CODE>AppClass</CODE> directive with the -affinity
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339 option, the <CODE>FCGI_PROCESS_ID</CODE> variable is set in the initial environment (not in the environment of
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340 a request). <CODE>FCGI_PROCESS_ID</CODE> is a decimal number in the range 0 to N - 1 where N is the number of
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341 processes (argument to the <CODE>-processes</CODE> option to <CODE>AppClass</CODE>). The process would use
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342 <CODE>FCGI_PROCESS_ID</CODE> in conjunction with other variables to locate session-related files or databases
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348 Per-Request Environment Variables
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350 <A NAME="9481"></A>
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352 In general, FastCGI uses the same per-request environment variables as CGI, and you access the values of
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353 environment variables in FastCGI applications just as you would in CGI applications. The only differences are
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360 <A NAME="9483"></A>
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363 In Authorizer FastCGI applications, the Web server unsets the <CODE>PATH_INFO</CODE>,
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364 <CODE>PATH_TRANSLATED</CODE>, and <CODE>CONTENT_LENGTH</CODE> variables. <A NAME="9484"></A>
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367 In Filter FastCGI applications, the Web server sets two additional environment variables:
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370 <A NAME="9486"></A>
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373 <CODE>FILE_LAST_MOD</CODE>: The Web server sets <CODE>FILE_LAST_MOD</CODE> to the date and time that
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374 filter input file was last modified. The format is the number of seconds since midnight (UTC),
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375 January 1, 1970. <A NAME="9488"></A>
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378 <CODE>FCGI_DATA_LENGTH</CODE>: The application reads at most <CODE>FCGI_DATA_LENGTH</CODE> bytes from
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379 the data stream before receiving the end-of-stream indication.
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382 <A NAME="9490"></A>
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385 FastCGI sets <CODE>FCGI_ROLE</CODE> for each request to <CODE>RESPONDER</CODE>, <CODE>AUTHORIZER</CODE>, or
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386 <CODE>FILTER</CODE>.
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390 Building FastCGI Applications in C
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392 <A NAME="9049"></A>
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394 The Software Development Toolkit that accompanies WebServer 2.0 contains two libraries, fcgi_stdio and
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395 fcgiapp, for building FastCGI applications in C.
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398 <A NAME="9723"></A> The fcgi_stdio library implements our philosophy of making FastCGI applications similar to
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399 CGI applications, and provides full binary compatibility between FastCGI applications and CGI applications:
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400 you can run the same C binary as either CGI or FastCGI.
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403 <A NAME="9545"></A> The fcgiapp library is more specific to FastCGI, and doesn't attempt the veneer of
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407 <A NAME="9731"></A> We recommend that you use the fcgi_stdio library, and this manual describes the routines
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408 in that library. The documentation for the fcgiapp library is in the code in the development kit.
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413 Building FastCGI Applications in Perl
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415 <A NAME="9581"></A>
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417 To build FastCGI applications in Perl, you need a FastCGI-savvy version of Perl, plus the FastCGI extension to
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418 Perl. We build FastCGI-savvy versions of the Perl interpreter for several common platforms and make them
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419 available on our Website. For details and examples, see Chapter <A HREF="ch3perl.htm#3659">3, "Developing
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420 FastCGI Applications in Perl," on page 17</A>.
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425 Building FastCGI Applications in Tcl
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427 <A NAME="9586"></A>
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429 To build FastCGI applications in Tcl, you need a FastCGI-savvy version of Tcl. We build FastCGI-savvy versions
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430 of the Tcl interpreter for several common platforms and make them available on our Website. For details and
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431 examples, see Chapter <A HREF="ch4tcl.htm#3659">4, "Developing FastCGI Applications in Tcl," on page
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437 Implementation Details
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439 <A NAME="8066"></A>
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441 The FastCGI application libraries are designed to shield you from the details of the FastCGI design. This
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442 section is designed for the curious reader who would like some low-level understanding. If you are not curious
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443 about the implementation, you can happily skip this section.
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446 <A NAME="8554"></A> As shown in the following figure, CGI applications use the three standard POSIX streams
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447 (<CODE>stdin</CODE>, <CODE>stdout</CODE>, and <CODE>stderr</CODE>), plus environment variables, to communicate
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448 with an HTTP server.
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451 <A NAME="8359"></A> <IMG ALT="error-file:TidyOut.log" SRC="ch1intra.gif">
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454 <A NAME="4295"></A>
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459 Figure 1: Flow of Data in CGI
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461 <A NAME="9001"></A>
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463 The fundamental difference between FastCGI and CGI is that FastCGI applications are long-lived, which means
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464 that the Web Server needs to rendezvous with a running application, rather than starting the application in
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465 order to explicitly communicate with it.
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468 <A NAME="9110"></A> The FastCGI implementation basically creates a bidirectional connection between two
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469 processes that have no relationship. FastCGI uses a single connection for all the data associated with an
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470 application -- stdin, stdout, stderr, and environment variables. The data on the connection is encapsulated
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471 using a FastCGI protocol that allows stdin and the environment variables to share the same half connection (on
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472 the way in) and stdout and stderr to share the half connection (on the way out).
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475 <A NAME="9020"></A> On the input side, the FastCGI application receives data on the connection, unpacks it to
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476 separate stdin from the environment variables and then invokes the application. On the output side, FastCGI
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477 wraps stdout and stderr with appropriate protocol headers, and sends the encapsulated data out to the server.
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480 <A NAME="9032"></A> Since a FastCGI application does not always run on the same node as the HTTP server, we
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481 support two implementations of the connection: a <EM>stream pipe</EM><A HREF="#9645"><SUP>1</SUP></A>, for
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482 communications on the same machine, and TCP streams, for communication when the client and the server are on
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483 different machines.
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486 <A NAME="8576"></A> <IMG ALT="error-file:TidyOut.log" SRC="ch1inta1.gif">
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491 Figure 2: Flow of Data in FastCGI when server and application are on different machines
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494 The fcgi_stdio Library: I/O Compatibility
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496 <A NAME="8977"></A>
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498 The implementation for I/O compatibility is that the library <CODE>fcgi_stdio.h</CODE> contains macros to
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499 translate the types and procedures defined in stdio.h into the appropriate FastCGI calls. For example,
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500 consider a FastCGI program written in C containing the following line of code:
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505 <A NAME="5877">fprintf(stdout, "<H2>Aerobic Juggling</H2>/n");
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508 <A NAME="9659"></A> <CODE>fcgi_stdio.h</CODE>
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510 header file contains the macro
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515 <A NAME="6403">#define fprintf FCGI_fprintf
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518 <A NAME="6402"></A>
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520 So the preprocessor translates the <CODE>fprintf</CODE> call into the following call:
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525 <A NAME="6411">FCGI_fprintf(stdout, "<H2>Aerobic Juggling</H2>/n");
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528 <A NAME="5888"></A> <CODE>FCGI_fprintf</CODE>
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530 takes the same arguments as <CODE>fprintf</CODE>.
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533 <A NAME="9664"></A> The implementation of FCGI_fprintf tests the file to see if it is a normal C stream or a
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534 FastCGI stream, and calls the appropriate implementation.
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537 <A NAME="6463"></A> The <CODE>fcgi_stdio.h</CODE> header file contains macros to translate calls to all ISO
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538 stdio.h routines (and all conventional Posix additions, such as <CODE>fileno</CODE>, <CODE>fdopen</CODE>,
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539 <CODE>popen</CODE>, and <CODE>pclose</CODE>) into their FastCGI equivalents.
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544 The fcgi_stdio Library: Binary compatibility
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546 <A NAME="9579"></A>
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548 The fcgi_stdio library provides full binary compatibility between FastCGI applications and CGI applications:
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549 you can run the same C binary as either CGI or FastCGI.
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552 <A NAME="9580"></A> The implementation is in FCGI_Accept: the FCGI_Accept function tests its environment to
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553 determine whether the application was invoked as a CGI program or an FastCGI program. If it was invoked as a
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554 CGI program, the request loop will satisfy a single client request and then exit, producing CGI behavior.
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557 <A NAME="8957"></A>
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564 "ap_guida.htm">[Bottom]</A>
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567 <SUP>1</SUP><A NAME="9645"></A>
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569 UNIX Network Programming, W. Richard Stevens, 1990 Prentice-Hall, Section 7.9
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