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1 =encoding utf-8
2
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3 =head1 NAME
4
5 PSGI - Perl Web Server Gateway Interface Specification
6
7 =head1 ABSTRACT
8
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9 This document specifies a standard interface between web servers and
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10 Perl web applications or frameworks. This interface is designed to promote web application
11 portability and reduce the duplication of effort by web application
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12 framework developers.
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13
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14 Please keep in mind that PSGI is not Yet Another web application
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15 framework. PSGI is a specification to decouple web server environments
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16 from web application framework code. Nor is PSGI a web application
17 API. Web application developers (end users) will not run their web
18 applications directly using the PSGI interface, but instead are
19 encouraged to use frameworks that support PSGI.
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20
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21 =head1 TERMINOLOGY
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22
23 =over 4
24
25 =item Servers
26
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27 A I<Server> is a web server that accepts HTTP requests, dispatches the
28 request to web applications, and returns the HTTP response to the
29 client. In the context of PSGI the server could be a Perl process running
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30 inside an HTTP server (e.g. mod_perl in Apache), a daemon process
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31 called from a web server (e.g. FastCGI daemon) or a pure perl HTTP
32 server.
33
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34 Servers are also called I<PSGI implementations> as well as
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35 I<Backends>.
36
37 =item Applications
38
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39 I<Applications> are web applications that accept an HTTP request
40 and return an HTTP response. In PSGI an application is a code reference.
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41
42 =item Middleware
43
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44 I<Middleware> is a PSGI application (a code reference) I<and> a
45 I<Server>. I<Middleware> looks like an I<application> when called from a
46 I<server>, and it in turn can call other I<applications>. It can be thought of
47 a I<plugin> to extend a PSGI application.
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48
49 =item Framework developers
50
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51 I<Framework developers> are the authors of web application frameworks. They
52 write adapters (or engines) which accept PSGI input, run a web
53 application, and return a PSGI response to the I<server>.
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54
55 =item Web application developers
56
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57 I<Web application developers> are developers who write code on top of a web
58 application framework. These developers should never have to deal with PSGI
59 directly.
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60
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61 =back
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62
e85dbb3 @miyagawa First draft of the PSGI spec and FAQ.
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63 =head1 SPECIFICATION
64
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65 =head2 Application
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66
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67 A PSGI application is a Perl code reference. It takes exactly one
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68 argument, the environment, and returns an array reference containing exactly
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69 three values.
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70
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71 my $app = sub {
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72 my $env = shift;
73 return [
74 '200',
75 [ 'Content-Type' => 'text/plain' ],
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76 [ "Hello World" ], # or IO::Handle-like object
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77 ];
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78 };
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79
80 =head3 The Environment
81
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82 The environment B<MUST> be a hash reference that includes CGI-like headers, as
83 detailed below. The application is free to modify the environment. The
84 environment B<MUST> include these keys (adopted from L<PEP
85 333|http://www.python.org/dev/peps/pep-0333/>,
86 L<Rack|http://rack.rubyforge.org/doc/files/SPEC.html> and
87 L<JSGI|http://jackjs.org/jsgi-spec.html>) except when they would normally be
88 empty.
89
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90 When an environment key is described as a boolean, its value B<MUST> conform
91 to Perl's notion of boolean-ness. This means that an empty string or an
92 explicit C<0> are both valid false values. If a boolean key is not present, an
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93 application B<MAY> treat this as a false value.
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94
95 The values for all CGI keys (named without a period) B<MUST> be a scalar
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96 string.
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97
98 See below for details.
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99
100 =over 4
101
102 =item *
103
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104 C<REQUEST_METHOD>: The HTTP request method, such as "GET" or
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105 "POST". This B<MUST NOT> be an empty string, and so is always
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106 required.
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107
108 =item *
109
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110 C<SCRIPT_NAME>: The initial portion of the request URL's I<path>,
111 corresponding to the application. This tells the application its
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112 virtual "location". This may be an empty string if the application
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113 corresponds to the server's root URI.
114
115 If this key is not empty, it B<MUST> start with a forward slash (C</>).
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116
117 =item *
118
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119 C<PATH_INFO>: The remainder of the request URL's I<path>, designating
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120 the virtual "location" of the request's target within the
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121 application. This may be an empty string if the request URL targets
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122 the application root and does not have a trailing slash. This value
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123 should be URI decoded by servers in order to be compatible with L<RFC 3875|http://www.ietf.org/rfc/rfc3875>.
124
125 If this key is not empty, it B<MUST> start with a forward slash (C</>).
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126
127 =item *
128
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129 C<REQUEST_URI>: The undecoded, raw request URL line. It is the raw URI
130 path and query part that appears in the HTTP C<GET /... HTTP/1.x> line
131 and doesn't contain URI scheme and host names.
132
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133 Unlike C<PATH_INFO>, this value B<SHOULD NOT> be decoded by servers. It is an
134 application's responsibility to properly decode paths in order to map URLs to
135 application handlers if they choose to use this key instead of C<PATH_INFO>.
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136
137 =item *
138
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139 C<QUERY_STRING>: The portion of the request URL that follows the C<?>,
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140 if any. This key B<MAY> be empty, but B<MUST> always be present, even if empty.
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141
142 =item *
143
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144 C<SERVER_NAME>, C<SERVER_PORT>: When combined with C<SCRIPT_NAME> and
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145 C<PATH_INFO>, these keys can be used to complete the URL. Note,
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146 however, that C<HTTP_HOST>, if present, should be used in preference
147 to C<SERVER_NAME> for reconstructing the request URL. C<SERVER_NAME>
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148 and C<SERVER_PORT> B<MUST NOT> be empty strings, and are always
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149 required.
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150
151 =item *
152
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153 C<SERVER_PROTOCOL>: The version of the protocol the client used to
154 send the request. Typically this will be something like "HTTP/1.0" or
155 "HTTP/1.1" and may be used by the application to determine how to
156 treat any HTTP request headers.
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157
158 =item *
159
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160 C<CONTENT_LENGTH>: The length of the content in bytes, as an
161 integer. The presence or absence of this key should correspond to the
162 presence or absence of HTTP Content-Length header in the request.
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163
164 =item *
165
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166 C<CONTENT_TYPE>: The request's MIME type, as specified by the client.
167 The presence or absence of this key should correspond to the presence
168 or absence of HTTP Content-Type header in the request.
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169
170 =item *
171
172 C<HTTP_*> Keys: These keys correspond to the client-supplied
173 HTTP request headers. The presence or absence of these keys should
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174 correspond to the presence or absence of the appropriate HTTP header
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175 in the request.
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176
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177 If there are multiple header lines sent with the same key, the server
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178 should treat them as if they were sent in one line and combine them
179 with C<, >, as in L<RFC 2616|http://www.ietf.org/rfc/rfc2616>.
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180
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181 =back
182
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183 In addition to the keys above, the PSGI environment B<MUST> also include these
184 PSGI-specific keys:
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185
186 =over 4
187
188 =item *
189
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190 C<psgi.version>: An array reference [1,0] representing this version of
191 PSGI. The first number is the major version and the second it the minor
192 version.
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193
194 =item *
195
196 C<psgi.url_scheme>: A string C<http> or C<https>, depending on the request URL.
197
198 =item *
199
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200 C<psgi.input>: the input stream. See below for details.
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201
202 =item *
203
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204 C<psgi.errors>: the error stream. See below for details.
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205
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206 =item *
207
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208 C<psgi.multithread>: This is a boolean value, which B<MUST> be true if the
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209 application may be simultaneously invoked by another thread in the same
210 process, false otherwise.
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211
212 =item *
213
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214 C<psgi.multiprocess>: This is a boolean value, which B<MUST> be true if an
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215 equivalent application object may be simultaneously invoked by another
216 process, false otherwise.
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217
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218 =item *
219
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220 C<psgi.run_once>: A boolean which is true if the server expects (but does not
221 guarantee!) that the application will only be invoked this one time during
222 the life of its containing process. Normally, this will only be true for a
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223 server based on CGI (or something similar).
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224
225 =item *
226
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227 C<psgi.nonblocking>: A boolean which is true if the server is calling the
228 application in an non-blocking event loop.
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229
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230 =item *
231
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232 C<psgi.streaming>: A boolean which is true if the server supports callback
233 style delayed response and streaming writer object.
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234
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235 =back
236
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237 The server or the application can store its own data in the environment as
238 well. These keys B<MUST> contain at least one dot, and B<SHOULD> be prefixed
239 uniquely. The C<psgi.> prefix is reserved for use with the PSGI core
240 implementation and officially blessed extensions. This prefix B<MUST NOT> be
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241 used by other servers or application.
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242
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243 The environment B<MUST NOT> contain keys named C<HTTP_CONTENT_TYPE> or
244 C<HTTP_CONTENT_LENGTH>.
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245
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246 One of C<SCRIPT_NAME> or C<PATH_INFO> B<MUST> be set. C<PATH_INFO> should be
247 C</> if C<SCRIPT_NAME> is empty. C<SCRIPT_NAME> B<MUST NOT> be C</>, but
248 B<MAY> be empty.
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249
250 =head3 The Input Stream
251
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252 The input stream in C<psgi.input> is an L<IO::Handle>-like object which
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253 streams the raw HTTP POST or PUT data. If it is a file handle then it
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254 B<MUST> be opened in binary mode. The input stream B<MUST> respond to
255 C<read> and B<MAY> implement C<seek>.
256
257 Perl's built-in filehandles or L<IO::Handle> based objects should work as-is
258 in a PSGI server. Application developers B<SHOULD NOT> inspect the type or
259 class of the stream. Instead, they B<SHOULD> simply call C<read> on the object.
260
261 Application developers B<SHOULD NOT> use Perl's built-in C<read> or iterator
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262 (C<< <$fh> >>) to read from the input stream. Instead, application
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263 developers should call C<read> as a method (C<< $fh->read >>) to allow for
264 duck typing.
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265
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266 Framework developers, if they know the input stream will be used with the
267 built-in read() in any upstream code they can't touch, B<SHOULD> use PerlIO or
268 a tied handle to work around with this problem.
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269
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270 The input stream objet is expected to provide a C<read> method:
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271
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272 =over 4
273
274 =item read
275
276 $input->read($buf, $len [, $offset ]);
277
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278 Returns the number of characters actually read, 0 at end of file, or
279 undef if there was an error.
280
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281 =back
282
283 It may also implement an optional C<seek> method.
284
285 =over 4
286
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287 =item seek
288
289 $input->seek($pos, $whence);
290
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291 Returns 1 on success, 0 otherwise.
292
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293 =back
294
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295 See the L<IO::Handle> documentation for more details on exactly how these
296 methods should work.
297
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298 =head3 The Error Stream
299
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300 The error stream in C<psgi.errors> is an L<IO::Handle>-like object to
301 print errors. The error stream must implement a C<print> method.
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302
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303 As with the input stream, Perl's built-in filehandles or L<IO::Handle> based
304 objects should work as-is in a PSGI server. Application developers B<SHOULD
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305 NOT> inspect the type or class of the stream. Instead, they B<SHOULD> simply
306 call C<print> on the object.
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307
308 =over 4
309
310 =item print
311
312 $errors->print($error);
313
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314 Returns true if successful.
315
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316 =back
317
318 =head3 The Response
319
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320 Applications B<MUST> return a response as either a three element array
321 reference, or a code reference for a delayed/streaming response.
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322
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323 The response array reference consists of the following elements:
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324
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325 =head4 Status
326
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327 An HTTP status code. This B<MUST> be an integer greater than or equal to 100,
328 and B<SHOULD> be an HTTP status code as documented in L<RFC
329 2616|http://www.w3.org/Protocols/rfc2616>.
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330
331 =head4 Headers
332
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333 The headers B<MUST> be an array reference (B<not> a hash reference)
334 of key/value pairs. This means it B<MUST> contain an even number of elements.
335
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336 The header B<MUST NOT> contain a key named C<Status>, onr any keys with C<:>
337 or newlines in their name. It B<MUST NOT> contain any keys that end in C<-> or
338 C<_>.
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339
340 All keys B<MUST> consist only of letters, digits, C<_> or C<->. All keys
341 B<MUST> start with a letter. The value of the header must be a scalar
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342 string. The value string B<MUST NOT> contain characters below ASCII chr(37)
343 except for chr(32) (whitespace).
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344
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345 If the same key name appears multiple times in an array ref, those
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346 header lines B<MUST> be sent to the client separately (e.g. multiple
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347 C<Set-Cookie> lines).
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348
349 =head4 Content-Type
350
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351 There B<MUST> be a C<Content-Type> except when the C<Status> is 1xx, 204
352 or 304, in which case there B<MUST NOT> be a content type.
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353
354 =head4 Content-Length
355
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356 There B<MUST NOT> be a C<Content-Length> header when the C<Status> is
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357 1xx, 204 or 304.
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358
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359 If the Status is not 1xx, 204 or 304 and there is no C<Content-Length> header,
360 a PSGI server B<MAY> calculate the content length by looking at the Body. This
361 value can then be appended to the list of headers returned by the application.
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362
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363 =head4 Body
364
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365 The response body B<MUST> be returned from the application as either an array
366 reference or a handle.
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367
368 =over 4
369
370 =item *
371
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372 If the body is an array reference, it is expected to contain an array of lines
373 which make up the body.
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374
375 my $body = [ "Hello\n", "World\n" ];
376
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377 Note that the elements in an array reference are B<NOT REQUIRED> to end
378 in a newline. A server B<SHOULD> write each elements as-is to the
379 client, and B<SHOULD NOT> care if the line ends with newline or not.
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380
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381 An array reference with a single value is valid. So C<[ $html ]> is a valid
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382 response body.
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383
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384 =item *
385
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386 The body can instead be a handle, either a Perl built-in filehandle or an
387 L<IO::Handle>-like object.
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388
389 open my $body, "</path/to/file";
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390 open my $body, "<:via(SomePerlIO)", ...;
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391 my $body = IO::File->new("/path/to/file");
392
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393 my $body = SomeClass->new(); # mock class that implements getline() and close()
394
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395 Servers B<SHOULD NOT> check the type or class of the body. Instead, they should
396 simply call C<getline> to iterate over the body, and
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397 call C<close> when done.
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398
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399 Servers B<MAY> check if the body is a real filehandle using C<fileno> and
400 C<Scalar::Util::reftype>. If the body is real filehandle, the server B<MAY>
401 optimize using techniques like I<sendfile(2)>.
402
403 The body object B<MAY> also respond to a C<path> method. This method is
404 expected to return the path to a file accessible by the server. This allows
405 the server to use this information instead of a file descriptor number to
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406 serve the file.
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407
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408 Servers B<SHOULD> set the C<$/> special variable to the buffer size when
409 reading content from C<$body> using the C<getline> method. This is done by
410 setting C<$/> with a reference to an integer (C<$/ = \8192>).
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411
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412 If the body filehandle is a Perl built-in filehandle L<IO::Handle> object,
413 they will respect this value. Similarly, an object which provides the same API
414 B<MAY> also respect this special variable, but are not required to do so.
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415
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416 =back
417
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418 =head2 Delayed Response and Streaming Body
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419
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420 The PSGI interface allows applications and servers to provide a
421 callback-style response instead of the three-element array
422 reference. This allows for a delayed response and a streaming body
423 (server push).
424
425 This interface B<SHOULD> be implemented by PSGI servers, and
426 C<psgi.streaming> environment B<MUST> be set to true in such servers.
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427
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428 To enable a delayed response, the application B<SHOULD> return a
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429 callback as its response. An application B<MAY> check if the
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430 C<psgi.streaming> environment is true and falls back to the direct
431 response if it isn't.
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432
433 This callback will be called with I<another> subroutine reference (referred to
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434 as the I<responder> from now on) as its only argument. The I<responder>
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435 should in turn be called with the standard three element array reference
436 response. This is best illustrated with an example:
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437
438 my $app = sub {
439 my $env = shift;
440
441 # Delays response until it fetches content from the network
442 return sub {
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443 my $responder = shift;
444
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445 fetch_content_from_server(sub {
446 my $content = shift;
447 $responder->([ 200, $headers, [ $content ] ]);
448 });
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449 };
450 };
451
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452 An application B<MAY> omit the third element (the body) when calling
453 the I<responder>. If the body is omitted, the I<responder> B<MUST>
454 return I<yet another> object which implements C<write> and C<close>
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455 methods. Again, an example illustrates this best.
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456
457 my $app = sub {
458 my $env = shift;
459
460 # immediately starts the response and stream the content
461 return sub {
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462 my $responder = shift;
463 my $writer = $responder->([ 200, [ 'Content-Type', 'application/json' ]]);
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464
465 wait_for_events(sub {
466 my $new_event = shift;
467 if ($new_event) {
468 $writer->write($new_event->as_json . "\n");
469 } else {
470 $writer->close;
471 }
472 });
473 };
474 };
475
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476 This delayed response and streaming API is useful if you want to
477 implement a non-blocking I/O based server streaming or long-poll Comet
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478 push technology, but could also be used to implement unbuffered writes
479 in a blocking server.
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480
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481 =head2 Middleware
482
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483 A I<middleware> component takes another PSGI application and runs it. From the
484 perspective of a server, a middleware component is a PSGI application. From
485 the perspective of the application being run by the middleware component, the
486 middleware is the server. Generally, this will be done in order to implement
487 some sort of pre-processing on the PSGI environment hash or post-processing on
488 the response.
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489
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490 Here's a simple example that appends a special HTTP header
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491 I<X-PSGI-Used> to any PSGI application.
492
493 # $app is a simple PSGI application
494 my $app = sub {
495 my $env = shift;
496 return [ '200', [ 'Content-Type' => 'text/plain' ], [ "Hello World" ] ];
497 };
498
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499 # $xheader is a piece of middleware that wraps $app
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500 my $xheader = sub {
501 my $env = shift;
502 my $res = $app->($env);
503 push @{$res->[1]}, 'X-PSGI-Used' => 1;
504 return $res;
505 };
506
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507 Middleware B<MUST> behave exactly like a PSGI application from the perspective
508 of a server. Middleware B<MAY> decide not to support the streaming interface
509 discussed earlier, but B<SHOULD> pass through the response types that it doesn't
510 understand.
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511
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512 =head1 CHANGELOGS
513
514 1.1: 2010.02.xx
515
516 =over 4
517
518 =item *
519
520 C<psgi.streaming> B<SHOULD> be implemented by PSGI servers, rather than B<MAY>.
521
522 =item *
523
524 PSGI keys C<psgi.run_once>, C<psgi.nonblocking> and C<psgi.streaming>
525 B<MUST> be set by PSGI servers.
526
527 =item *
528
529 Removed C<poll_cb> from writer methods.
530
531 =back
532
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533 =head1 ACKNOWLEDGEMENTS
534
535 Some parts of this specification are adopted from the following specifications.
536
537 =over 4
538
539 =item *
540
541 PEP333 Python Web Server Gateway Interface L<http://www.python.org/dev/peps/pep-0333>
542
543 =item *
544
545 Rack L<http://rack.rubyforge.org/doc/SPEC.html>
546
547 =item *
548
549 JSGI Specification L<http://jackjs.org/jsgi-spec.html>
550
551 =back
552
553 I'd like to thank authors of these great documents.
554
555 =head1 AUTHOR
556
557 Tatsuhiko Miyagawa E<lt>miyagawa@bulknews.netE<gt>
558
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559 =head1 CONTRIBUTORS
560
561 The following people have contributed to the PSGI specification and
562 Plack implementation by commiting their code, sending patches,
563 reporting bugs, asking questions, suggesting useful advices,
564 nitpicking, chatting on IRC or commenting on my blog (in no particular
565 order):
566
567 Tokuhiro Matsuno
568 Kazuhiro Osawa
569 Yuval Kogman
570 Kazuho Oku
571 Alexis Sukrieh
048f9a3 @miyagawa update Dann's name. README links to the site
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572 Takatoshi Kitano
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573 Stevan Little
574 Daisuke Murase
575 mala
576 Pedro Melo
577 Jesse Luehrs
578 John Beppu
579 Shawn M Moore
580 Mark Stosberg
581 Matt S Trout
582 Jesse Vincent
583 Chia-liang Kao
584 Dave Rolsky
585 Hans Dieter Pearcey
586 Randy J Ray
587 Benjamin Trott
588 Max Maischein
589 Slaven Rezić
590 Marcel Grünauer
591 Masayoshi Sekimura
592 Brock Wilcox
593 Piers Cawley
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594 Daisuke Maki
595 Kang-min Liu
2b08fe3 @miyagawa oops
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596 Yasuhiro Matsumoto
5623139 @miyagawa changed the Content-Length from SHOULD to MAY
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597 Ash Berlin
31284b9 @miyagawa Added Artur for his advice around gearman/schwartz
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598 Artur Bergman
ba2001a @miyagawa more questions around HTTP::Engine confusions
miyagawa authored
599 Simon Cozens
600 Scott McWhirter
a87c263 @miyagawa mentions return value of psgi.input and psgi.errors methods
miyagawa authored
601 Jiro Nishiguchi
01b3c47 @miyagawa Sorry!
miyagawa authored
602 Masahiro Chiba
1ef1c9c @miyagawa Added patspam for his webgui work
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603 Patrick Donelan
d7fc7bb @miyagawa Added frodwith for helping us sort out the non-blocking from POE deve…
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604 Paul Driver
93432fb @miyagawa Added rafl
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605 Florian Ragwitz
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606
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607 =head1 COPYRIGHT AND LICENSE
608
609 Copyright Tatsuhiko Miyagawa, 2009.
610
611 This document is licensed under the Creative Commons license by-sa.
612
613 =cut
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