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PSGI - Perl Web Server Gateway Interface Specification


This document specifies a standard interface between web servers and Perl web applications or frameworks, to promote web application portability and reduce the duplicated efforts by web application framework developers.

Keep in mind that PSGI is not Yet Another web application framework. PSGI is a specification to decouple web server environments from web application framework code. PSGI is also not the web application API. Web application developers (end users) are not supposed to run their web applications directly using the PSGI interface, but instead are encouraged to use frameworks that support PSGI, or use the helper implementations like Plack (more on that later).



Servers are web servers that accept HTTP requests, dispatch the requests to the web applications and return the HTTP response to the clients. In PSGI specification it's a Perl process that's running inside an HTTP server (e.g. mod_perl in Apache), a daemon process called from a web server (e.g. FastCGI daemon) or a pure perl HTTP server.

Servers are also called PSGI implementations as well as Backends.


Applications are web applications that actually get HTTP requests and return HTTP response. In PSGI it's a code reference: see below.


Middleware is a PSGI application, which is a code reference, but also runs like a server to run other applications. It can be thought of a plugin to extend PSGI application: see below.

Framework developers

Framework developers are authors of web application frameworks. They need to write adapters (or engines) to read PSGI input, then run the application logic and returns PSGI response to the server.

Web application developers

Web application developers are developers who write code that uses one of the web application framework that uses PSGI interface. They usually don't need to deal with nor care about PSGI protocol at all.



A PSGI application is a Perl code reference. It takes exactly one argument, the environment and returns an array reference of exactly three values.

  sub app {
      my $env = shift;
      return [
          [ 'Content-Type' => 'text/plain' ],
          [ "Hello World" ], # or IO::Handle-like object

The Environment

The environment MUST be a hash reference that includes CGI-like headers. The application is free to modify the environment. The environment is required to include these variables (adopted from PEP333, Rack and JSGI) except when they'd be empty, but see below:

In addition to this, the PSGI environment MUST include these PSGI-specific variables:

The PSGI environment MAY include these optional PSGI variables:

The server or the application can store its own data in the environment, too. The keys MUST contain at least one dot, and should be prefixed uniquely. The prefix psgi. is reserved for use with the PSGI core implementation and other accepted extensions and MUST NOT be used otherwise. The environment MUST NOT contain the keys HTTP_CONTENT_TYPE or HTTP_CONTENT_LENGTH (use the versions without HTTP_). The CGI keys (named without a period) MUST have a scalar variable containing strings. There are the following restrictions:

The Input Stream

The input stream in psgi.input is an IO::Handle-like object which streams the raw HTTP POST or PUT data. If it is a file handle then it MUST be opened in binary mode. The input stream MUST respond to read and MAY implement seek.

The built-in filehandle or IO::Handle based objects should work fine everywhere. Application developers SHOULD NOT inspect the type or class of the stream, but instead just call read to duck type.

Application developers SHOULD NOT use the built-in read function to read from the input stream, because read function only works with the real IO object (a glob ref based file handle or PerlIO) and makes duck typing difficult. Web application framework developers, if they know the input stream will be used with the built-in read() in any upstream code they can't touch, SHOULD use PerlIO or tie handle to work around with this problem.

  $input->read($buf, $len [, $offset ]);

Returns the number of characters actually read, 0 at end of file, or undef if there was an error.

  $input->seek($pos, $whence);

Returns 1 on success, 0 otherwise.

The Error Stream

The error stream in psgi.errors is an IO::Handle-like object to print errors. The error stream must implement print.

The built-in filehandle or IO::Handle based objects should work fine everywhere. Application developers SHOULD NOT inspect the type or class of the stream, but instead just call print to duck type.


Returns true if successful.

The Response

The response MUST be a three element array reference if the application wants to directly return the HTTP response.

An application MAY choose to return other type of responses such as a code reference, to delay the response only if the server supports the streaming (See below).


HTTP status code, is an integer and MUST be greater than or equal to 100.


The headers must be an array reference (and NOT a hash reference!) containing key and value pairs. Its number of elements MUST be even. The header MUST NOT contain a Status key, contain keys with : or newlines in their name, contain keys that end in - or _ but only contain keys that consist of letters, digits, _ or - and start with a letter. The value of the header must be a scalar value that contain a string. The value string MUST NOT contain characters below chr(37) except chr(32) (whitespace).

If the same key name appears multiple times in an array ref, those header lines MUST be sent to the client separately (e.g. multiple Set-Cookie lines).


There MUST be a Content-Type except when the Status is 1xx, 204 or 304, in which case there MUST be none given.


There MUST NOT be a Content-Length header when the Status is 1xx, 204 or 304.

If the Status is not 1xx, 204 or 304 and there is no Content-Length header, servers MAY calculate the content length by looking at Body, in case it can be calculated (i.e. if it's an array ref of body chunk or a real file handle), and append to the outgoing headers.


The response body is returned from the application in one of following two types of scalar variable.

Delayed Reponse and Streaming Body

PSGI interface allows applications and servers optionally handle callback-style response (instead of three-element array reference) to delay the HTTP response and stream content (server push).

To enable delayed response, an application SHOULD check if psgi.streaming environment is true, and in that case, MAY return a callback that is passed another callback (response starter) as its first argument, and pass the three element response to the callback.

  my $app = sub {
      my $env = shift;

      # Delays response until it fetches content from the network
      return sub {
          my $respond = shift;
          fetch_content_from_server(sub {
              my $content = shift;
              # ...
              $respond->([ 200, $headers, [ $content ] ]);

Similarly, an application MAY omit the third element (the body) in the callback to get a response writer object, that implements write, poll_cb and close method to push the response body.

  my $app = sub {
      my $env = shift;

      # immediately starts the response and stream the content
      return sub {
          my $respond = shift;
          my $writer = $respond->([ 200, [ 'Content-Type', 'application/json' ]]);

          wait_for_events(sub {
              my $new_event = shift;
              if ($new_event) {
                  $writer->write($new_event->as_json . "\n");
                  # Or:
                  # $writer->poll_cb(sub { $_[0]->write($new_event->as_json . "\n") });
              } else {

Delayed response and streaming should be useful if you want to implement non-blocking I/O based server streaming or long-poll Comet push technology. IO::Handle-like object is pull, while this streaming response implements push.

This interface is optional: An applciation SHOULD check if the server supports streaming. Servers MAY decide to not accept this streaming response and throws an exception. Servers MUST set psgi.streaming to true if this interface is supported. Servers MUST return a writer object if the third argument (response body) is omitted or not defined in the response starter callback arguments.


Middleware is itself a PSGI application but it takes an existing PSGI application and runs it like a server, mostly to do pre-processing on $env or post-processing on the response objects.

Here's a simple example that appends special HTTP header X-PSGI-Used to any PSGI application.

  # $app is a simple PSGI application
  my $app = sub {
      my $env = shift;
      return [ '200', [ 'Content-Type' => 'text/plain' ], [ "Hello World" ] ];

  # $xheader is a middleware to wrap $app
  my $xheader = sub {
      my $env = shift;
      my $res = $app->($env);
      push @{$res->[1]}, 'X-PSGI-Used' => 1;
      return $res;

Middleware itself MUST behave exactly like a PSGI application: take $env and return $res. Middleware MAY decide not to support the streaming interface (see above) but SHOULD pass through the response types that it doesn't understand.


Some parts of this specification are adopted from the following specifications.

I'd like to thank authors of these great documents.


Tatsuhiko Miyagawa <>


The following people have contributed to the PSGI specification and Plack implementation by commiting their code, sending patches, reporting bugs, asking questions, suggesting useful advices, nitpicking, chatting on IRC or commenting on my blog (in no particular order):

  Tokuhiro Matsuno
  Kazuhiro Osawa
  Yuval Kogman
  Kazuho Oku
  Alexis Sukrieh
  Takatoshi Kitano
  Stevan Little
  Daisuke Murase
  Pedro Melo
  Jesse Luehrs
  John Beppu
  Shawn M Moore
  Mark Stosberg
  Matt S Trout
  Jesse Vincent
  Chia-liang Kao
  Dave Rolsky
  Hans Dieter Pearcey
  Randy J Ray
  Benjamin Trott
  Max Maischein
  Slaven Rezić
  Marcel Grünauer
  Masayoshi Sekimura
  Brock Wilcox
  Piers Cawley
  Daisuke Maki
  Kang-min Liu
  Yasuhiro Matsumoto
  Ash Berlin
  Artur Bergman
  Simon Cozens
  Scott McWhirter
  Jiro Nishiguchi
  Masahiro Chiba
  Patrick Donelan
  Paul Driver


Copyright Tatsuhiko Miyagawa, 2009.

This document is licensed under the Creative Commons license by-sa.

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