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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. This interface is designed to promote web application portability and reduce the duplication of effort by web application framework developers.

Please 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. Nor is PSGI a web application API. Web application developers (end users) will not run their web applications directly using the PSGI interface, but instead are encouraged to use frameworks that support PSGI.


Web Servers

Web servers accept HTTP requests issued by web clients, dispatching those requests to web applications if configured to do so, and return HTTP responses to the request-initiating clients.

PSGI Server

A PSGI Server is a Perl program providing an environment for a PSGI application to run in.

PSGI specifying an interface for web applications and the main purpose of web applications being to be served to the Internet, a PSGI Server will most likely be either: part of a web server (like Apache mod_perl), connected to a web server (with FastCGI, SCGI), invoked by a web server (as in plain old CGI), or be a standalone web server itself, written entirely or partly in Perl.

There is, however, no requirement for a PSGI Server to actually be a web server or part of one, as PSGI only defines an interface between the server and the application, not between the server and the world.

A PSGI Server is often also called PSGI Application Container because it is similar to a Java Servlet container, which is Java process providing an environment for Java Servlets.


Web applications accept HTTP requests and return HTTP responses.

PSGI applications are web applications conforming to the PSGI interface, prescribing they take the form of a code reference with defined input and output.

For simplicity, PSGI Applications will also be referred to as Applications for the remainder of this document.


Middleware is a PSGI application (a code reference) and a Server. Middleware looks like an application when called from a server, and it in turn can call other applications. It can be thought of a plugin to extend a PSGI application.

Framework developers

Framework developers are the authors of web application frameworks. They write adapters (or engines) which accept PSGI input, run a web application, and return a PSGI response to the server.

Web application developers

Web application developers are developers who write code on top of a web application framework. These developers should never have to deal with PSGI directly.



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

  my $app = sub {
      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, as detailed below. The application is free to modify the environment. The environment MUST include these keys (adopted from PEP 333, Rack and JSGI) except when they would normally be empty.

When an environment key is described as a boolean, its value MUST conform to Perl's notion of boolean-ness. This means that an empty string or an explicit 0 are both valid false values. If a boolean key is not present, an application MAY treat this as a false value.

The values for all CGI keys (named without a period) MUST be a scalar string.

See below for details.

A server should attempt to provide as many other CGI variables as are applicable. Note, however, that an application that uses any CGI variables other than the ones listed above are necessarily non-portable to web servers that do not support the relevant extensions.

In addition to the keys above, the PSGI environment MUST also include these PSGI-specific keys:

The server or the application can store its own data in the environment as well. These keys MUST contain at least one dot, and SHOULD be prefixed uniquely.

The psgi. prefix is reserved for use with the PSGI core specification, and psgix. prefix is reserved for officially blessed extensions. These prefixes MUST NOT be used by other servers or application. See psgi-extensions for the list of officially approved extensions.

The environment MUST NOT contain keys named HTTP_CONTENT_TYPE or HTTP_CONTENT_LENGTH.

One of SCRIPT_NAME or PATH_INFO MUST be set. When REQUEST_URI is /, PATH_INFO should be / and SCRIPT_NAME should be empty. SCRIPT_NAME MUST NOT be /, but MAY be empty.

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.

Perl's built-in filehandles or IO::Handle based objects should work as-is in a PSGI server. Application developers SHOULD NOT inspect the type or class of the stream. Instead, they SHOULD simply call read on the object.

Application developers SHOULD NOT use Perl's built-in read or iterator (<$fh>) to read from the input stream. Instead, application developers should call read as a method ($fh->read) to allow for duck typing.

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 a tied handle to work around with this problem.

The input stream object is expected to provide a read method:

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

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

It may also implement an optional seek method. If psgix.input.buffered environment is true, it MUST implement the seek method.

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

Returns 1 on success, 0 otherwise.

See the IO::Handle documentation for more details on exactly how these methods should work.

The Error Stream

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

As with the input stream, Perl's built-in filehandles or IO::Handle based objects should work as-is in a PSGI server. Application developers SHOULD NOT inspect the type or class of the stream. Instead, they SHOULD simply call print on the object.


Returns true if successful.

The Response

Applications MUST return a response as either a three element array reference, or a code reference for a delayed/streaming response.

The response array reference consists of the following elements:


An HTTP status code. This MUST be an integer greater than or equal to 100, and SHOULD be an HTTP status code as documented in RFC 2616.


The headers MUST be an array reference (not a hash reference) of key/value pairs. This means it MUST contain an even number of elements.

The header MUST NOT contain a key named Status, nor any keys with : or newlines in their name. It MUST NOT contain any keys that end in - or _.

All keys MUST consist only of letters, digits, _ or -. All keys MUST start with a letter. The value of the header MUST be a scalar string and defined. The value string MUST NOT contain characters below octal 037 i.e. chr(31).

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 NOT be a content type.


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, a PSGI server MAY calculate the content length by looking at the Body. This value can then be appended to the list of headers returned by the application.


The response body MUST be returned from the application as either an array reference or a handle containing the response body as byte strings. The body MUST be encoded into appropriate encodings and MUST NOT contain wide characters (> 255).

Delayed Response and Streaming Body

The PSGI interface allows applications and servers to provide a callback-style response instead of the three-element array reference. This allows for a delayed response and a streaming body (server push).

This interface SHOULD be implemented by PSGI servers, and psgi.streaming environment MUST be set to true in such servers.

To enable a delayed response, the application SHOULD return a callback as its response. An application MAY check if the psgi.streaming environment is true and falls back to the direct response if it isn't.

This callback will be called with another subroutine reference (referred to as the responder from now on) as its only argument. The responder should in turn be called with the standard three element array reference response. This is best illustrated with an example:

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

      # Delays response until it fetches content from the network
      return sub {
          my $responder = shift;

          fetch_content_from_server(sub {
              my $content = shift;
              $responder->([ 200, $headers, [ $content ] ]);

An application MAY omit the third element (the body) when calling the responder. If the body is omitted, the responder MUST return yet another object which implements write and close methods. Again, an example illustrates this best.

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

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

          wait_for_events(sub {
              my $new_event = shift;
              if ($new_event) {
                  $writer->write($new_event->as_json . "\n");
              } else {

This delayed response and streaming API is useful if you want to implement a non-blocking I/O based server streaming or long-poll Comet push technology, but could also be used to implement unbuffered writes in a blocking server.


A middleware component takes another PSGI application and runs it. From the perspective of a server, a middleware component is a PSGI application. From the perspective of the application being run by the middleware component, the middleware is the server. Generally, this will be done in order to implement some sort of pre-processing on the PSGI environment hash or post-processing on the response.

Here's a simple example that appends a 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 piece of middleware that wraps $app
  my $xheader = sub {
      my $env = shift;
      my $res = $app->($env);
      push @{$res->[1]}, 'X-PSGI-Used' => 1;
      return $res;

Middleware MUST behave exactly like a PSGI application from the perspective of a server. Middleware MAY decide not to support the streaming interface discussed earlier, but SHOULD pass through the response types that it doesn't understand.


1.1: 2010.02.xx


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

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


Tatsuhiko Miyagawa <miyagawa@bulknews.net>


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
  Florian Ragwitz


Copyright Tatsuhiko Miyagawa, 2009-2011.

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

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