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LWP::Protocol::PSGI - Override LWP's HTTP/HTTPS backend with your own PSGI application


  use LWP::UserAgent;
  use LWP::Protocol::PSGI;

  # $app can be any PSGI application: Mojolicious, Catalyst or your own
  my $app = do {
      use Dancer;
      set apphandler => 'PSGI';
      get '/search' => sub {
          return 'searching for ' . params->{q};

  # Register the $app to handle all LWP requests

  # can hijack any code or module that uses LWP::UserAgent underneath, with no changes
  my $ua  = LWP::UserAgent->new;
  my $res = $ua->get("");
  print $res->content; # "searching for bar"

  # Only hijacks specific host (and port)
  LWP::Protocol::PSGI->register($psgi_app, host => 'localhost:3000');

  my $ua = LWP::UserAgent->new;
  $ua->get("http://localhost:3000/app"); # this routes $app
  $ua->get("");     # this doesn't - handled with actual HTTP requests


LWP::Protocol::PSGI is a module to hijack any code that uses LWP::UserAgent underneath such that any HTTP or HTTPS requests can be routed to your own PSGI application.

Because it works with any code that uses LWP, you can override various WWW::*, Net::* or WebService::* modules such as WWW::Mechanize, without modifying the calling code or its internals.

  use WWW::Mechanize;
  use LWP::Protocol::PSGI;


  my $mech = WWW::Mechanize->new;
  $mech->get(""); # $my_psgi_app runs


This module is extremely handy if you have tests that run HTTP requests against your application and want them to work with both internal and external instances.

  # in your .t file
  use Test::More;
  use LWP::UserAgent;

  unless ($ENV{TEST_LIVE}) {
      require LWP::Protocol::PSGI;
      my $app = Plack::Util::load_psgi("app.psgi");

  my $ua = LWP::UserAgent->new;
  my $res = $ua->get("");
  is $res->code, 200;
  like $res->content, qr/Hello/;

This test script will by default route all HTTP requests to your own PSGI app defined in $app, but with the environment variable TEST_LIVE set, runs the requests against the live server.

You can also combine Plack::App::Proxy with LWP::Protocol::PSGI to route all requests made in your test against a specific server.

  use LWP::Protocol::PSGI;
  use Plack::App::Proxy;

  my $app = Plack::App::Proxy->new(remote => "http://testapp.local:3000")->to_app;

  my $ua = LWP::UserAgent->new;
  my $res = $ua->request(""); # this hits testapp.local:3000


  LWP::Protocol::PSGI->register($app, %options);
  my $guard = LWP::Protocol::PSGI->register($app, %options);

Registers an override hook to hijack HTTP requests. If called in a non-void context, returns a guard object that automatically resets the override when it goes out of context.

      my $guard = LWP::Protocol::PSGI->register($app);
      # hijack the code using LWP with $app

  # now LWP uses the original HTTP implementations

When %options is specified, the option limits which URL and hosts this handler overrides. You can either pass host or uri to match requests, and if it doesn't match, the handler falls back to the original LWP HTTP protocol implementor.

  LWP::Protocol::PSGI->register($app, host => '');
  LWP::Protocol::PSGI->register($app, host => qr/\.google\.com$/);
  LWP::Protocol::PSGI->register($app, uri => sub { my $uri = shift; ... });

The options can take either a string, where it does a complete match, a regular expression or a subroutine reference that returns boolean given the value of host (only the hostname) or uri (the whole URI, including query parameters).


Resets all the overrides for LWP. If you use the guard interface described above, it will be automatically called for you.


Mock vs Protocol handlers

There are similar modules on CPAN that allows you to emulate LWP requests and responses. Most of them are implemented as a mock library, which means it doesn't go through the LWP guts and just gives you a wrapper for receiving HTTP::Request and returning HTTP::Response back.

LWP::Protocol::PSGI is implemented as an LWP protocol handler and it allows you to use most of the LWP extensions to add capabilities such as manipulating headers and parsing cookies.


Test::LWP::UserAgent has the similar concept of overriding LWP request method with particular PSGI applications. It has more features and options such as passing through the requests to the native LWP handler, while LWP::Protocol::PSGI only allows one to map certain hosts and ports.

Test::LWP::UserAgent requires you to change the instantiation of UserAgent from LWP::UserAgent->new to Test::LWP::UserAgent->new somehow and it's your responsibility to do so. This mechanism gives you more control which requests should go through the PSGI app, and it might not be difficult if the creation is done in one place in your code base. However it might be hard or even impossible when you are dealing with third party modules that calls LWP::UserAgent inside.

LWP::Protocol::PSGI affects the LWP calling code more globally, while having an option to enable it only in a specific block, thus there's no need to change the UserAgent object manually, whether it is in your code or CPAN modules.


Tatsuhiko Miyagawa <>


Copyright 2011- Tatsuhiko Miyagawa


This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.


Plack::Client LWP::UserAgent