Plack::Middleware::Debug - display information about the current request/response


  enable "Debug";


The debug middleware offers a configurable set of panels that displays information about the current request and response. The information is generated only for responses with a status of 200 (OK) and a Content-Type that contains text/html or application/xhtml+xml and is embedded in the HTML that is sent back to the browser.

Note that the code is injected directly before the closing tag (</body>) so if there is no such tag, the debug panel will not be injected at all.

To enable the middleware, just use Plack::Builder as usual in your .psgi file:

    use Plack::Builder;

    builder {
        enable 'Debug', panels => [ qw(DBITrace Memory Timer) ];

The Debug middleware takes an optional panels argument whose value is expected to be a reference to an array of panel specifications. If given, only those panels will be enabled. If you don't pass a panels argument, the default list of panels - Environment, Response, Timer, Memory, Session and DBITrace - will be enabled, each with their default settings, and automatically disabled if their target modules or middleware components are not loaded.

Each panel specification can take one of three forms:

A string

This is interpreted as the base name of a panel in the Plack::Middeware::Debug:: namespace, unless preceded by +, in which case it's interpreted as an absolute name similar to how Plack::Builder handles such names, e.g. +My::Plack::Middleware::Debug::Something.

The panel class is loaded and a panel object is created with its default settings.

An array reference

If you need to pass arguments to the panel object as it is created, you may use this form (But see below).

The first element of the array reference has to be the panel base name. The remaining elements are key/value pairs to be passed to the panel.

For example:

    builder {
        enable 'Debug', panels =>
          [ qw(Environment Response Timer Memory),
            [ 'DBITrace', level => 2 ]

Because each panel is a middleware component, you can write this way as well:

    builder {
        enable 'Debug'; # load defaults
        enable 'Debug::DBITrace', level => 2;

Note that the <enable 'Debug'> line should come before other Debug panels because of the order middleware components are executed.

Custom middleware

You can also pass a Panel middleware component. This might be useful if you have custom debug panels in your framework or web application.


The Debug middleware is designed to be easily extensible. You might want to write a custom debug panel for your framework or for your web application. Each debug panel is also a Plack middleware component and is easy to write one.

Let's look at the anatomy of the Timer debug panel. Here is the code from that panel:

  package Plack::Middleware::Debug::Timer;
  use Time::HiRes;

  use parent qw(Plack::Middleware::Debug::Base);

  sub run {
      my($self, $env, $panel) = @_;

      my $start = [ Time::HiRes::gettimeofday ];

      return sub {
          my $res = shift;

          my $end = [ Time::HiRes::gettimeofday ];
          my $elapsed = sprintf '%.6f s', Time::HiRes::tv_interval $start, $end;

                  [ Start  => $self->format_time($start),
                    End    => $self->format_time($end),
                    Elapsed => $elapsed ],

  sub format_time { ... }

To write a new debug panel, place it in the Plack::Middleware::Debug:: namespace. In our example, the Timer panel lives in the Plack::Middleware::Debug::Timer package.

The only thing your panel should do is to subclass Plack::Middleware::Debug::Base. This does most of the things a middleware component should do as a Plack middleware, so you only need to override run method to profile and create the panel content.

  sub run {
      my($self, $env, $panel) = @_;

      # Do something before the application runs

      return sub {
          my $res = shift;

          # Do something after the application returns


You can create as many lexical variables as you need and reference that in the returned callback as a closure, and update the content of of the $panel which is Plack::Middleware::Debug::Panel object.

In our Timer example we want to list three key/value pairs: the start time, the end time and the elapsed time. We use the render_list_pairs() method to place the pairs in the order we want. There is also a render_hash() and render_lines() method, to render a hash keys and values, as well as just text lines (e.g. log messages).


Please report any bugs or feature requests through the web interface at


See perlmodinstall for information and options on installing Perl modules.


The latest version of this module is available from the Comprehensive Perl Archive Network (CPAN). Visit to find a CPAN site near you. Or see

The development version lives at Instead of sending patches, please fork this project using the standard git and github infrastructure.


Marcel Grunauer, <>

Tatsuhiko Miyagawa, <>


Copyright 2009 by Marcel Grünauer

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


The debug middleware is heavily influenced (that is, adapted from) the Django Debug Toolbar - see