Dancer2::Manual::Migration - Migrating from Dancer to Dancer2


version 1.1.0

Migration from Dancer 1 to Dancer2

This document covers some changes that users will need to be aware of while upgrading from Dancer (version 1) to Dancer2.

Launcher script

The default launcher script bin/ in Dancer looked like this:

#!/usr/bin/env perl
use Dancer;
use MyApp;

In Dancer2 it is available as bin/app.psgi and looks like this:

#!/usr/bin/env perl

use strict;
use warnings;
use FindBin;
use lib "$FindBin::Bin/../lib";

use MyApp;

So you need to remove the use Dancer; part, replace the dance; command by MyApp->to_app; (where MyApp is the name of your application), and add the following lines:

use strict;
use warnings;
use FindBin;
use lib "$FindBin::Bin/../lib";

There is a Dancer Advent Calendar article covering the to_app keyword and its usage.


You specify a different location to the directory used for serving static (public) content by setting the public_dir option. In that case, you have to set static_handler option also.


1. In Dancer2, each module is a separate application with its own namespace and variables. You can set the application name in each of your Dancer2 application modules. Different modules can be tied into the same app by setting the application name to the same value.

For example, to set the appname directive explicitly:


package MyApp;
use Dancer2;
use MyApp::Admin

hook before => sub {
    var db => 'Users';

get '/' => sub {...};



package MyApp::Admin;
use Dancer2 appname => 'MyApp';

# use a lexical prefix so we don't override it globally
prefix '/admin' => sub {
    get '/' => sub {...};


Without the appname directive, MyApp::Admin would not have access to variable db. In fact, when accessing /admin, the before hook would not be executed.

See Dancer2::Cookbook for details.

2. To speed up an app in Dancer2, install the recommended modules listed in the "Performance Improvements" in Dancer2::Manual::Deployment section.


The request object (Dancer2::Core::Request) is now deferring much of its code to Plack::Request to be consistent with the known interface to PSGI requests.

Currently the following attributes pass directly to Plack::Request:

address, remote_host, protocol, port, method, user, request_uri, script_name, content_length, content_type, content_encoding, referer, and user_agent.

If previous attributes returned undef for no value beforehand, they will return whatever Plack::Request defines now, which just might be an empty list.

For example:

my %data = (
    referer    => request->referer,
    user_agent => request->user_agent,

should be replaced by:

my %data = (
    referer    => request->referer    || '',
    user_agent => request->user_agent || '',

Plugins: plugin_setting

plugin_setting returns the configuration of the plugin. It can only be called in register or on_plugin_import.


Dancer2 requires all routes defined via a string to begin with a leading slash /.

For example:

get '0' => sub {
    return "not gonna fly";

would return an error. The correct way to write this would be to use get '/0'

Route parameters

The params keyword which provides merged parameters used to allow body parameters to override route parameters. Now route parameters take precedence over query parameters and body parameters.

We have introduced route_parameters to retrieve parameter values from the route matching. Please refer to Dancer2::Manual for more information.


Dancer2 recommends the use of Plack::Test.

For example:

use strict;
use warnings;
use Test::More tests => 2;
use Plack::Test;
use HTTP::Request::Common;

    package App::Test; # or whatever you want to call it
    get '/' => sub { template 'index' };

my $test = Plack::Test->create( App::Test->to_app );
my $res  = $test->request( GET '/' );

ok( $res->is_success, '[GET /] Successful' );
like( $res->content, qr{<title>Test2</title>}, 'Correct title' );

Other modules that could be used for testing are:


The logger_format in the Logger role (Dancer2::Core::Role::Logger) is now log_format.

read_logs can no longer be used, as with Dancer2::Test. Instead, Dancer2::Logger::Capture could be used for testing, to capture all logs to an object.

For example:

use strict;
use warnings;
use Test::More import => ['!pass'];
use Plack::Test;
use HTTP::Request::Common;
use Ref::Util qw<is_coderef>;

    package App;
    use Dancer2;

    set log       => 'debug';
    set logger    => 'capture';

    get '/' => sub {
        debug 'this is my debug message';
        return 1;

my $app = Dancer2->psgi_app;
ok( is_coderef($app), 'Got app' );

test_psgi $app, sub {
    my $cb = shift;

    my $res = $cb->( GET '/' );
    is $res->code, 200;

    my $trap = App->dancer_app->logger_engine->trapper;

    is_deeply $trap->read, [
        { level => 'debug', message => 'this is my debug message' }

Exports: Tags

The following tags are not needed in Dancer2:

use Dancer2 qw(:syntax);
use Dancer2 qw(:tests);
use Dancer2 qw(:script);

The plackup command should be used instead. It provides a development server and reads the configuration options in your command line utilities.


Templating engine changes

Whereas in Dancer1, the following were equivalent for Template::Toolkit:

template 'foo/bar'
template '/foo/bar'

In Dancer2, when using Dancer2::Template::TemplateToolkit, the version with the leading slash will try to locate /foo/bar relative to your filesystem root, not relative to your Dancer application directory.

The Dancer2::Template::Simple engine is unchanged in this respect.

Whereas in Dancer1, template engines have the methods:


In Dancer2, you should instead write:


You may not need these unless you are writing a templating engine.


You no longer need to implement the loaded method. It is simply unnecessary.


Now the Simple session engine is turned on by default, unless you specify a different one.



You cannot set the public directory with setting now. Instead you will need to call config:

# before
setting( 'public_dir', 'new_path/' );

# after
config->{'public_dir'} = 'new_path';


The warnings configuration option, along with the environment variable DANCER_WARNINGS, have been removed and have no effect whatsoever.

They were added when someone requested to be able to load Dancer without the warnings pragma, which it adds, just like Moose, Moo, and other modules provide.

If you want this to happen now (which you probably shouldn't be doing), you can always control it lexically:

use Dancer2;
no warnings;

You can also use Dancer2 within a narrower scope:

{ use Dancer2 }
use strict;
# warnings are not turned on

However, having warnings turned it is very recommended.


The configuration server_tokens has been introduced in the reverse (but more sensible, and Plack-compatible) form as no_server_tokens.



If you want to use Template::Toolkit instead of the built-in simple templating engine you used to enable the following line in the config.yml file.

template: "template_toolkit"

That was enough to get started. The start_tag and end_tag it used were the same as in the simple template <% and %> respectively.

If you wanted to further customize the Template::Toolkit you could also enable or add the following:

     encoding:  'utf8'
     start_tag: '[%'
     end_tag:   '%]'

In Dancer 2 you can also enable Template::Toolkit with the same configuration option:

template: "template_toolkit"

But the default start_tag and end_tag are now [% and %], so if you used the default in Dancer 1 now you will have to explicitly change the start_tag and end_tag values. The configuration also got an extra level of depth. Under the engine key there is a template key and the template_toolkit key comes below that. As in this example:

      start_tag: '<%'
      end_tag:   '%>'

In a nutshell, if you used to have

template: "template_toolkit"

You need to replace it with

template: "template_toolkit"
      start_tag: '<%'
      end_tag:   '%>'

Session engine

The session engine is configured in the engine section.

Dancer2 also adds two attributes for session:

See Dancer2::Core::Role::SessionFactory for more detailed documentation for these options, or the particular session engine for other supported options.

session: Simple

      cookie_name: dance.set
      cookie_duration: '24 hours'
      is_secure: 1
      is_http_only: 1

Plack Middleware

In Dancer1 you could set up Plack Middleware using a plack_middlewares key in your config.yml file. Under Dancer2 you will instead need to invoke middleware using Plack::Builder, as demonstrated in Dancer2::Manual::Deployment.


Calling Keywords Explicitly

In Dancer1, keywords could be imported individually into a package:

package MyApp;
use Dancer qw< get post params session >;

get '/foo' { ... };

Any keywords you did't export could be called explicitly:

package MyApp;
use Dancer qw< get post params session >;
use List::Util qw< any >;

Dancer::any sub { ... };

Dancer2's DSL is implemented differently. Keywords only exist in the namespace of the package which uses Dancer2, i.e. there is no Dancer2::any, only e.g. MyApp::any.

If you only want individual keywords, you can write a shim as follows:

package MyApp::DSL;
use Dancer2 appname => 'MyApp';

use Exporter qw< import >;

our @EXPORT = qw< get post ... >

Then in other packages:

package MyApp;

use MyApp::DSL qw< get post >;

MyApp::DSL::any sub { ... };


This keyword does not exist in Dancer2. However, the same information can be found in config->{'appdir'}.


This keyword is no longer required. Dancer2 loads the environment automatically and will not reload any other environment when called with load. (It's a good thing.)


This keyword doesn't exist in Dancer2.


In Dancer a session was created and a cookie was sent just by rendering a page using the template function. In Dancer2 one needs to actually set a value in a session object using the session function in order to create the session and send the cookie.

The session keyword has multiple states:

In Dancer 1 it wasn't possible to delete a key, but in Dancer2 we can finally delete:

# these two are equivalent
session $key => undef;

my $session = session;

You can retrieve the whole session hash with the data method:


To destroy a session, instead of writing:


In Dancer2, we write:

app->destroy_session if app->has_session

If you make changes to the session in an after hook, your changes will not be written to storage, because writing sessions to storage also takes place in an (earlier) after hook.


Dancer Core Developers


This software is copyright (c) 2023 by Alexis Sukrieh.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.