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Manual Reference Pages  -  DANCER2::MANUAL::MIGRATION (3)

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



version 0.166001

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 <B>separate applicationB> 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 dont 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. The following modules can be used to speed up an app in Dancer2:
o URL::Encode::XS
o CGI::Deurl::XS
o HTTP::Parser::XS
o Scope::Upper
They would need to be installed separately. This is because Dancer2 does not incorporate any C code, but it can get C-code compiled as a module. Thus, these modules can be used for speed improvement provided:
o You have access to a C interpreter
o You don’t need to fatpack your application


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.


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:
o Test::TCP
o Test::WWW::Mechanize::PSGI

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;

        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;
    is( ref $app, CODE, 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.


o Engines receive a logging callback

Engines now receive a logging callback named log_cb. Engines can use it to log anything in run-time, without having to worry about what logging engine is used.

This is provided as a callback because the logger might be changed in run-time and we want engines to be able to always reach the current one without having a reference back to the core application object.

The logger engine doesn’t have the attribute since it is the logger itself.

o Engines handle encoding consistently

All engines are now expected to handle encoding on their own. User code is expected to be in internal Perl representation.

Therefore, all serializers, for example, should deserialize to the Perl representation. Templates, in turn, encode to UTF-8 if requested by the user, or by default.

One side-effect of this is that from_yaml will call YAML’s Load function with decoded input.


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.



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 extral 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.
o session_name changed to cookie_name.
o session_domain changed to cookie_domain.
o session_expires changed to cookie_duration.
o session_secure changed to is_secure.
o session_is_http_only changed to is_http_only.
Dancer2 also adds two attributes for session:
o cookie_path The path of the cookie to create for storing the session key. Defaults to /.
o session_duration Duration in seconds before sessions should expire, regardless of cookie expiration. If set, then SessionFactories should use this to enforce a limit on session validity.
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



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 actully 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:
o No arguments

Without any arguments, the session keyword returns a Dancer2::Core::Session object, which has methods for read, write, and delete.

    my $session = session;
    $session->write( $key => $value );

o Single argument (key)

If a single argument is provided, it is treated as the key, and it will retrieve the value for it.

    my $value = session $key;

o Two arguments (key, value)

If two arguments are provided, they are treated as a key and a value, in which case the session will assign the value to the key.

    session $key => $value;

o Two arguments (key, undef)

If two arguments are provided, but the second is <B>undefB>, the key will be deleted from the session.

    session $key => undef;

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;


Dancer Core Developers


This software is copyright (c) 2015 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.

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