3 Catalyst::Manual::Intro - Introduction to Catalyst
7 This is a brief overview of why and how to use Catalyst. It explains how
8 Catalyst works and shows how to get a simple application up and running
11 =head2 What is Catalyst?
13 Catalyst is an elegant web application framework, extremely flexible yet
14 extremely simple. It's similar to Ruby on Rails, Spring (Java) and
15 L<Maypole>, upon which it was originally based.
19 Catalyst follows the Model-View-Controller (MVC) design pattern,
20 allowing you to easily separate concerns, like content, presentation,
21 and flow control, into separate modules. This separation allows you to
22 modify code that handles one concern without affecting code that handles
23 the others. Catalyst promotes the re-use of existing Perl modules that
24 already handle common web application concerns well.
26 Here's how the M, V, and C map to those concerns, with examples of
27 well-known Perl modules you may want to use for each.
33 Access and modify content (data). L<Class::DBI>, L<Plucene>, L<Net::LDAP>...
37 Present content to the user. L<Template Toolkit|Template>,
38 L<Mason|HTML::Mason>, L<HTML::Template>...
42 Control the whole request phase, check parameters, dispatch actions, flow
47 If you're unfamiliar with MVC and design patterns, you may want to check
48 out the original book on the subject, I<Design Patterns>, by Gamma,
49 Helm, Johnson, and Vlissides, also known as the Gang of Four (GoF). You
50 can also just Google it. Many, many web application frameworks are
51 based on MVC, including all those listed above.
55 Catalyst is much more flexible than many other frameworks. We'll talk
56 more about this later, but rest assured you can use your favorite Perl
57 modules with Catalyst.
61 =item * B<Multiple Models, Views, and Controllers>
63 To build a Catalyst application, you handle each type of concern inside
64 special modules called L</Components>. Often this code will be very
65 simple, just calling out to Perl modules like those listed above under
66 L</MVC>. Catalyst handles these components in a very flexible way. Use
67 as many Models, Views, and Controllers as you like, using as many
68 different Perl modules as you like, all in the same application. Want to
69 manipulate multiple databases, and retrieve some data via LDAP? No
70 problem. Want to present data from the same Model using L<Template
71 Toolkit|Template> and L<PDF::Template>? Easy.
73 =item * B<Reuseable Components>
75 Not only does Catalyst promote the re-use of already existing Perl
76 modules, it also allows you to re-use your Catalyst components in
77 multiple Catalyst applications.
79 =item * B<Unrestrained URL-to-Action Dispatching>
81 Catalyst allows you to dispatch any URLs to any application L<Actions>,
82 even through regular expressions! Unlike most other frameworks, it
83 doesn't require mod_rewrite or class and method names in URLs.
85 With Catalyst you register your actions and address them directly. For
89 my ( $self, $context ) = @_;
90 $context->response->body('Hello World!');
93 Now http://localhost:3000/hello prints "Hello World!".
95 =item * B<Support for CGI, mod_perl, Apache::Request>
97 Use L<Catalyst::Engine::Apache> or L<Catalyst::Engine::CGI>.
103 The best part is that Catalyst implements all this flexibility in a very
108 =item * B<Building Block Interface>
110 Components interoperate very smoothly. For example, Catalyst
111 automatically makes a L<Context> object available to every
112 component. Via the context, you can access the request object, share
113 data between components, and control the flow of your
114 application. Building a Catalyst application feels a lot like snapping
115 together toy building blocks, and everything just works.
117 =item * B<Component Auto-Discovery>
119 No need to C<use> all of your components. Catalyst automatically finds
122 =item * B<Pre-Built Components for Popular Modules>
124 See L<Catalyst::Model::CDBI> for L<Class::DBI>, or L<Catalyst::View::TT>
125 for L<Template Toolkit|Template>.
127 =item * B<Built-in Test Framework>
129 Catalyst comes with a built-in, lightweight http server and test
130 framework, making it easy to test applications from the command line.
132 =item * B<Helper Scripts>
134 Catalyst provides helper scripts to quickly generate running starter
135 code for components and unit tests. See L<Catalyst::Helper>.
141 Here's how to install Catalyst and get a simple application up and
142 running, using the helper scripts described above.
146 $ perl -MCPAN -e 'install Task::Catalyst'
153 $ script/myapp_create.pl controller Library::Login
157 $ script/myapp_server.pl
159 Now visit these locations with your favorite browser or user agent to see
164 =item http://localhost:3000/
166 =item http://localhost:3000/library/login/
174 Let's see how Catalyst works, by taking a closer look at the components
175 and other parts of a Catalyst application.
177 =head3 Application Class
179 In addition to the Model, View, and Controller components, there's a
180 single class that represents your application itself. This is where you
181 configure your application, load plugins, define application-wide
182 actions, and extend Catalyst.
187 use Catalyst qw/-Debug/;
190 name => 'My Application',
192 # You can put anything else you want in here:
193 my_configuration_variable => 'something',
196 sub default : Private {
197 my ( $self, $context ) = @_;
198 $context->response->body('Catalyst rocks!');
203 For most applications, Catalyst requires you to define only one config
210 Name of your application.
214 Optionally, you can specify a B<root> parameter for templates and static
215 data. If omitted, Catalyst will try to auto-detect the directory's
216 location. You can define as many parameters as you want for plugins or
217 whatever you need. You can access them anywhere in your application via
218 C<$context-E<gt>config-E<gt>{$param_name}>.
222 Catalyst automatically blesses a Context object into your application
223 class and makes it available everywhere in your application. Use the
224 Context to directly interact with Catalyst and glue your L<Components>
225 together. For example, if you need to use the Context from within a
226 Template Toolkit template, it's already there:
228 <h1>Welcome to [% c.config.name %]!</h1>
230 As illustrated in our URL-to-Action dispatching example, the Context is
231 always the second method parameter, behind the Component object
232 reference or class name itself. Previously we called it C<$context> for
233 clarity, but most Catalyst developers just call it C<$c>:
236 my ( $self, $c ) = @_;
237 $c->res->body('Hello World!');
240 The Context contains several important objects:
244 =item * L<Catalyst::Request>
249 The request object contains all kinds of request-specific information, like
250 query parameters, cookies, uploads, headers, and more.
252 $c->req->params->{foo};
253 $c->req->cookies->{sessionid};
254 $c->req->headers->content_type;
257 =item * L<Catalyst::Response>
262 The response is like the request, but contains just response-specific
265 $c->res->body('Hello World');
266 $c->res->status(404);
267 $c->res->redirect('http://oook.de');
269 =item * L<Catalyst::Config>
275 =item * L<Catalyst::Log>
279 $c->log->debug('Something happened');
280 $c->log->info('Something you should know');
285 $c->stash->{foo} = 'bar';
289 The last of these, the stash, is a universal hash for sharing data among
290 application components. For an example, we return to our 'hello' action:
293 my ( $self, $c ) = @_;
294 $c->stash->{message} = 'Hello World!';
295 $c->forward('show_message');
298 sub show_message : Private {
299 my ( $self, $c ) = @_;
300 $c->res->body( $c->stash->{message} );
303 Note that the stash should be used only for passing data in an
304 individual request cycle; it gets cleared at a new request. If you need
305 to maintain more persistent data, use a session.
309 A Catalyst controller is defined by its actions. An action is a sub with
310 a special attribute. You've already seen some examples of actions in
311 this document. The URL (for example http://localhost.3000/foo/bar)
312 consists of two parts, the base (http://localhost:3000/ in this example)
313 and the path (foo/bar). Please note that the trailing slash after the
314 hostname[:port] always belongs to base and not to the action.
316 Catalyst supports several types of actions:
322 package MyApp::Controller::My::Controller;
323 sub bar : Path('foo/bar') { }
325 Literal C<Path> actions will act relative to their current
326 namespace. The above example matches only
327 http://localhost:3000/my/controller/foo/bar. If you start your path with
328 a forward slash, it will match from the root. Example:
330 package MyApp::Controller::My::Controller;
331 sub bar : Path('/foo/bar') { }
333 Matches only http://localhost:3000/foo/bar.
335 package MyApp::Controller::My::Controller;
338 By leaving the C<Path> definition empty, it will match on the namespace
339 root. The above code matches http://localhost:3000/my/controller.
343 sub bar : Regex('^item(\d+)/order(\d+)$') { }
345 Matches any URL that matches the pattern in the action key, e.g.
346 http://localhost:3000/item23/order42. The '' around the regexp is
347 optional, but perltidy likes it. :)
349 Regex matches act globally, i.e. without reference to the namespace from
350 which it is called, so that a C<bar> method in the
351 C<MyApp::Controller::Catalog::Order::Process> namespace won't match any
352 form of C<bar>, C<Catalog>, C<Order>, or C<Process> unless you
353 explicitly put this in the regex. To achieve the above, you should
354 consider using a C<LocalRegex> action.
356 =item * B<LocalRegex>
358 sub bar : LocalRegex('^widget(\d+)$') { }
360 LocalRegex actions act locally. If you were to use C<bar> in
361 C<MyApp::Controller::Catalog>, the above example would match urls like
362 http://localhost:3000/catalog/widget23.
364 If you omit the "C<^>" from your regex, then it will match any depth
365 from the controller and not immediately off of the controller name. The
366 following example differs from the above code in that it will match
367 http://localhost:3000/catalog/foo/widget23 as well.
369 package MyApp::Controller::Catalog;
370 sub bar : LocalRegex('widget(\d+)$') { }
372 For both LocalRegex and Regex actions, if you use capturing parentheses
373 to extract values within the matching URL, those values are available in
374 the C<$c-E<gt>req-E<gt>snippets> array. In the above example, "widget23"
375 would capture "23" in the above example, and
376 C<$c-E<gt>req-E<gt>snippets-E<gt>[0]> would be "23". If you want to pass
377 arguments at the end of your URL, you must use regex action keys. See
378 L</URL Path Handling> below.
385 Matches http://localhost:3000/foo. The function name is mapped directly
386 to the application base.
388 =item * B<Namespace-Prefixed>
390 package MyApp::Controller::My::Controller;
393 Matches http://localhost:3000/my/controller/foo.
395 This action type indicates that the matching URL must be prefixed with a
396 modified form of the component's class (package) name. This modified
397 class name excludes the parts that have a pre-defined meaning in
398 Catalyst ("MyApp::Controller" in the above example), replaces "::" with
399 "/", and converts the name to lower case. See L</Components> for a full
400 explanation of the pre-defined meaning of Catalyst component class
405 sub foo : Private { }
407 Matches no URL, and cannot be executed by requesting a URL that
408 corresponds to the action key. Private actions can be executed only
409 inside a Catalyst application, by calling the C<forward> method:
413 See L</Flow Control> for a full explanation of C<forward>. Note that, as
414 discussed there, when forwarding from another component, you must use
415 the absolute path to the method, so that a private C<bar> method in your
416 C<MyApp::Controller::Catalog::Order::Process> controller must, if called
417 from elsewhere, be reached with
418 C<$c-E<gt>forward('/catalog/order/process/bar')>.
422 B<Note:> After seeing these examples, you probably wonder what the point
423 is of defining names for regex and path actions. Actually, every public
424 action is also a private one, so you have one unified way of addressing
425 components in your C<forward>s.
427 =head4 Built-in Private Actions
429 In response to specific application states, Catalyst will automatically
430 call these built-in private actions in your application class:
434 =item * B<default : Private>
436 Called when no other action matches. Could be used, for example, for
437 displaying a generic frontpage for the main app, or an error page for
438 individual controllers.
440 If C<default> isn't acting how you would expect, look at using a
441 L<Literal> C<Path> action (with an empty path string). The difference is
442 that C<Path> takes arguments relative from the namespace and C<default>
443 I<always> takes arguments relative from the root, regardless of what
446 =item * B<index : Private>
448 C<index> is much like C<default> except that it takes no arguments
449 and it is weighted slightly higher in the matching process. It is
450 useful as a static entry point to a controller, e.g. to have a static
453 =item * B<begin : Private>
455 Called at the beginning of a request, before any matching actions are
458 =item * B<end : Private>
460 Called at the end of a request, after all matching actions are called.
464 =head4 Built-in actions in controllers/autochaining
466 Package MyApp::Controller::Foo;
467 sub begin : Private { }
468 sub default : Private { }
469 sub auto : Private { }
471 You can define built-in private actions within your controllers as
472 well. The actions will override the ones in less-specific controllers,
473 or your application class. In other words, for each of the three
474 built-in private actions, only one will be run in any request
475 cycle. Thus, if C<MyApp::Controller::Catalog::begin> exists, it will be
476 run in place of C<MyApp::begin> if you're in the C<catalog> namespace,
477 and C<MyApp::Controller::Catalog::Order::begin> would override this in
480 In addition to the normal built-in actions, you have a special action
481 for making chains, C<auto>. Such C<auto> actions will be run after any
482 C<begin>, but before your action is processed. Unlike the other
483 built-ins, C<auto> actions I<do not> override each other; they will be
484 called in turn, starting with the application class and going through to
485 the I<most> specific class. I<This is the reverse of the order in which
486 the normal built-ins override each other>.
488 Here are some examples of the order in which the various built-ins
493 =item for a request for C</foo/foo>
497 MyApp::Controller::Foo::default # in the absence of MyApp::Controller::Foo::Foo
500 =item for a request for C</foo/bar/foo>
502 MyApp::Controller::Foo::Bar::begin
504 MyApp::Controller::Foo::auto
505 MyApp::Controller::Foo::Bar::auto
506 MyApp::Controller::Foo::Bar::default # for MyApp::Controller::Foo::Bar::foo
507 MyApp::Controller::Foo::Bar::end
511 The C<auto> action is also distinguished by the fact that you can break
512 out of the processing chain by returning 0. If an C<auto> action returns
513 0, any remaining actions will be skipped, except for C<end>. So, for the
514 request above, if the first auto returns false, the chain would look
519 =item for a request for C</foo/bar/foo> where first C<auto> returns
522 MyApp::Controller::Foo::Bar::begin
524 MyApp::Controller::Foo::Bar::end
528 An example of why one might use this is an authentication action: you
529 could set up a C<auto> action to handle authentication in your
530 application class (which will always be called first), and if
531 authentication fails, returning 0 would skip any remaining methods
534 B<Note:> Looking at it another way, C<auto> actions have to return a
535 true value to continue processing! You can also C<die> in the autochain
536 action; in that case, the request will go straight to the finalize
537 stage, without processing further actions.
539 =head4 URL Path Handling
541 You can pass variable arguments as part of the URL path. In this case,
542 you must use regex action keys with '^' and '$' anchors, and the
543 arguments must be separated with forward slashes (/) in the URL. For
544 example, suppose you want to handle C</foo/$bar/$baz>, where C<$bar> and
547 sub foo : Regex('^foo$') { my ($self, $context, $bar, $baz) = @_; }
549 But what if you also defined actions for C</foo/boo> and C</foo/boo/hoo>?
551 sub boo : Path('foo/boo') { .. }
552 sub hoo : Path('foo/boo/hoo') { .. }
554 Catalyst matches actions in most specific to least specific order:
558 /foo # might be /foo/bar/baz but won't be /foo/boo/hoo
560 So Catalyst would never mistakenly dispatch the first two URLs to the
563 =head4 Parameter Processing
565 Parameters passed in the URL query string are handled with methods in
566 the L<Catalyst::Request> class. The C<param> method is functionally
567 equivalent to the C<param> method of C<CGI.pm> and can be used in
568 modules that require this.
570 # http://localhost:3000/catalog/view/?category=hardware&page=3
571 my $category = $c->req->param('category');
572 my $current_page = $c->req->param('page') || 1;
574 # multiple values for single parameter name
575 my @values = $c->req->param('scrolling_list');
577 # DFV requires a CGI.pm-like input hash
578 my $results = Data::FormValidator->check($c->req->params, \%dfv_profile);
582 You control the application flow with the C<forward> method, which
583 accepts the key of an action to execute. This can be an action in the
584 same or another Catalyst controller, or a Class name, optionally
585 followed by a method name. After a C<forward>, the control flow will
586 return to the method from which the C<forward> was issued.
588 A C<forward> is similar to a method call. The main differences are that
589 it wraps the call in an C<eval> to allow exception handling; it
590 automatically passes along the context object (C<$c> or C<$context>);
591 and it allows profiling of each call (displayed in the log with
595 my ( $self, $c ) = @_;
596 $c->stash->{message} = 'Hello World!';
597 $c->forward('check_message'); # $c is automatically included
600 sub check_message : Private {
601 my ( $self, $c ) = @_;
602 return unless $c->stash->{message};
603 $c->forward('show_message');
606 sub show_message : Private {
607 my ( $self, $c ) = @_;
608 $c->res->body( $c->stash->{message} );
611 A C<forward> does not create a new request, so your request
612 object (C<$c-E<gt>req>) will remain unchanged. This is a
613 key difference between using C<forward> and issuing a
616 You can pass new arguments to a C<forward> by adding them
617 in an anonymous array. In this case C<$c-E<gt>req-E<gt>args>
618 will be changed for the duration of the C<forward> only; upon
619 return, the original value of C<$c-E<gt>req-E<gt>args> will
623 my ( $self, $c ) = @_;
624 $c->stash->{message} = 'Hello World!';
625 $c->forward('check_message',[qw/test1/]);
626 # now $c->req->args is back to what it was before
629 sub check_message : Private {
630 my ( $self, $c ) = @_;
631 my $first_argument = $c->req->args[0]; # now = 'test1'
635 As you can see from these examples, you can just use the method name as
636 long as you are referring to methods in the same controller. If you want
637 to forward to a method in another controller, or the main application,
638 you will have to refer to the method by absolute path.
640 $c->forward('/my/controller/action');
641 $c->forward('/default'); # calls default in main application
643 Here are some examples of how to forward to classes and methods.
646 my ( $self, $c ) = @_;
647 $c->forward(qw/MyApp::Model::Hello say_hello/);
651 my ( $self, $c ) = @_;
652 $c->forward('MyApp::Model::Hello'); # no method: will try 'process'
655 package MyApp::Model::Hello;
658 my ( $self, $c ) = @_;
659 $c->res->body('Hello World!');
663 my ( $self, $c ) = @_;
664 $c->res->body('Goodbye World!');
667 Note that C<forward> returns to the calling action and continues
668 processing after the action finishes. If you want all further processing
669 in the calling action to stop, use C<detach> instead, which will execute
670 the C<detach>ed action and not return to the calling sub. In both cases,
671 Catalyst will automatically try to call process() if you omit the
676 Catalyst has an uncommonly flexible component system. You can define as many
677 L<Models>, L<Views>, and L<Controllers> as you like.
679 All components must inherit from L<Catalyst::Base>, which provides a simple
680 class structure and some common class methods like C<config> and C<new>
683 package MyApp::Controller::Catalog;
686 use base 'Catalyst::Base';
688 __PACKAGE__->config( foo => 'bar' );
692 You don't have to C<use> or otherwise register Models, Views, and
693 Controllers. Catalyst automatically discovers and instantiates them
694 when you call C<setup> in the main application. All you need to do is
695 put them in directories named for each Component type. Notice that you
696 can use some very terse aliases for each one.
700 =item * B<MyApp/Model/>
704 =item * B<MyApp/View/>
708 =item * B<MyApp/Controller/>
716 To show how to define views, we'll use an already-existing base class for the
717 L<Template Toolkit|Template>, L<Catalyst::View::TT>. All we need to do is
718 inherit from this class:
720 package MyApp::View::TT;
723 use base 'Catalyst::View::TT';
727 (You can also generate this automatically by using the helper script:
729 script/myapp_create.pl view TT TT
731 where the first C<TT> tells the script that the name of the view should
732 be C<TT>, and the second that it should be a Template Toolkit view.)
734 This gives us a process() method and we can now just do
735 $c->forward('MyApp::View::TT') to render our templates. The base class
736 makes process() implicit, so we don't have to say
737 C<$c-E<gt>forward(qw/MyApp::View::TT process/)>.
740 my ( $self, $c ) = @_;
741 $c->stash->{template} = 'hello.tt';
745 my ( $self, $c ) = @_;
746 $c->forward('MyApp::View::TT');
749 You normally render templates at the end of a request, so it's a perfect
750 use for the global C<end> action.
752 Also, be sure to put the template under the directory specified in
753 C<$c-E<gt>config-E<gt>{root}>, or you'll be forced to look at our
754 eyecandy debug screen. ;)
758 To show how to define models, again we'll use an already-existing base
759 class, this time for L<Class::DBI>: L<Catalyst::Model::CDBI>.
761 But first, we need a database.
765 id INTEGER PRIMARY KEY,
770 id INTEGER PRIMARY KEY,
771 foo INTEGER REFERENCES foo,
775 INSERT INTO foo (data) VALUES ('TEST!');
778 % sqlite /tmp/myapp.db < myapp.sql
780 Now we can create a CDBI component for this database.
782 package MyApp::Model::CDBI;
785 use base 'Catalyst::Model::CDBI';
788 dsn => 'dbi:SQLite:/tmp/myapp.db',
794 Catalyst automatically loads table layouts and relationships. Use the
795 stash to pass data to your templates.
800 use Catalyst '-Debug';
803 name => 'My Application',
804 root => '/home/joeuser/myapp/root'
810 my ( $self, $c ) = @_;
811 $c->stash->{template} ||= 'index.tt';
812 $c->forward('MyApp::View::TT');
816 my ( $self, $c, $id ) = @_;
817 $c->stash->{item} = MyApp::Model::CDBI::Foo->retrieve($id);
822 # Then, in a TT template:
823 The id is [% item.data %]
825 Models do not have to be part of your Catalyst application; you
826 can always call an outside module that serves as your Model:
830 my ( $self, $c ) = @_;
831 $c->stash->{template} = 'list.tt';
832 use Some::Outside::CDBI::Module;
833 my @records = Some::Outside::CDBI::Module->retrieve_all;
834 $c->stash->{records} = \@records;
837 But by using a Model that is part of your Catalyst application, you gain
838 several things: you don't have to C<use> each component, Catalyst will
839 find and load it automatically at compile-time; you can C<forward> to
840 the module, which can only be done to Catalyst components; and only
841 Catalyst components can be fetched with
842 C<$c-E<gt>model('SomeModel')>.
844 Happily, since many people have existing Model classes that they
845 would like to use with Catalyst (or, conversely, they want to
846 write Catalyst models that can be used outside of Catalyst, e.g.
847 in a cron job), it's trivial to write a simple component in
848 Catalyst that slurps in an outside Model:
850 package MyApp::Model::Catalog;
851 use base qw/Catalyst::Base Some::Other::CDBI::Module::Catalog/;
854 and that's it! Now C<Some::Other::CDBI::Module::Catalog> is part of your
855 Cat app as C<MyApp::Model::Catalog>.
859 Multiple controllers are a good way to separate logical domains of your
862 package MyApp::Controller::Login;
864 sub sign-in : Local { }
865 sub new-password : Local { }
866 sub sign-out : Local { }
868 package MyApp::Controller::Catalog;
873 package MyApp::Controller::Cart;
876 sub update : Local { }
877 sub order : Local { }
881 Catalyst has a built-in http server for testing! (Later, you can easily
882 use a more powerful server, e.g. Apache/mod_perl, in a production
885 Start your application on the command line...
887 script/myapp_server.pl
889 ...then visit http://localhost:3000/ in a browser to view the output.
891 You can also do it all from the command line:
893 script/myapp_test.pl http://localhost/
901 Join #catalyst on irc.perl.org.
905 http://lists.rawmode.org/mailman/listinfo/catalyst
906 http://lists.rawmode.org/mailman/listinfo/catalyst-dev
910 Sebastian Riedel, C<sri@oook.de>
911 David Naughton, C<naughton@umn.edu>
912 Marcus Ramberg, C<mramberg@cpan.org>
913 Jesse Sheidlower, C<jester@panix.com>
914 Danijel Milicevic, C<me@danijel.de>
918 This program is free software, you can redistribute it and/or modify it under
919 the same terms as Perl itself.