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Catalyst::Manual::AbouUser Contributed Perl DocumentCatalyst::Manual::About(3)

NAME
       Catalyst::Manual::About - The philosophy of Catalyst

DESCRIPTION
       This document is a basic introduction to the why of Catalyst. It does
       not teach you how to write Catalyst applications; for an introduction
       to that please see Catalyst::Manual::Intro. Rather, it explains the
       basics of what Catalyst is typically used for, and why you might want
       to use Catalyst to build your applications.

   What is Catalyst? The short summary
       Catalyst is a web application framework. This means that you use it to
       help build applications that run on the web, or that run using
       protocols used for the web. Catalyst is designed to make it easy to
       manage the various tasks you need to do to run an application on the
       web, either by doing them itself, or by letting you "plug in" existing
       Perl modules that do what you need. There are a number of things you
       typically do with a web application. For example:

       ·   Interact with a web server

	   If you're on the web, you're relying on a web server, a program
	   that sends files over the web. There are a number of these, and
	   your application has to do the right thing to make sure that your
	   program works with the web server you're using. If you change your
	   web server, you don't want to have to rewrite your entire
	   application to work with the new one.

       ·   Do something based on a URI

	   It's typical for web applications to use URIs as a main way for
	   users to interact with the rest of the application; various
	   elements of the URI will indicate what the application needs to do.
	   Thus,
	   "http://www.mysite.com/add_record.cgi?name=John&title=President"
	   will add a person named "John" whose title is "President" to your
	   database, and "http://www.mysite.com/catalog/display/23" will go to
	   a "display" of item 23 in your catalog, and
	   "http://www.mysite.com/order_status/7582" will display the status
	   of order 7582, and "http://www.mysite.com/add_comment/?page=8" will
	   display a form to add a comment to page 8. Your application needs
	   to have a regular way of processing these URIs so it knows what to
	   do when such a request comes in.

       ·   Interact with a data store

	   You probably use a database to keep track of your information. Your
	   application needs to interact with your database, so you can
	   create, edit, and retrieve your data.

       ·   Handle forms

	   When a user submits a form, you receive it, process it to make sure
	   it's been filled in properly, and then do something based on the
	   result--submit an order, update a record, send e-mail, or return to
	   the form if there's an error.

       ·   Display results

	   If you have an application running on the web, people need to see
	   things. You usually want your application displayed on a web
	   browser, in which case you will probably be using a template system
	   to help generate HTML code. But you might need other kinds of
	   display, such as PDF files, or other forms of output, such as RSS
	   feeds or e-mail.

       ·   Manage users

	   You might need the concept of a "user", someone who's allowed to
	   use your system, and is allowed to do certain things only. Perhaps
	   normal users can only view or modify their own information;
	   administrative users can view or modify anything; normal users can
	   only order items for their own account; normal users can view
	   things but not modify them; order-processing users can send records
	   to a different part of the system; and so forth. You need a way of
	   ensuring that people are who they say they are, and that people
	   only do the things they're allowed to do.

       ·   Develop the application itself

	   When you're writing or modifying the application, you want to have
	   access to detailed logs of what it is doing. You want to be able to
	   write tests to ensure that it does what it's supposed to, and that
	   new changes don't break the existing code.

       Catalyst makes it easy to do all of these tasks, and many more. It is
       extremely flexible in terms of what it allows you to do, and very fast.
       It has a large number of "components" and "plugins" that interact with
       existing Perl modules so that you can easily use them from within your
       application.

       ·   Interact with a web server?

	   Catalyst lets you use a number of different ones, and even comes
	   with a built-in server for testing or local deployment.

       ·   Do something based on a URI?

	   Catalyst has extremely flexible systems for figuring out what to do
	   based on a URI.

       ·   Interact with a data store?

	   Catalyst has many plugins for different databases and database
	   frameworks, and for other non-database storage systems.

       ·   Handle forms?

	   Catalyst has plugins available for several form creation and
	   validation systems that make it easy for the programmer to manage.

       ·   Display results?

	   Catalyst has plugins available for a number of template modules and
	   other output packages.

       ·   Manage users?

	   Catalyst has plugins that handle sessions, authentication, and
	   authorization, in any way you need.

       ·   Developing the application?

	   Catalyst has detailed logging built-in, which you can configure as
	   necessary, and supports the easy creation of new tests--some of
	   which are automatically created when you begin writing a new
	   application.

       What isn't Catalyst?

       Catalyst is not an out-of-the-box solution that allows you to set up a
       complete working e-commerce application in ten minutes. (There are,
       however, several systems built on top of Catalyst that can get you very
       close to a working app.)

       Catalyst is designed for flexibility and power; to an extent, this
       comes at the expense of simplicity. Programmers have many options for
       almost everything they need to do, which means that any given need can
       be done in many ways, and finding the one that's right for you, and
       learning the right way to do it, can take time. TIMTOWDI works both
       ways.

       Catalyst is not designed for end users, but for working programmers.

   Web programming: The Olden Days
       Perl has long been favored for web applications. There are a wide
       variety of ways to use Perl on the web, and things have changed over
       time. It's possible to handle everything with very raw Perl code:

	   print "Content-type: text/html\n\n<center><h1>Hello
	   World!</h1></center>";

       for example, or

	   my @query_elements = split(/&/, $ENV{'QUERY_STRING'});
	   foreach my $element (@query_elements) {
	       my ($name, $value) = split(/=/, $element);
	       # do something with your parameters, or kill yourself
	       # in frustration for having to program like this
	   }

       Much better than this is to use Lincoln Stein's great CGI module, which
       smoothly handles a wide variety of common tasks--parameter parsing,
       generating form elements from Perl data structures, printing http
       headers, escaping text, and very many more, all with your choice of
       functional or object-oriented style. While CGI was revolutionary and is
       still widely used, it has various drawbacks that make it unsuitable for
       larger applications: it is slow; your code with it generally combines
       application logic and display code; and it makes it very difficult to
       handle larger applications with complicated control flow.

       A variety of frameworks followed, of which the most widely used is
       probably CGI::Application, which encourages the development of modular
       code, with easy-to-understand control-flow handling, the use of plugins
       and templating systems, and the like. Other systems include AxKit,
       which is designed for use with XML running under mod_perl;
       Maypole--upon which Catalyst was originally based--designed for the
       easy development of powerful web databases; Jifty, which does a great
       deal of automation in helping to set up web sites with many complex
       features; and Ruby on Rails (see <http://www.rubyonrails.org>), written
       of course in Ruby and among the most popular web development systems.
       It is not the purpose of this document to criticize or even briefly
       evaluate these other frameworks; they may be useful for you and if so
       we encourage you to give them a try.

   The MVC pattern
       MVC, or Model-View-Controller, is a model currently favored for web
       applications. This design pattern is originally from the Smalltalk
       programming language. The basic idea is that the three main areas of an
       application--handling application flow (Controller), processing
       information (Model), and outputting the results (View)--are kept
       separate, so that it is possible to change or replace any one without
       affecting the others, and so that if you're interested in one
       particular aspect, you know where to find it.

       Discussions of MVC often degenerate into nitpicky arguments about the
       history of the pattern, and exactly what "usually" or "should" go into
       the Controller or the Model. We have no interest in joining such a
       debate. In any case, Catalyst does not enforce any particular setup;
       you are free to put any sort of code in any part of your application,
       and this discussion, along with others elsewhere in the Catalyst
       documentation, are only suggestions based on what we think works well.
       In most Catalyst applications, each branch of MVC will be made of up of
       several Perl modules that can handle different needs in your
       application.

       The purpose of the Model is to access and modify data. Typically the
       Model will interact with a relational database, but it's also common to
       use other data sources, such as the Xapian search engine or an LDAP
       server.

       The purpose of the View is to present data to the user. Typical Views
       use a templating module to generate HTML code, using Template Toolkit,
       Mason, HTML::Template, or the like, but it's also possible to generate
       PDF output, send e-mail, etc., from a View. In Catalyst applications
       the View is usually a small module, just gluing some other module into
       Catalyst; the display logic is written within the template itself.

       The Controller is Catalyst itself. When a request is made to Catalyst,
       it will be received by one of your Controller modules; this module will
       figure out what the user is trying to do, gather the necessary data
       from a Model, and send it to a View for display.

       A simple example

       The general idea is that you should be able to change things around
       without affecting the rest of your application. Let's look at a very
       simple example (keeping in mind that there are many ways of doing this,
       and what we're discussing is one possible way, not the only way).
       Suppose you have a record to display. It doesn't matter if it's a
       catalog entry, a library book, a music CD, a personnel record, or
       anything else, but let's pretend it's a catalog entry. A user is given
       a URL such as "http://www.mysite.com/catalog/display/2782". Now what?

       First, Catalyst figures out that you're using the "catalog" Controller
       (how Catalyst figures this out is entirely up to you; URL dispatching
       is extremely flexible in Catalyst). Then Catalyst determines that you
       want to use a "display" method in your "catalog" Controller. (There
       could be other "display" methods in other Controllers, too.) Somewhere
       in this process, it's possible that you'll have authentication and
       authorization routines to make sure that the user is registered and is
       allowed to display a record. The Controller's "display" method will
       then extract "2782" as the record you want to retrieve, and make a
       request to a Model for that record. The Controller will then look at
       what the Model returns: if there's no record, the Controller will ask
       the View to display an error message, otherwise it will hand the View
       the record and ask the View to display it. In either case, the View
       will then generate an HTML page, which Catalyst will send to the user's
       browser, using whatever web server you've configured.

       How does this help you?

       In many ways. Suppose you have a small catalog now, and you're using a
       lightweight database such as SQLite, or maybe just a text file. But
       eventually your site grows, and you need to upgrade to something more
       powerful--MySQL or Postgres, or even Oracle or DB2. If your Model is
       separate, you only have to change one thing, the Model; your Controller
       can expect that if it issues a query to the Model, it will get the
       right kind of result back.

       What about the View? The idea is that your template is concerned almost
       entirely with display, so that you can hand it off to a designer who
       doesn't have to worry about how to write code. If you get all the data
       in the Controller and then pass it to the View, the template isn't
       responsible for any kind of data processing. And if you want to change
       your output, it's simple: just write a new View. If your Controller is
       already getting the data you need, you can pass it in the same way, and
       whether you display the results to a web browser, generate a PDF, or
       e-mail the results back to the user, the Controller hardly changes at
       all--it's up to the View.

       And throughout the whole process, most of the tools you need are either
       part of Catalyst (the parameter-processing routines that extract "2782"
       from the URL, for example) or are easily plugged into it (the
       authentication routines, or the plugins for using Template Toolkit as
       your View).

       Now, Catalyst doesn't enforce very much at all. Template Toolkit is a
       very powerful templating system, and you can connect to a database,
       issue queries, and act on them from within a TT-based View, if you
       want. You can handle paging (i.e. retrieving only a portion of the
       total records possible) in your Controller or your Model. In the above
       example, your Controller looked at the query result, determining
       whether to ask the View for a no-result error message, or for a result
       display; but it's perfectly possible to hand your query result directly
       to the View, and let your template decide what to do. It's up to you;
       Catalyst doesn't enforce anything.

       In some cases there might be very good reasons to do things a certain
       way (issuing database queries from a template defeats the whole purpose
       of separation-of-concerns, and will drive your designer crazy), while
       in others it's just a matter of personal preference (perhaps your
       template, rather than your Controller, is the better place to decide
       what to display if you get an empty result). Catalyst just gives you
       the tools.

SEE ALSO
       Catalyst, Catalyst::Manual::Intro

AUTHORS
       Catalyst Contributors, see Catalyst.pm

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

perl v5.14.2			  2011-02-16	    Catalyst::Manual::About(3)
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