--- /dev/null
+=head1 NAME
+
+Catalyst::Manual::About - The philosophy of Catalyst
+
+=head1 DESCRIPTION
+
+This document is a basic introduction to the I<why> of Catalyst. It does
+not teach you how to write Catalyst applications; for an introduction to
+that please see L<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.
+
+=head2 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:
+
+=over 4
+
+=item * 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.
+
+=item * 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,
+C<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 C<http://www.mysite.com/catalog/display/23> will go to a "display"
+of item 23 in your catalog, and
+C<http://www.mysite.com/order_status/7582> will display the status of
+order 7582, and C<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.
+
+=item * 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.
+
+=item * 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.
+
+=item * 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.
+
+=item * 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.
+
+=item * 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.
+
+=back
+
+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 very large number of "plugins" that interact with existing Perl
+modules so that you can easily use them from within your
+application.
+
+=over 4
+
+=item * 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.
+
+=item * Do something based on a URI?
+
+Catalyst has extremely flexible systems for figuring out what to do
+based on a URI.
+
+=item * Interact with a data store?
+
+Catalyst has many plugins for different databases and database
+frameworks, and for other non-database storage systems.
+
+=item * Handle forms?
+
+Catalyst has plugins available for several form creation and validation
+systems that make it easy for the programmer to manage.
+
+=item * Display results?
+
+Catalyst has plugins available for a number of template modules and
+other output packages.
+
+=item * Manage users?
+
+Catalyst has plugins that handle sessions, authentication, and
+authorization, in any way you need.
+
+=item * 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.
+
+=back
+
+=head3 What B<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.
+
+=head2 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 L<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 L<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 L<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
+L<AxKit>, which is designed for use with XML running under mod_perl;
+L<Maypole>--upon which Catalyst was originally based--designed for the
+easy development of powerful web databases; L<Jifty>, which does a great
+deal of automation in helping to set up web sites with many complex
+features; and Ruby on Rails (see L<http://www.rubyonrails.org>), written
+of course in Ruby and among the most popular web development systems. Is
+it 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.
+
+=head2 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 B<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 L<Xapian|Catalyst::Model::Xapian>
+search engine or an LDAP server.
+
+The purpose of the B<View> is to present data to the user. Typical Views
+use a templating module to generate HTML code, using L<Template
+Toolkit|Template>, L<Mason|HTML::Mason>, L<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.
+
+=head3 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 C<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
+I<extremely> flexible in Catalyst). Then Catalyst determines that you
+want to use a C<display> method in your "catalog" Controller. (There
+could be other C<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 C<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.
+
+=head1 AUTHOR
+
+Jesse Sheidlower, C<jester@panix.com>
+
+=head1 SEE ALSO
+
+L<Catalyst>, L<Catalyst::Manual::Intro>
+
+=head1 COPYRIGHT
+
+This program is free software, you can redistribute it and/or modify it
+under the same terms as Perl itself.
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