=head1 NAME

Catalyst::Manual::About - Basic explanation 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 data 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

So that 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.

=item * Interact with a data store

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

=item * Handle forms

When a user submits a form, you process it, and make sure it's been
filled in properly, and then then do something based on the
result--submitting an order, updating a record, sending e-mail, or going
back to the form if there's an error.

=item * Display results

You have an application running on the web, people need to see
things. You usually want to display things on a web browser; you will
probably be using a template system to help generate HTML code; you
might need other kinds of display, such as PDF files or RSS feeds.

=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 using them from within your application.

=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.) It is not designed for end users, but for
working programmers.

=head2 Some background

=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.

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 (and others elsewhere in the Catalyst documentation) is
only a suggestion 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<Plucene> search
engine, an LDAP server, etc.

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 email, etc., from a
View. In Catalyst 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 sees that you want to
use a "display" method in your Controller. 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 display 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 even 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 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. 

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, the plugins for using Template Toolkit as your
View).

Now, Catalyst doesn't enforce very much at all. You can connect to a
database, issue queries, and act on them from within your 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. It's up to you. In
some cases there might be very good reasons to do things a certain way
(issuing database queries from a template seems to defeat 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.