Commit | Line | Data |
aa2b0d97 |
1 | =head1 NAME |
2 | |
3 | Catalyst::Manual::About - Basic explanation of Catalyst |
4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | This document is a basic introduction to the I<why> of Catalyst. It does |
8 | not teach you how to write Catalyst applications; for an introduction to |
9 | that please see L<Catalyst::Manual::Intro>. Rather, it explains the |
10 | basics of what Catalyst is typically used for, and why you might want |
11 | to use Catalyst to build your applications. |
12 | |
13 | =head2 What is Catalyst? The short summary |
14 | |
15 | Catalyst is a web application framework. This means that you use it to |
16 | help build applications that run on the web, or that run using protocols |
17 | used for the web. Catalyst is designed to make it easy to manage the |
18 | various tasks you need to do to run an application on the web, either by |
19 | doing them itself, or by letting you "plug in" existing Perl modules |
20 | that do what you need. There are a number of things you typically do |
21 | with a web application, for example: |
22 | |
23 | =over 4 |
24 | |
25 | =item * Interact with a web server |
26 | |
27 | If you're on the web, you're relying on a web server, a program that |
28 | sends files over the web. There are a number of these, and your |
29 | application has to do the right thing to make sure that your data works |
30 | with the web server you're using. If you change your web server, you |
31 | don't want to have to rewrite your entire application to work with the |
32 | new one. |
33 | |
34 | =item * Do something based on a URI |
35 | |
4100e391 |
36 | It's typical for web applications to use URIs as a main way for users to |
37 | interact with the rest of the application; various elements of the URI |
38 | will indicate what the application needs to do. Thus, |
39 | C<http://www.mysite.com/add_record.cgi?name=John&title=President> will |
40 | add a person named "John" whose title is "President" to your database, |
41 | and C<http://www.mysite.com/catalog/display/23> will go to a "display" |
42 | of item 23 in your catalog, and |
95bb1e81 |
43 | C<http://www.mysite.com/order_status/7582> will display the status of |
44 | order 7582, and C<http://www.mysite.com/add_comment/?page=8> will |
4100e391 |
45 | display a form to add a comment to page 8. Your application needs to |
46 | have a regular way of processing these URIs so it knows what to do |
47 | when such a request comes in. |
aa2b0d97 |
48 | |
49 | =item * Interact with a data store |
50 | |
51 | You probably use a database to keep track of your information. Your |
4100e391 |
52 | application needs to interact with your database, so you can create, |
53 | edit, and retrieve your data. |
aa2b0d97 |
54 | |
55 | =item * Handle forms |
56 | |
57 | When a user submits a form, you process it, and make sure it's been |
58 | filled in properly, and then then do something based on the |
59 | result--submitting an order, updating a record, sending e-mail, or going |
60 | back to the form if there's an error. |
61 | |
62 | =item * Display results |
63 | |
64 | You have an application running on the web, people need to see |
65 | things. You usually want to display things on a web browser; you will |
66 | probably be using a template system to help generate HTML code; you |
67 | might need other kinds of display, such as PDF files or RSS feeds. |
68 | |
69 | =item * Manage users |
70 | |
71 | You might need the concept of a "user", someone who's allowed to use |
72 | your system, and is allowed to do certain things only. Perhaps normal |
73 | users can only view or modify their own information; administrative |
74 | users can view or modify anything; normal users can only order items for |
75 | their own account; normal users can view things but not modify them; |
76 | order-processing users can send records to a different part of the |
77 | system; and so forth. You need a way of ensuring that people are who |
78 | they say they are, and that people only do the things they're allowed to |
79 | do. |
80 | |
81 | =item * Develop the application itself |
82 | |
83 | When you're writing or modifying the application, you want to have |
84 | access to detailed logs of what it is doing. You want to be able to |
85 | write tests to ensure that it does what it's supposed to, and that new |
86 | changes don't break the existing code. |
87 | |
88 | =back |
89 | |
90 | Catalyst makes it easy to do all of these tasks, and many more. It is |
91 | extremely flexible in terms of what it allows you to do, and very fast. |
92 | It has a very large number of "plugins" that interact with existing Perl |
4100e391 |
93 | modules so that you can easily use them from within your |
94 | application. Interact with a web server? Catalyst lets you use a number |
95 | of different ones, and even comes with a built-in server for testing or |
96 | local deployment. Do something based on a URI? Catalyst has extremely |
97 | flexible systems for figuring out what to do based on a URI. Interact |
98 | with a data store? Catalyst has many plugins for different databases |
99 | and database frameworks, and for other non-database storage |
100 | systems. Handle forms? Catalyst has plugins available for several form |
101 | creation and validation systems that make it easy for the programmer to |
102 | manage. Display results? Catalyst has plugins available for a number of |
103 | template modules and other output packages. Manage users? Catalyst has |
104 | plugins that handle sessions, authentication, and authorization, in any |
105 | way you need. Developing the application? Catalyst has detailed logging |
106 | built-in, which you can configure as necessary, and supports the easy |
107 | creation of new tests--some of which are automatically created when you |
108 | begin writing a new application. |
aa2b0d97 |
109 | |
110 | =head3 What B<isn't> Catalyst? |
111 | |
112 | Catalyst is not an out-of-the-box solution that allows you to set up a |
113 | complete working e-commerce application in ten minutes. (There are, |
114 | however, several systems built on top of Catalyst that can get you very |
115 | close to a working app.) It is not designed for end users, but for |
116 | working programmers. |
117 | |
4100e391 |
118 | =head2 Web programming: The Olden Days |
119 | |
120 | Perl has long been favored for web applications. There are a wide |
121 | variety of ways to use Perl on the web, and things have changed over |
122 | time. It's possible to handle everything with very raw Perl code: |
123 | C<print "Content-type: text/html\n\n<center><h1>Hello |
124 | World!</h1></center>";> for example, or |
125 | |
126 | my @query_elements = split(/&/, $ENV{'QUERY_STRING'}); |
127 | foreach my $element (@query_elements) { |
128 | my ($name, $value) = split(/=/, $element); |
129 | # do something with your parameters, or kill yourself |
130 | # in frustration for having to program like this |
131 | } |
132 | |
133 | Much better than this is to use Lincoln Stein's great L<CGI> module, |
134 | which smoothly handles a wide variety of common tasks--parameter |
135 | parsing, generating form elements from Perl data structures, printing |
136 | http headers, escaping text, and very many more, all with your choice of |
137 | functional or object-oriented style. While L<CGI> was revolutionary and |
138 | is still widely used, it has various drawbacks that make it unsuitable |
139 | for larger applications: it is slow, your code with it generally |
140 | combines application logic and display code, and it makes it very |
141 | difficult to handle larger applications with complicated control flow. |
142 | |
143 | A variety of frameworks followed, of which the most widely used is |
144 | probably L<CGI::Application>, which encourages the development of |
145 | modular code, with easy-to-understand control-flow handling, the use of |
146 | plugins and templating systems, and the like. Other systems include |
147 | L<AxKit>, which is designed for use with XML running under mod_perl, and |
148 | L<Maypole>--upon which Catalyst was originally based--designed for the |
149 | easy development of powerful web databases. Is it not the purpose of |
150 | this document to criticize or even evaluate these other frameworks; they |
151 | may be useful for you and if so we encourage you to give them a try. |
152 | |
aa2b0d97 |
153 | =head2 The MVC pattern |
154 | |
95bb1e81 |
155 | MVC, or Model-View-Controller, is a model currently favored for web |
156 | applications. This design pattern is originally from the Smalltalk |
157 | programming language. The basic idea is that the three main areas of an |
158 | application--handling application flow (Controller), processing |
159 | information (Model), and outputting the results (View)--are kept |
160 | separate, so that it is possible to change or replace any one without |
4100e391 |
161 | affecting the others, and so that if you're interested in one particular |
162 | aspect, you know where to find it. |
95bb1e81 |
163 | |
164 | Discussions of MVC often degenerate into nitpicky arguments about the |
165 | history of the pattern, and exactly what "usually" or "should" go into |
166 | the Controller or the Model. We have no interest in joining such a |
167 | debate. In any case, Catalyst does not enforce any particular setup; you |
168 | are free to put any sort of code in any part of your application, and |
169 | this discussion (and others elsewhere in the Catalyst documentation) is |
170 | only a suggestion based on what we think works well. In most Catalyst |
171 | applications, each branch of MVC will be made of up of several Perl |
172 | modules that can handle different needs in your application. |
173 | |
174 | The purpose of the B<Model> is to access and modify data. Typically |
175 | the Model will interact with a relational database, but it's also |
176 | common to use other data sources, such as the L<Plucene> search |
177 | engine, an LDAP server, etc. |
178 | |
179 | The purpose of the B<View> is to present data to the user. Typical Views |
180 | use a templating module to generate HTML code, using L<Template |
181 | Toolkit|Template>, L<Mason|HTML::Mason>, L<HTML::Template>, or the like, |
182 | but it's also possible to generate PDF output, send email, etc., from a |
183 | View. In Catalyst the View is usually a small module, just gluing some |
184 | other module into Catalyst; the display logic is written within the |
185 | template itself. |
186 | |
187 | The Controller is Catalyst itself. When a request is made to Catalyst, |
188 | it will be received by one of your Controller modules; this module |
189 | will figure out what the user is trying to do, gather the necessary |
190 | data from a Model, and send it to a View for display. |
191 | |
192 | =head3 A simple example |
193 | |
194 | The general idea is that you should be able to change things around |
195 | without affecting the rest of your application. Let's look at a very |
196 | simple example (keeping in mind that there are many ways of doing this, |
197 | and what we're discussing is one possible way, not the only |
198 | way). Suppose you have a record to display. It doesn't matter if it's a |
199 | catalog entry, a library book, a music CD, a personnel record, or |
200 | anything else, but let's pretend it's a catalog entry. A user is given a |
201 | URL such as C<http://www.mysite.com/catalog/display/2782>. Now what? |
202 | |
203 | First, Catalyst figures out that you're using the "catalog" Controller |
204 | (how Catalyst figures this out is entirely up to you; URL dispatching is |
205 | I<extremely> flexible in Catalyst). Then Catalyst sees that you want to |
206 | use a "display" method in your Controller. Somewhere in this process, |
207 | it's possible that you'll have authentication and authorization routines |
208 | to make sure that the user is registered and is allowed to display a |
209 | record. The Controller's display method will then extract "2782" as the |
210 | record you want to retrieve, and make a request to a Model for that |
211 | record. The Controller will then look at what the Model returns: if |
212 | there's no record, the Controller will ask the View to display an error |
213 | message, otherwise it will hand the View the record and ask the View to |
214 | display it. In either case, the View will then generate an HTML page, |
215 | which Catalyst will display to the user's browser, using whatever web |
216 | server you've configured. |
217 | |
218 | How does this help you? |
219 | |
220 | In many ways. Suppose you have a small catalog now, and you're using a |
221 | lightweight database such as SQLite, or even a text file. But eventually |
222 | your site grows, and you need to upgrade to something more |
223 | powerful--MySQL or Postgres, or even Oracle or DB2. If your Model is |
224 | separate, you only have to change one thing, the Model; your Controller |
225 | can expect that if it issues a query to the Model, it will get the right |
226 | kind of result back. |
227 | |
228 | What about the View? The idea is that your template is concerned almost |
229 | entirely with display, so that you can hand it off to a designer who |
230 | doesn't have to worry about how to write code. If you get all the data |
231 | in the Controller and then pass it to the View, the template isn't |
232 | responsible for any kind of data processing. And if you want to change |
233 | your output, it's simple: just write a new View. If your Controller is |
234 | already getting the data you need, you pass it in the same way, and |
235 | whether you display the results to a web browser, generate a PDF, or |
236 | e-mail the results back to the user, the Controller hardly changes at |
237 | all. |
238 | |
239 | And throughout the whole process, most of the tools you need are either |
240 | part of Catalyst (the parameter-processing routines that extract "2782" |
241 | from the URL, for example) or are easily plugged into it (the |
242 | authentication routines, the plugins for using Template Toolkit as your |
243 | View). |
244 | |
245 | Now, Catalyst doesn't enforce very much at all. You can connect to a |
246 | database, issue queries, and act on them from within your View, if you |
247 | want. You can handle paging (i.e. retrieving only a portion of the total |
248 | records possible) in your Controller or your Model. It's up to you. In |
249 | some cases there might be very good reasons to do things a certain way |
250 | (issuing database queries from a template seems to defeat the whole |
251 | purpose of separation-of-concerns, and will drive your designer crazy), |
252 | while in others it's just a matter of personal preference (perhaps your |
253 | template, rather than your Controller, is the better place to decide |
254 | what to display if you get an empty result). Catalyst just gives you the |
255 | tools. |
256 | |
aa2b0d97 |
257 | =head1 AUTHOR |
258 | |
259 | Jesse Sheidlower, C<jester@panix.com> |
260 | |
261 | =head1 SEE ALSO |
262 | |
263 | L<Catalyst>, L<Catalyst::Manual::Intro> |
264 | |
265 | =head1 COPYRIGHT |
266 | |
267 | This program is free software, you can redistribute it and/or modify it |
268 | under the same terms as Perl itself. |