1Maypole::Manual::About(U3s)er Contributed Perl DocumentatMiaoynpole::Manual::About(3)
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6 Maypole::Manual::About - Introduction to Maypole
7
9 This chapter serves as a gentle introduction to Maypole and setting up
10 Maypole applications. We look at what Maypole is, how to get it up and
11 running, and how to start thinking about building Maypole applications.
12 What is Maypole?
14 Presumably you have some idea of what Maypole is all about, or
15 otherwise you wouldn't be reading this manual. But Maypole is good at
16 many different things, and you may have accidentally focussed on one
17 aspect of Maypole while missing the big picture.
18 For instance, you may know that Maypole is extremely good at putting
20 web front-ends onto databases. This is true, but it's only a part of
21 what Maypole does. You may have heard that Maypole is a web application
22 framework, which is true, but it doesn't mean very much. There are a
23 huge number of things that Maypole can do, because it's very much a
24 blank slate. You can make it do what you will. In this manual, we'll be
25 making it act as a front-end to a database, as a social network site,
26 as an intranet portal, and many other things besides. It is a
27 framework.
28 I like to think that Maypole is a way of going from a URL to a method
30 call to some output. If you have a URL like "/product/order/12",
31 Maypole is a way of having it load up product number 12, call an
32 "order" method, and produce a page about what it's just done. The
33 reason Maypole is such a big deal is because it does all this for you.
34 You no longer have to care about your web server. You hardly have to
35 care about your database. You don't have to care about templating
36 modules, parsing CGI parameters, or anything else. You only need to
37 care about business logic, and the business logic in this instance is
38 how you "order" a product, and what you need to display about it once
39 you've done so. This is what programming should be: only caring about
40 the work that distinguishes one program from another.
41 It does this using a technique called MVC for web applications.
43 What is MVC for web applications?
45 Maypole was originally called "Apache::MVC", reflecting its basis in
46 the Model-View-Controller design pattern. (I had to change it firstly
47 because Maypole isn't tied to Apache, and secondly because
48 "Apache::MVC" is a really dull name.) It's the same design pattern that
49 forms the foundation of similar projects in other languages, such as
50 Java's Struts framework.
51 This design pattern is found primarily in graphical applications; the
53 idea is that you have a Model class which represents and manipulates
54 your data, a View class which is responsible for displaying that data
55 to the user, and a Controller class which controls the other classes in
56 response to events triggered by the user. This analogy doesn't
57 correspond precisely to a web-based application, but we can take an
58 important principle from it. As Template Toolkit author Andy Wardley
59 explains:
60 What the MVC-for-the-web crowd are really trying to achieve is a clear
62 separation of concerns. Put your database code in one place, your
63 application code in another, your presentation code in a third place.
64 That way, you can chop and change different elements at will,
65 hopefully without affecting the other parts (depending on how well your
66 concerns are separated, of course). This is common sense and good practice.
67 MVC achieves this separation of concerns as a by-product of clearly
68 separating inputs (controls) and outputs (views).
69 This is what Maypole does. It has a number of database drivers, a
71 number of front-end drivers and a number of templating presentation
72 drivers. In common cases, Maypole provides precisely what you need for
73 all of these areas, and you get to concentrate on writing just the
74 business logic of your application. This is one of the reasons why
75 Maypole lets you develop so rapidly: because most of the time, you
76 don't need to do any development at all.
77 The Beer Database example
79 Throughout this manual, we're going to be referring back to a
80 particular application so that we can give concrete examples for the
81 concepts we're talking about. We could say ""related_accessors" returns
82 a list of accessors which can be called to return a list of objects in
83 a has-a relationship to the original", or we could say "if we call
84 "related_accessors" while viewing a "brewery", it returns "beers",
85 because we can call "beers" on a "brewery" object to get a list of that
86 brewery's beers."
87 Because Maypole is all about beer. If you look carefully, you can
89 probably see men playing cricket on the village green. The first ever
90 Maypole application was written to help me keep track of the many
91 different ales available in my area - their styles, their tastes, their
92 breweries, prices and so on. Then the more I thought about it, the more
93 I thought it was a particularly good data model for demonstrating
94 different kinds of database relationships.
95 We have a "brewery" table, which has several "beer"s. We'll call this a
97 has-many relationship. The beers each have a "style"; styles are stored
98 in a separate table, so "beer" has-a "style". Beers are in several pubs
99 and a pub has several beers, so beers and pubs are in a many-to-many
100 relationship. We use a link table called "handpump" to relate pubs to
101 beers.
102 All in all, this gives us a schema like the following:
104 create table brewery (
106 id int not null auto_increment primary key,
107 name varchar(30),
108 url varchar(50),
109 notes text
110 );
111 create table beer (
113 id int not null auto_increment primary key,
114 brewery integer,
115 style integer,
116 name varchar(30),
117 url varchar(120),
118 score integer(2),
119 price varchar(12),
120 abv varchar(10),
121 notes text
122 );
123 create table handpump (
125 id int not null auto_increment primary key,
126 beer integer,
127 pub integer
128 );
129 create table pub (
131 id int not null auto_increment primary key,
132 name varchar(60),
133 url varchar(120),
134 notes text
135 );
136 create table style (
138 id int not null auto_increment primary key,
139 name varchar(60),
140 notes text
141 );
142 If you have "DBD::SQLite" available, then a database like this will be
144 created when Maypole was installed. Let's now see how to set it up with
145 a web interface.
146 Setting up Maypole
148 The first thing we need for a Maypole interface to a database is to
149 have a database. If you don't have one, now would be a good time to
150 create one, using the schema above. If you're creating a database by
151 hand, don't forget to grant permissions for your Apache server to
152 access it as well as yourself (typically a user name like "www-data" or
153 "wwwrun").
154 The next thing we need is a module which is going to do all the work.
156 Thankfully, it doesn't need to do all the work itself. It's going to be
157 a subclass of "Maypole" or a Maypole front-end like "Apache::MVC". It
158 roughly corresponds to the controller in an MVC design, and is also
159 referred to as the driver, handler or request.
160 Here's the driver class for our beer database application. We're not
162 going to go into much detail about it here; we'll do that in the Beer
163 Database chapter. For now, simply admire its brevity, as you realise
164 this is all the code you need to write for a simple database front-end:
165 package BeerDB;
167 use Maypole::Application;
168 BeerDB->setup("dbi:SQLite:t/beerdb.db");
169 BeerDB->config->uri_base("http://localhost/beerdb");
170 BeerDB->config->template_root("/path/to/templates");
171 BeerDB->config->rows_per_page(10);
172 BeerDB->config->display_tables([qw[beer brewery pub style]]);
173 BeerDB::Brewery->untaint_columns( printable => [qw/name notes url/] );
174 BeerDB::Style->untaint_columns( printable => [qw/name notes/] );
175 BeerDB::Beer->untaint_columns(
176 printable => [qw/abv name price notes/],
177 integer => [qw/style brewery score/],
178 date => [ qw/date/],
179 );
180 use Class::DBI::Loader::Relationship;
182 BeerDB->config->{loader}->relationship($_) for (
183 "a brewery produces beers",
184 "a style defines beers",
185 "a pub has beers on handpumps");
186 1;
187 There's a version of this program in the examples/ directory in the
189 Maypole files that you downloaded in the ~root/.cpan/ build area. This
190 defines the "BeerDB" application. To set it up as a mod_perl handler,
191 just tell the Apache configuration about it:
192 <Location /beerdb>
194 SetHandler perl-script
195 PerlHandler BeerDB
196 </Location>
197 To use it as a CGI script, put it in your cgi-bin directory, together
199 with a small file called beer.cgi:
200 #!/usr/bin/perl
202 use strict;
203 use warnings;
204 use BeerDB;
205 BeerDB->run();
206 and change one line in "BeerDB.pm":
208 BeerDB->config->uri_base("http://localhost/cgi-bin/beer.cgi");
210 And now we need some templates. As we'll see in the chapter on views,
212 there are several types of template. We're going to copy the whole lot
213 from the templates/ directory of the Maypole source package into the
214 /beerdb directory under our web root. Make the "template_root" in
215 "BeerDB" agree with your path.
216 And that's it. We should now be able to go to
218 "http://localhost/beerdb/" or "http://localhost/cgi-bin/beer.cgi/" and
219 see a menu of things to browse; "http://localhost/beerdb/beer/list"
220 will give a list of beers. There might not be any yet. There's a box
221 that lets you add them.
222 If you have any problems getting to this point, you might want to look
224 at <http://maypole.perl.org>. There's a FAQ and a link to a mailing
225 list.
226 Play about with the site. Add some beers. Maybe go out and buy some
228 beers to review if you need some inspiration. Don't be ill on my
229 carpet.
230 Phases of a Maypole request
232 Now you should have a feel for what Maypole can do. The important thing
233 to know at this point is that this is by no means all that Maypole can
234 do. What you've seen in the beer database example is all that Maypole
235 can do if you don't customize it at all.
236 Remember that, for instance, we don't ever tell Maypole what tables our
238 database has, or what columns each table has. We don't tell Maypole
239 what those tables should be called or how to display them. We don't
240 tell Maypole what to do - that we want to list, search, edit and delete
241 beers and breweries. Maypole just works that out for itself. We can
242 customize it and have Maypole do all sorts of interesting things with
243 our database, and most of the rest of this manual will be about how to
244 do that.
245 In order to do that, we need to look at what Maypole's actually doing.
247 Here's a quick overview, there's more detail in the Workflow chapter.
248 As mentioned, Maypole is responsible for turning a URL into an object,
250 a method call, and some templated output.
251 Maypole's process revolves around the concept of the Maypole request
253 object. This is a little like Apache's request object, but at a much
254 higher level - in fact, in "mod_perl"-based Maypole front-ends, the
255 Apache request object is incorporated in the Maypole request object.
256 All that Maypole does is gradually flesh out this object until it
257 contains something in the "output" member, and then it is dispatched
258 back to the front-end for output.
259 So to start with, we take the Apache request (or CGI object, or other
261 way of isolating what's going on) and break it down. For instance, we
262 turn the URL "/beer/view/1" into
263 {
265 table => "beer",
266 action => "view",
267 args => [ 1 ]
268 }
269 Then Maypole will check that "beer" is a real table, and find the class
271 that models it. It also checks whether or not we're allowed to call the
272 "view" method over the network:
273 {
275 table => "beer",
276 action => "view",
277 args => [ 1 ],
278 model_class => "BeerDB::Beer"
279 }
280 Then there's a user-defined authentication method, which by default
282 just lets us do anything. Now we hand over to the model class, which
283 loads up the object, and decides what template we want to use:
284 {
286 table => "beer",
287 action => "view",
288 args => [ ],
289 objects => [ BeerDB::Beer->retrieve(1) ],
290 model_class => "BeerDB::Beer",
291 template => "view"
292 }
293 Then it calls "BeerDB::Beer->view", passing in the request object as a
295 parameter, and passes the whole lot to the view class for templating.
296 In the next two chapters, we'll look at how Maypole's default model and
297 view classes generally do what you want them to do.
298 Links
300 Contents, Next Maypole Model Classes
301perl v5.34.0 2021-07-22 Maypole::Manual::About(3)