=head1 Introduction to the Maypole Request Model

This chapter serves as a gentle introduction to Maypole and setting up
Maypole applications. We look at Maypole is, how to get it up and
running, and how to start thinking about building Maypole applications.

=head2 What is Maypole?

Presumably you have some idea of what Maypole is all about, or otherwise
you wouldn't be reading this manual. But Maypole is good at many
different things, and you may have accidentally focussed on one aspect
of Maypole while missing the big picture.

For instance, you may know that Maypole is extremely good at putting web
front-ends onto databases. This is true, but it's only a part of what
Maypole does. You may have heard that Maypole is a web application
framework, which is true, but it doesn't mean very much. There are a
huge number of things that Maypole can do, because it's very much a
blank slate. You can make it do what you will. In this manual, we'll be
making it act as a front-end to a database, as a social network site, as
an intranet portal, and many other things besides. It is a framework.

I like to think that Maypole is a way of going from a URL to a method
call to some output. If you have a URL like C</product/order/12>,
Maypole is a way of having it load up product number 12, call an
C<order> method, and produce a page about what it's just done. The
reason Maypole is such a big deal is because it does all this for you.
You no longer have to care about your web server. You hardly have to
care about your database. You don't have to care about templating
modules, parsing CGI parameters, or anything else. You only need to care
about business logic, and the business logic in this instance is how you
C<order> a product, and what you need to display about it once you've
done so. This is what programming should be: only caring about the work
that distinguishes one program from another.

It does this using a technique called MVC for web applications.

=head2 What is MVC for web applications?

Maypole was originally called C<Apache::MVC>, reflecting its basis in
the Model-View-Controller design pattern. (I had to change it firstly
because Maypole isn't tied to Apache, and secondly because
C<Apache::MVC> is a really dull name.) It's the same design pattern that
forms the foundation of similar projects in other languages, such as
Java's Struts framework.

This design pattern is found primarily in graphical applications; the
idea is that you have a Model class which represents and manipulates
your data, a View class which is responsible for displaying that data to
the user, and a Controller class which controls the other classes in
response to events triggered by the user. This analogy doesn't
correspond precisely to a web-based application, but we can take an
important principle from it. As Andy Wardley explains:

    What the MVC-for-the-web crowd are really trying to achieve is a clear
    separation of concerns.  Put your database code in one place, your 
    application code in another, your presentation code in a third place.  
    That way, you can chop and change different elements at will,
    hopefully without affecting the other parts (depending on how well your
    concerns are separated, of course).  This is common sense and good practice.
    MVC achieves this separation of concerns as a by-product of clearly 
    separating inputs (controls) and outputs (views). 

This is what Maypole does. It has a number of database drivers, a number
of front-end drivers and a number of templating presentation drivers.
In common cases, Maypole provides precisely what you need for all of
these areas, and you get to concentrate on writing just the business
logic of your application. This is one of the reasons why Maypole lets
you develop so rapidly: because most of the time, you don't need to do
any development at all.

=head2 Installing Maypole

The first thing you're going to need to do to get Maypole running is to
install it. Maypole needs an absolute shedload of Perl modules from CPAN
to do its job. I am unrepentant about this. Maypole does a lot of work,
so that you don't have to. This is called code re-use, and if we're
serious about code re-use, then Maypole should be re-using as much code
as possible in terms of Perl modules. In another sense, this gives the
impression that Maypole doesn't actually do all that much itself,
because all it's doing is gluing together already-existing code. Well,
welcome to code re-use.

The downside of code re-use is, of course, that you then have to install
a shedload of Perl modules from CPAN. If you're using OpenBSD or
FreeBSD, the wonderful ports system will be your friend. There's a
Maypole port in C<p5-Maypole>. Just type C<make install>.

Debian users, hang in there. There's a package coming.

For other Unices, the L<CPANPLUS> or C<CPAN> modules will help with
this. If you don't have C<CPANPLUS> installed, my recommendation is to
use C<perl -MCPAN -e install CPANPLUS> to install it and then throw
C<CPAN.pm> away. In any case, one of these two should get all that
Maypole needs:

    % perl -MCPANPLUS -e 'install Maypole'
    % perl -MCPAN -e 'install Maypole'

I don't know if Maypole works on Windows. I'm not sure I care.

You're also going to need a database server and a web server. For
databases, I recommend SQLite (if you install the C<DBD::SQLite> module,
you get the SQLite library for free) for prototyping and mysql for
production; heavier duty users should use Postgresql or Oracle - Maypole
should be happy with them all. Maypole is happiest when running under
Apache C<mod_perl>, with the C<Apache::Request> module installed, but as
I said, it is a blank slate, and everything is customizable. There is a
C<CGI::Maypole> frontend available to run as a standalone CGI script.

=head2 The Beer Database example

Throughout this manual, we're going to be referring back to a particular
application so that we can give concrete examples for the concepts we're
talking about. We could say "C<related_accessors> returns a list of
accessors which can be called to return a list of objects in a has-a
relationship to the original", or we could say "if we call
C<related_accessors> on while viewing C<brewery>, it returns C<beers>,
because we can call C<beers> on a C<brewery> object to get a list of
that berwery's beers." 

Because Maypole is all about beer. If you look carefully, you can
probably see men playing cricket on the village green. The first
ever Maypole application was written to help me keep track of the many
different ales available in my area - their styles, their tastes, their
breweries, prices and so on. Then the more I thought about it, the more
I thought it was a particularly good data model for demonstrating
different kinds of database relationships.

We have a C<brewery> table, which has several C<beer>s. We'll call this
a has-many relationship. The beers each have a C<style>; styles are
stored in a separate table, so C<beer> has-a C<style>. Beers are in
several pubs and a pub has several beers, so beers and pubs are in a
many-to-many relationship. We use a link table called C<handpump> to
relate pubs to beers.

All in all, this gives us a schema like the following:

    create table brewery (
        id int not null auto_increment primary key,
        name varchar(30),
        url varchar(50),
        notes text
    );

    create table beer (
        id int not null auto_increment primary key,
        brewery integer,
        style integer, 
        name varchar(30),
        url varchar(120),
        score integer(2),
        price varchar(12),
        abv varchar(10),
        notes text
    );

    create table handpump (
        id int not null auto_increment primary key,
        beer integer,
        pub integer
    );

    create table pub (
        id int not null auto_increment primary key,
        name varchar(60),
        url varchar(120),
        notes text
    );

    create table style (
        id int not null auto_increment primary key,
        name varchar(60),
        notes text
    );

If you have C<DBD::SQLite> available, then a database like this will
be created when Maypole was installed. Let's now see how to set it up
with a web interface.

=head2 Setting up Maypole

The first thing we need for a Maypole interface to a database is to
have a database. If you don't have one, now would be a good time to
create one, using the schema above.

The next thing we need is a module which is going to do all the work.
Thankfully, it doesn't need to do B<all> the work itself. It's going to be a 
subclass of C<Maypole> or a Maypole front-end like C<Apache::MVC>. 

Here's the driver class for our beer database application. We're not
going to go into much detail about it here; we'll do that in L<Beer.pod>.
For now, simply admire its brevity, as you realise this is all the code
you need to write for a simple database front-end:

    package BeerDB;
    use base 'Apache::MVC';
    BeerDB->setup("dbi:SQLite:t/beerdb.db");
    BeerDB->config->{uri_base} = "http://localhost/beerdb/";
    BeerDB->config->{rows_per_page} = 10;
    BeerDB->config->{display_tables} = [qw[beer brewery pub style]];
    BeerDB::Brewery->untaint_columns( printable => [qw/name notes url/] );
    BeerDB::Style->untaint_columns( printable => [qw/name notes/] );
    BeerDB::Beer->untaint_columns(
        printable => [qw/abv name price notes/],
        integer => [qw/style brewery score/],
        date => [ qw/date/],
    );

    use Class::DBI::Loader::Relationship;
    BeerDB->config->{loader}->relationship($_) for (
        "a brewery produces beers",
        "a style defines beers",
        "a pub has beers on handpumps");
    1;

This defines the C<BeerDB> application, which, as it inherits from 
C<Apache::MVC>, will be a mod_perl handler. This means we need to
tell the Apache configuration about it:

    <Location /beerdb>
        SetHandler perl-script
        PerlHandler BeerDB
    </Location>

And now we need some templates. As we'll see in the chapter on views,
L<View.pod>, there are several types of template. We're going to copy
the whole lot from the F<templates/> directory of the Maypole source
package into the F</beerdb> directory under our web root.

And that's it. We should now be able to go to C<http://localhost/beerdb/>
and see a menu of things to browse; C<http://localhost/beerdb/beer/list>
will give a list of beers. There might not be any yet. There's a box
that lets you add them.

If you have any problems getting to this point, you might want to look
at L<http://wiki.simon-cozens.org/index.cgi?InstallationIssues>.

Play about with the site. Add some beers. Maybe go out and buy some beers
to review if you need some inspiration. Don't be ill on my carpet.

=head2 Phases of a Maypole request

Now you should have a feel for what Maypole can do. The important thing
to know at this point is that this is by no means B<all> that Maypole
can do. What you've seen in the beer database example is all that Maypole
can do if you don't customize it at all.

Remember that, for instance, we don't ever tell Maypole what tables our
database has, or what columns each table has. We don't tell Maypole what
those tables should be called or how to display them. We don't tell Maypole
what to do - that we want to list, search, edit and delete beers and breweries.
Maypole just works that out for itself. We can customize it and have Maypole
do all sorts of interesting things with our database, and most of the rest
of this manual will be about how to do that.

In order to do that, we need to look at what Maypole's actually doing.

As mentioned, Maypole is responsible for turning a URL into an object, a
method call, and some templated output. Here's a handy diagram to
explain how it does that:

=for html
<IMG SRC="maypole_process2.png">

Maypole's process revolves around the concept of the Maypole request
object. This is a little like Apache's request object, but at a much
higher level - in fact, in C<mod_perl>-based Maypole front-ends, the
Apache request object is incorporated in the Maypole request object. All
that Maypole does is gradually flesh out this object until it contains
something in the C<output> member, and then it is dispatched back to the
front-end for output.

So to start with, we take the Apache request (or CGI object, or other
way of isolating what's going on) and break it down. For instance, we
turn the URL C</beer/view/1> into

    {
        table => "beer",
        action => "view",
        args => [ 1 ]
    }

Then Maypole will check that C<beer> is a real table, and find the class
that models it. It also checks whether or not we're allowed to call the
C<view> method over the network:

    {
        table => "beer",
        action => "view",
        args => [ 1 ],
        model_class => "BeerDB::Beer"
    }

Then there's a user-defined authentication method, which by default just
lets us do anything. Now we hand over to the model class, which loads up
the object, and decides what template we want to use:

    {
        table => "beer",
        action => "view",
        args => [ ],
        objects => [ BeerDB::Beer->retrieve(1) ],
        model_class => "BeerDB::Beer",
        template => "view"
    }

Then it calls C<BeerDB::Beer-E<gt>view>, passing in the request object
as a parameter, and passes the whole lot to the view class for templating.
In the next two chapters, we'll look at how Maypole's default model and
view classes generally do what you want them to do.