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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.

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 /product/order/12, Maypole is a way of having it load up product number 12, call an 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 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.

What is MVC for web applications?

Maypole was originally called 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 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.

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 p5-Maypole. Just type make install.

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

For other Unices, the CPANPLUS or CPAN modules will help with this. If you don't have CPANPLUS installed, my recommendation is to use perl -MCPAN -e install CPANPLUS to install it and then throw 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 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 mod_perl, with the Apache::Request module installed, but as I said, it is a blank slate, and everything is customizable. There is a CGI::Maypole frontend available to run as a standalone CGI script.

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 "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 related_accessors on while viewing brewery, it returns beers, because we can call beers on a 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 brewery table, which has several beers. We'll call this a has-many relationship. The beers each have a style; styles are stored in a separate table, so beer has-a 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 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 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.

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 all the work itself. It's going to be a subclass of Maypole or a Maypole front-end like 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 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 BeerDB application, which, as it inherits from 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, View.pod, there are several types of template. We're going to copy the whole lot from the templates/ directory of the Maypole source package into the /beerdb directory under our web root.

And that's it. We should now be able to go to http://localhost/beerdb/ and see a menu of things to browse; 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 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.

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 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:

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 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 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 /beer/view/1 into

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

Then Maypole will check that beer is a real table, and find the class that models it. It also checks whether or not we're allowed to call the 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 BeerDB::Beer->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.

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