Mapping the concepts

When I started looking at social networking sites, I began by identifying the concepts which were going to make up the tables of the application. At its most basic, a site like Orkut or Flox has two distinct concepts: a user, and a connection between two users. Additionally, there's the idea of an invitation to a new user, which can be extended, accepted, declined or ignored. These three will make up the key tables; there are an extra two tables in Flox, but they're essentially enumerations that are a bit easier to edit: each user has an affiliation to a particular college or department, and a status in the university. (Undergraduate, graduate, and so on.)

For this first run-through, we're going to ignore the ideas of societies and communities, and end up with a schema like so:

    CREATE TABLE user (
        id int not null auto_increment primary key,
        first_name varchar(50),
        last_name varchar(50),
        email varchar(255),
        profile text,
        password varchar(255),
        affiliation int,
        unistatus int,
        status ENUM("real", "invitee"),
        photo blob,
        photo_type varchar(30)
    );

    CREATE TABLE connection (
        id int not null auto_increment primary key,
        from_user int,
        to_user int,
        status ENUM("offered", "confirmed")
    );

    CREATE TABLE invitation (
        id char(32) not null primary key,
        issuer int,
        recipient int,
        expires date
    );

Plus the definition of our two auxiliary tables:

    CREATE TABLE affiliation (
        id int not null auto_increment primary key,
        name varchar(255)
    );

    CREATE TABLE unistatus (
        id int not null auto_increment primary key,
        name varchar(255)
    );

Notice that, for simplicity, invitations and friendship connections are quite similar: they are extended from one user to another. This means that people who haven't accepted an invite yet still have a place in the user table, with a different "status". Similarly, a connection between users can be offered, and when it is accepted, its status is changed to "confirmed" and a reciprocal relationship put in place.

We also have some idea, based on what we want to happen, of what pages and actions we're going to define. Leaving the user aside for the moment, we want an action which extends an invitation from the current user to a new user. We want a page the new user can go to in order to accept that invitation. Similarly, we want an action which offers a friendship connection to an existing user, and a page the user can go to to accept or reject it. This gives us five pages so far:

    invitation/issue
    invitation/accept

    user/befriend
    connection/accept
    connection/reject

Notice that the "befriend" action is performed on a user, not a connection. This is distinct from "invitation/issue" because when befriending, we have a real user on the system that we want to do something to. This makes sense if you think of it in terms of object oriented programming - we could say

    Flox::Connection->create(to => $user)

but it's clearer to say

    $user->befriend

Similarly, we could say

    Flox::User->create({ ... })->issue_invitation_to

but it's clearer to say

    Flox::Invitation->issue( to => Flox::User->create({ ... }) )

because it more accurately reflects the principal subject and object of these actions.

Returning to look at the user class, we want to be able to view a user's profile, edit one's own profile, set up the profile for the first time, upload pictures and display pictures. We also need to handle the concepts of logging in and logging out.

As usual, though, we'll start with a handler class which sets up the database:

    package Flox;
    use Maypole::Application;
    Flox->setup("dbi:mysql:flox");
    Flox->config->display_tables([qw[user invitation connection]]);
    1;

Very simple, as these things are meant to be. Now let's build on it.

Users and Authentication

We also want to be able to refer to the current user from the templates, so we use the overridable "additional_data" method in the driver class to give us a "my" template variable:

    sub additional_data { 
        my $r = shift; $r->template_args->{my} = $r->user; 
    }

I've called it "my" rather than "me" because we it lets us check "[% my.name %]", and so on.

Viewing a user

The only difference from a programming point of view between this action and the default "view" action is that, if no user ID is given, then we want to view "me", the current user. Remembering that the default view action does nothing, our "Flox::User::view" action only needs to do nothing plus ensure it has a user in the "objects" slot, putting "$r->{user}" in there if not:

    sub view :Exported {
        my ($class, $r) = @_;
        $r->objects([ $r->user ]) unless @{ $r->objects || [] };
    }

Maypole, unfortunately, is very good at making programming boring. The downside of having to write very little code at all is that we now have to spend most of our time writing nice HTML for the templates.

Pictures of Users

    sub view_picture :Exported {
        my ($self, $r) = @_;
        my $user = $r->objects->[0] || $r->user;
        if ($r->content_type($user->photo_type)) {
           $r->output($user->photo);
        } else {
           # Read no-photo photo
           $r->content_type("image/png");
           $r->output(slurp_file("images/no-photo.png"));
        }
    }

We begin by getting a user object, just like in the "view" action: either the user whose ID was passed in on the URL, or the current user. Then we check if a "photo_type" has been set in this user's record. If so, then we'll use that as the content type for this request, and the data in the "photo" attribute as the data to send out. The trick here is that setting "$r->{output}" overrides the whole view class processing and allows us to write the content out directly.

In our template, we can now say

    <IMG SRC="[%base%]/user/view_picture/[% user.id %]">

and the appropriate user's mugshot will appear.

However, if we're throwing big chunks of data around like "photo", it's now worth optimizing the "User" class to ensure that only pertitent data is fetched by default, and "photo" and friends are only fetched on demand. The "lazy population" section of Class::DBI's man page explains how to group the columns by usage so that we can optimize fetches:

    Flox::User->columns(Primary   => qw/id/);
    Flox::User->columns(Essential => qw/status/);
    Flox::User->columns(Helpful   => qw/ first_name last_name email password/)
    Flox::User->columns(Display   => qw/ profile affiliation unistatus /);
    Flox::User->columns(Photo     => qw/ photo photo_type /);

This means that the status and ID columns will always be retrieved when we deal with a user; next, any one of the name, email or password columns will cause that group of data to be retrieved; if we go on to display more information about a user, we also load up the profile, affiliation and university status; finally, if we're throwing around photos, then we load in the photo type and photo data.

These groupings are somewhat arbitrary, and there needs to be a lot of profiling to determine the most efficient groupings of columns to load, but they demonstrate one principle about working in Maypole: this is the first time in dealing with Maypole that we've had to explicitly list the columns of a table, but Maypole has so far Just Worked. There's a difference, though, between Maypole just working and Maypole working well, and if you want to optimize your application, then you need to start putting in the code to do that. The beauty of Maypole is that you can do as much or as little of such optimization as you want or need.

So now we can view users and their photos. It's time to allow the users to edit their profiles and upload a new photo.

Editing user profiles

    Flox::User->untaint_columns(
        html      => [qw/first_name last_name profile/],
        printable => [qw/password/],
        integer   => [qw/affiliation unistatus /],
        email     => [qw/email/]
    );

The next stage was the ability to upload a photo. We unleash the "Uploading files" recipe, with an additional check to make sure the photo is of a sensible size:

    use constant MAX_IMAGE_SIZE => 512 * 1024;
    sub do_upload :Exported {
        my ($class, $r) = @_;
        my $user = $r->user;
        my $upload = $r->ar->upload("picture");
        if ($upload) {
            my $ct = $upload->info("Content-type");
            return $r->error("Unknown image file type $ct")
                if $ct !~ m{image/(jpeg|gif|png)};
            return $r->error("File too big! Maximum size is ".MAX_IMAGE_SIZE)
                if $upload->size > MAX_IMAGE_SIZE;

            my $fh = $upload->fh;
            my $image = do { local $/; <$fh> };

            use Image::Size;
            my ($x, $y) = imgsize(\$image);
            return $r->error("Image too big! ($x, $y) Maximum size is 350x350")
                if $y > 350 or $x > 350;
            $r->user->photo_type($ct);
            $r->user->photo($image);
        }

        $r->objects([ $user ]);
        $r->template("view");
    }

Now we've gone as far as we want to go about user editing at the moment. Let's have a look at the real meat of a social networking site: getting other people involved, and registering connections between users.

Invitations

    sub issue :Exported {}

and the template proceeds as normal:

    [% PROCESS header %]
    <h2> Invite a friend </h2>

    <FORM ACTION="[%base%]/invitation/do_edit/" METHOD="post">
    <TABLE>

Now we use the "Catching errors in a form" recipe from the Request Cookbook and write our form template:

    <TR><TD>
    First name: <INPUT TYPE="text" NAME="forename"
    VALUE="[%request.params.forename%]">
    </TD>
    <TD>
    Last name: <INPUT TYPE="text" NAME="surname"
    VALUE="[%request.params.surname%]"> 
    </TD></TR>
    [% IF errors.forename OR errors.surname %]
        <TR>
        <TD><SPAN class="error">[% errors.forename %]</SPAN> </TD>
        <TD><SPAN class="error">[% errors.surname %]</SPAN> </TD>
        </TR>
    [% END %]
    <TR>
    ...

Now we need to work on the "do_edit" action. This has to validate the form parameters, create the invited user, create the row in the "invitation" table, and send an email to the new user asking them to join.

We'd normally use "create_from_cgi" to do the first two stages, but this time we handle the untainting manually, because there are a surprising number of things we need to check before we actually do the create. So here's the untainting of the parameters:

    sub do_edit :Exported {
        my ($self, $r) = @_;
        my $h = CGI::Untaint->new(%{$r->params});
        my (%errors, %ex);
        for (qw( email forename surname )) {
            $ex{$_} = $h->extract(
                    "-as_".($_ eq "email" ? "email" : "printable") => $_
            ) or $errors{$_} = $h->error;
        }

Next, we do the usual dance of throwing the user back at the form in case of errors:

        if (keys %errors) {
            $r->template_args->{message} =
                "There was something wrong with that...";
            $r->template_args->{errors} = \%errors;
            $r->template("issue");
            return;
        }

We've introduced a new template variable here, "message", which we'll use to display any important messages to the user.

The first check we need to do is whether or not we already have a user with that email address. If we have, and they're a real user, then we abort the invite progress and instead redirect them to viewing that user's profile.

        my ($user) = Flox::User->search({ email => $ex{email} });
        if ($user) {
            if ($user->status eq "real") {
                $r->template_args->{message} =
                    "That user already seems to exist on Flox. ".
                    "Is this the one you meant?";

                $self->redirect_to_user($r, $user);
            }

Where "redirect_to_user" looks like this:

    sub redirect_to_user {
        my ($self, $r, $user) = @_;
        $r->objects([ $user ]);
        $r->template("view");
        $r->model_class("Flox::User"); # Naughty.
    }

This is, as the comment quite rightly points out, naughty. We're currently doing a "/invitation/do_edit/" and we want to turn this into a "/user/view/xxx", changing the table, template and arguments all at once. To do this, we have to change the Maypole request object's idea of the model class, since this determines where to look for the template: if we didn't, we'd end up with "invitation/view" instead of "user/view".

Ideally, we'd do this with a Apache redirect, but we want to get that "message" in there as well, so this will have to do. This isn't good practice; we put it into a subroutine so that we can fix it up if we find a better way to do it.

Anyway back in the "do_edit" action, this is what we should do if a user already exists on the system and has accepted an invite already. What if we're trying to invite a user but someone else has invited them first and they haven't replied yet?

             } else {
                # Put it back to the form
                $r->template_args->{message} =
                    "That user has already been invited; " .
                    "please wait for them to accept";
                $r->template("issue");
             }
             return;
        }

Race conditions suck.

Okay. Now we know that the user doesn't exist, and so can create the new one:

        my $new_user = Flox::User->create({
            email      => $ex{email},
            first_name => $ex{forename},
            last_name  => $ex{surname},
            status     => "invitee"
        });

We want to give the invitee a URL that they can go to in order to accept the invite. Now we don't just want the IDs of our invites to be sequential, since someone could get one invite, and then guess the rest of the invite codes. We provide a relatively secure MD5 hash as the invite ID:

        my $random = md5_hex(time.(0+{}).$$.rand);

For additional security, we're going to have the URL in the form "/invitation/accept/id/from_id/to_id", encoding the user ids of the two users. Now we can send email to the invitee to ask them to visit that URL:

        my $newid = $new_user->id;
        my $myid  = $r->user->id;
        _send_mail(to   => $ex{email},
                   url  => "$random/$myid/$newid", 
                   user => $r->user);

I'm not going to show the "_send_mail" routine, since it's boring. We haven't actually created the "Invitation" object yet, so let's do that now.

        Flox::Invitation->create({
            id        => $random,
            issuer    => $r->user,
            recipient => $new_user,
            expires   => Time::Piece->new(time + LIFETIME)->datetime
        });

You can also imagine a daily cron job that cleans up the "Invitation" table looking for invitations that ever got replied to within their lifetime:

   ($_->expires > localtime && $_->delete)
       for Flox::Invitation->retrieve_all;

Notice that we don't check whether the ID is already used. We could, but, you know, if MD5 sums start colliding, we have much bigger problems on our hands.

Anyway, now we've got the invitation created, we can go back to whence we came: viewing the original user:

        $self->redirect_to_user($r, $r->user);

Now our invitee has an email, and goes click on the URL. What happens?

XXX

Friendship Connections

Links

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