# PODNAME: Config::Model::Manual::ModelCreationAdvanced
# ABSTRACT: Creating a model with advanced features

=head1 Introduction

The page L<Config::Model::Manual::ModelCreationIntroduction> explains
what is a configuration tree and a configuration model and how to
create a simple configuration model.

But a configuration model can be more complex and define interactions
between elements with the following features:

=over

=item *

Model warp. For instance, Xorg driver options change depending on
driver name (C<nvidia>, C<radeon>...)

=item *

Simple computation from other elements (used for upgrades)

=item *

References. For instance, in C<Xorg::Device::Radeon>, C<Monitor-DVI-0>
name must refer to one of the monitors declared in C<Monitor> section.

=back

Caveat: Xorg examples are based on Xorg 1.4 and may not be valid for
Xorg 1.5 or 1.6

=head1 Model plugin

Config::Model can also use model plugins. Each model can be augmented by model snippets
stored into directory C<< <model_name>.d >>. All files found there are merged to existing model.

For instance, this model in file C<.../Config/Model/models/Fstab/Fsline.pl>:

 {
    name => "Fstab::Fsline",
    element => [
	fs_vfstype => {
            type => 'leaf',
            value_type => 'enum',
            choice => [ qw/ext2 ext3/ ],
        },
        fs_mntopts => {
            type => 'warped_node',
            follow => { 'f1' => '- fs_vfstype' },
            rules => [
                '$f1 eq \'ext2\'', { 'config_class_name' => 'Fstab::Ext2FsOpt' },
                '$f1 eq \'ext3\'', { 'config_class_name' => 'Fstab::Ext3FsOpt' },
            ],
        }
    ]
 }

can be augmented with the content of C<.../Config/Model/models/Fstab/Fsline.d/addext4.pl>:

 {
    name => "Fstab::Fsline",
    element => [
	fs_vfstype => { choice => [ qw/ext4/ ], },
        fs_mntopts => {
            rules => [
                q!$f1 eq 'ext4'!, { 'config_class_name' => 'Fstab::Ext4FsOpt' },
            ],
        },
    ]
 } ;

Then, the merged model will feature C<fs_vfstype> with choice C<ext2 ext4 ext4>.
Likewise, C<fs_mntopts> will feature rules for the 3 filesystems.

Under the hood, L<Config::Model/augment_config_class> method is used to load model snippets.

=head1 Model warp

From a user's point of view, model warp looks like the structure
or properties of the configuration is changing (or adapting)
dynamically depending on the values being entered. For instance, when
changing a driver name from C<fglrx> to C<radeon>, some options
disappear from the GUI and some other options pop-in.

Model warping need not be that spectacular and can have more subtle
effect like changing a default value.

Of course, there's no magic, model warp properties needs to be
prepared and declared in the model.

=head2 Warped value

Let's start simple with value warp: the properties of a single value
is changed dynamically. Let's imagine a configuration file with 2
values: I<size> which can be set to I<big> or I<small> and I<length>
whose maximum value is 10 when size is small and 50 when size is
big. (this may be dumb, but it's for the sake of the example).

So the basic model without warp is

 element => [
              size => { type => 'leaf',
                        value_type => 'enum',
                        choice     => ['big','small'],
                      },
              length => { type => 'leaf',
                          value_type => 'integer',
                          max => '10',
                        },
            ]

Now we need to declare the relationship between I<size> and I<length> to
be able to change dynamically the I<max> property.

This setup is made of 2 specifications:

=over

=item *

what is the element that triggers the change (called I<warp master>
in the doc)

=item *

what is the effect of the warp master change

=back

The first is done with a declaration of the I<path> to I<follow> to find
the warp master (associated to a variable). The second is a set of
value properties:

 element => [ 
   size => { 
     type => 'leaf',
     value_type => 'enum',
     choice     => ['big','small'],
   },

   length => { 
     type => 'leaf',
     value_type => 'integer',
     warp => {                         # change specification
       follow => {                     # declare what trigger the change
         size_type => '- size'         # size_type: go 1 level above and fetch
                                       #            size value
       },
       rules  => {                     # how to apply change
         '$size_type eq "small"' => {  # set max to 10 when size is small
            max => 10 
         },
         '$size_type eq "big" ' => {   # set max to 50 when size is big
             max => 50 },
         },
       },
     }
  ]

=head2 Warp in or warp out an element

Here's a real use case scenario from OpenSsh.

C<ssh_config> enables a user to set up a tunnel through ssh. The input
of this tunnel can listen to localhost (default) or to other hosts.
These other hosts are specified by the I<bind_adress> part of the
C<LocalForward> parameter.

But this bind address is ignored if C<GatewayPorts> is false (which is
the default).

In order to present only meaningful parameters to the user,
I<bind_address> parameter must be hidden when C<GatewayPorts> is false
and shown when C<GatewayPorts> is true.

Here's the recipe. First create a boolean element for C<GatewayPorts>:

 GatewayPorts => {
    type => 'leaf',
    value_type => 'boolean',
    upstream_default => 0,
 },

And C<LocalForward> that provides I<bind_address> parameter:

 LocalForward => {
   type => 'list',
   cargo => {
     type => 'node',
     config_class_name => 'Ssh::PortForward'
   },
   summary => 'Local port forwarding',
 }

In C<Ssh::PortForward> configuration class, declare I<bind_address> with the warp
instructions:

 bind_address => {
   type => 'leaf',
   value_type => 'uniline',
   level => 'hidden',             # by default, is hidden from user
   warp => {                      # instructions to show bind_address
     follow => {                  # specify what does trigger the change
        gp => '- - GatewayPorts'  # gp: go to 2 levels above in tree ('- -') and
                                  #     fetch GatewayPorts value
     },
     rules => [                   # specify how to apply the change triggered by gp
       '$gp' => {                 # apply change when $gp is true
           level => 'normal'      # set level to normal (instead of 'hidden'). This change
                                  #     will show this parameter in the UI
       }
     ]
   },
 },

=head2 warped node

Sometimes, warping a value line by line is not practical. For
instance, in C</etc/fstab> the mount options of a file system change
drastically from one file system to another. In this case, it's better
to swap a configuration class with another.

For instance, swap C<vfat> mount options with C<ext3> mount options
when a file system is changed from C<vfat> to C<ext3>.

Here's how this can be done. First declare the C<fstype> parameter:

 fs_vfstype => {
   type => 'leaf',
   mandatory => 1,
   value_type => 'enum',
   choice => [ 'auto', 'davfs', 'vfat', 'ext2', 'ext3', ] , # etc ...
 }

Then declare C<mntopts> as a B<warped_node> (not a simple C<node>))
that uses C<fs_vfstype> to swap one config class with another:

 fs_mntopts => {
   type => 'warped_node', # a shape-shifting node
   follow => {
     f1 => '- fs_vfstype' , # use fs_vfstype as a trigger
   },
   rules => [
     # condition     => effect: config class to swap in

     "$f1 eq 'proc'" => { config_class_name => 'Fstab::CommonOptions' },
     "$f1 eq 'auto'" => { config_class_name => 'Fstab::CommonOptions' },
     "$f1 eq 'vfat'" => { config_class_name => 'Fstab::CommonOptions' },
     "$f1 eq 'swap'" => { config_class_name => 'Fstab::SwapOptions'   },
     "$f1 eq 'ext3'" => { config_class_name => 'Fstab::Ext3FsOpt'     },
     # etc ...
   ]
  }

=head1 References

=head1 Computation and migrations

=head2 Cascaded warp

Config::Model also supports cascaded warps: A warped value is
dependent on another value which is itself a warped value.

=head1 Feedback welcome

Feel free to send comments and suggestion about this page at

 config-model-users at lists dot sourceforge dot net.

=head1 AUTHORS

Dominique Dumont <ddumont at cpan.org>
