NAME

Config::Model::Manual::ModelCreationIntroduction - Introduction to model creation with Config::Model

VERSION

version 2.155

Introduction

This page describes how to write a simple configuration model. Creation of more complex models are described in Creating a model with advanced features.

Note that this document shows a lot of Perl data structure to highlight the content of a model. A Perl data structure is very similar to a JSON structure. The only thing you need to know are:

Curly braces { ... } contain a dictionary of key, value pairs (a hash in Perl land))
Square brackets [ ... ] contain a list of items (array or list in Perl land)

Some definitions

configuration file: Text file where configuration data are stored. This configuration file is used by an application -- the target application
configuration tree: The semantic content of the configuration file stored in a tree representation
configuration model: Structure and constraints of the configuration tree. Like a schema for the configuration tree
target application: The application that uses the configuration file. The application can be of type system (i.e. the configuration file is located in /etc), user (i.e. the configuration file is located in a user directory like ~/.config) or application (the configuration file is in or below the current directory)
end user: User of the target application
application developer: Target application developer
model developer: People developing the configuration model. Not necessarily the application developer

What is a configuration tree?

Most configuration files are actually organized mostly as a tree structure. Depending on the syntax of the file, this structure may be obvious to see (e.g. for XML, Apache) or not so obvious (Xorg syntax, INI syntax).

For some files like approx.conf or adduser.conf, this tree structure is quite flat. It looks much like a rake than a tree, but still, it's a tree.

For instance, this approx.conf:

$pdiffs     1
$max_wait   14
debian     http://ftp.fr.debian.org/debian

can have this tree representation:

root
|--pdiff=1
|--max_wait=14
`--distrib(debian)=http://ftp.fr.debian.org/debian

Other configuration files like apache2.conf or xorg.conf have a structure that look more like a tree.

For instance, consider this xorg.conf snippet:

Section "Device"
   Identifier     "Device0"
   Driver         "nvidia"
EndSection

Section "Screen"
   Identifier     "Screen0"
   Device         "Device0"
   Option         "AllowGLXWithComposite" "True"
   Option         "DynamicTwinView" "True"
   SubSection     "Display"
       Depth       24
   EndSubSection
EndSection

Knowing that Xorg.conf can have several Device or Screen sections identified by their Identifiers, the configuration can be represented in this tree as:

root
|--Device(Device0)
|  `--Driver=nvidia
`--Screen(Screen0)
   |--Device=Device0
   |--Option
   |  |--AllowGLXWithComposite=True
   |  `--DynamicTwinView=True
   `--Display
      `--Depth=24

One may argue that some Xorg parameter refer to others (i.e.Device and Monitor value in Screen section) and so they cannot be represented as a tree. That's right, there are some more complex relations that are added to the tree structure. This will be covered in more details when dealing with complex models.

In some other case, the structure of a tree is not fixed. For instance, Device options in Xorg.conf are different depending on the value of the Device Driver. In this case, the structure of the configuration tree must be adapted (morphed) depending on a parameter value.

Just like XML data can have Schema to validate their content, the configuration tree structure needs to have its own schema to validate its content. Since the tree structure cannot be represented as a static tree without reference, XML like schema are not enough to validate configuration data.

Config::Model provides a kind of schema for configuration data that takes care of the cross references mentioned above and of the dynamic nature of the configuration tree required for Xorg (and others).

What is a model?

A configuration model defines the configuration tree structure:

A model defines one or more configuration class
At least one class is required to define the configuration tree root
Each class contains several elements. An element can be:
- A leaf to represent one configuration parameter
- A list of hash of leaves to represent several parameter
- A node to hold a node of a configuration tree
- A list or hash of nodes

These basic relations enable to define the main parts of a configuration tree.

If we refer to the approx.conf example mentioned above, one only class is required (let's say the Approx class). This class must contain (see approx.conf man page):

A boolean leaf for pdiff (1 if not specified)
An integer leaf for max_wait (10 seconds unless specified otherwise)
A hash of string leaves for distrib (no default).

A configuration model is stored this way by Config::Model:

{
 name => 'Approx',
 element => [
     pdiffs => {
         type => 'leaf',
         value_type => 'boolean',
         upstream_default => '1'
     },
     max_wait => {
         type => 'leaf',
         value_type => 'integer',
         upstream_default => '10'
     },
     distributions => {
         type => 'hash',
         index_type => 'string' ,
         cargo => {
             value_type => 'uniline',
             type => 'leaf',
         },
     }
  ]
}

The Xorg example leads to a slightly more complex model with several classes:

Xorg (root class)
Xorg::Device
Xorg::Screen
Xorg::Screen::Option for the Screen options
Xorg::Screen::Display for theDisplay subsection

The root class is declared this way:

{
 name => 'Xorg',
 element => [
             Device => {
                        type => 'hash',
                        index_type => 'string'
                        cargo => {
                                   type => 'node',
                                   config_class_name => 'Xorg::Device'
                                 },
                       },
             Screen => {
                        type => 'hash',
                        index_type => 'string'
                        cargo => {
                                  type => 'node',
                                  config_class_name => 'Xorg::Screen'
                                 },
                       },
          ]
}

TheXorg::Screen class is:

{
 name => 'Xorg::Screen',
 element => [
              Device => {
                          type' => 'leaf',
                          value_type => 'uniline',
                        },
              Display => {
                           type => 'hash',
                           index_type => 'integer'
                           cargo => {
                                      type => 'node',
                                      config_class_name => 'Xorg::Screen::Display'
                                    },
                         }
             Option => {
                         type => 'node',
                         config_class_name => 'Xorg::Screen::Option'
                       },
             ]
 }

It's now time to detail how the elements of a class are constructed.

Model analysis

To define the required configuration classes, you should read the documentation of the target application to :

Find the structure of the configuration tree
Identify configuration parameters, their constraints and relations

Last but not least, you should also find several valid examples of your application configuration. These examples can be used as non-regression tests and to verify that the application documentation was understood.

Model declaration

Configuration class declaration

Since writing the data structure shown below is not fun (even with Perl), you are encouraged to use the model editor provided by cme using cme meta edit command (provided by Config::Model::Itself). This commands provides a GUI to create or update your model.

When saving, cme writes the data structure in the correct directory.

Configuration class declaration (the hard way)

In summary, configuration documentation is translated in a format usable by Config::Model:

The structure is translated into configuration classes
Configuration parameters are translated into elements
Constraints are translated into element attributes

All models files must be written in a specific directory. For instance, for model Xorg, you must create ./lib/Config/Model/models/Xorg.pl. Other classes like Xorg::Screen can be stored in their own file ./lib/Config/Model/models/Xorg/Screen.pl or included in Xorg.pl

A model file is a Perl file containing an array for hash ref. Each Hash ref contains a class declaration:

[ { name => 'Xorg', ... } , { name => 'Xorg::Screen', ... } ] ;

A class can have the following parameters:

name: mandatory name of the class
class_description: Description of the configuration class.
generated_by: Mention with a descriptive string if this class was generated by a program. This parameter is currently reserved for Config::Model::Itself model editor.
include: Include element description from another class.

For more details, see "Configuration_Model" in Config::Model.

For instance:

$ cat lib/Config/Model/models/Xorg.pl
[
  {
    name => 'Xorg',
    class_description => 'Top level Xorg configuration.',
    include => [ 'Xorg::ConfigDir'],
    element => [
                Files => {
                          type => 'node',
                          description => 'File pathnames',
                          config_class_name => 'Xorg::Files'
                         },
                # snip
               ]
  },
  {
    name => 'Xorg::DRI',
    element => [
                Mode => {
                         type => 'leaf',
                         value_type => 'uniline',
                         description => 'DRI mode, usually set to 0666'
                        }
               ]
  }
];

Common attributes for all elements

This first set of attributes helps the user by providing guidance (with level and status) and documentation (summary and description).

All elements (simple or complex) can have the following attributes:

description: full length description of the attribute
summary: one line summary of the above description
level: is important, normal or hidden. The level is used to set how configuration data is presented to the user in browsing mode. Important elements are shown to the user no matter what. hidden elements are explained with the warp notion.
status: is obsolete, deprecated or standard (default). Warnings are shown when using a deprecated element and an exception is raised when an obsolete element is used.

See "Configuration_class" in Config::Model for details.

Leaf elements

Leaf element is the most common type to represent configuration data. A leaf element represents a specific configuration parameter.

In more details, a leaf element have the following attributes (See "Value_model_declaration" in Config::Model::Value doc):

type: Set to leaf (mandatory)
value_type: Either boolean, integer, number, enum, string, uniline (i.e. a string without "\n") (mandatory)
min: Minimum value (for integer or number)
max: Maximum value (for integer or number)
choice: Possible values for an enum
mandatory: Whether the value is mandatory or not
default: Default value that must be written in the configuration file
upstream_default: Default value that is known by the target application and thus does not need to be written in the configuration file.

To know which attributes to use, you should read the documentation of the target application.

For instance, AddressFamily parameter (sshd_config(5)) is specified with: Specifies which address family should be used by sshd(8). Valid arguments are "any", "inet" (use IPv4 only), or "inet6" (use IPv6 only). The default is "any".

For Config::Model, AddressFamily is a type leaf element, value_type enum and the application falls back to any if this parameter is left blank in sshd_config file.

Thus the model of this element is :

AddressFamily => {
  type             => 'leaf',
  value_type       => 'enum',
  upstream_default => 'any',
  description      => 'Specifies which address family should be used by sshd(8).',
  choice           => [ 'any', 'inet', 'inet6' ]
}

Simple list or hash element

Some configuration parameters are in fact a list or a hash of parameters. For instance, approx.conf can feature a list of remote repositories:

# remote repositories
debian     http://ftp.fr.debian.org/debian
multimedia http://www.debian-multimedia.org

These repositorie URLs must be stored as a hash where the key is debian or multimedia and the associated value is a URL. But this hash must have something which is not explicit in approx.conf file: a parameter name. Approx man page mentions that: The name/value pairs [not beginning with '$' are used to map distribution names to remote repositories.. So let's use distribution as a parameter name.

The example is stored this way in the configuration tree:

root
|--distribution(debian)=http://ftp.fr.debian.org/debian
`--distribution(multimedia)=http://www.debian-multimedia.org

The model needs to declare that distribution is:

a type hash parameter
the hash key is a string
the values of the hash are of type leaf and value_type uniline

distribution => {
                  type => 'hash',
                  index_type => 'string',
                  cargo => {
                             type => 'leaf',
                             value_type => 'uniline',
                           },
                  summary => 'remote repositories',
                  description => 'The other name/value pairs are ...',
                }

For more details on list and hash elements, see hash or list model declaration man page.

node element

A node element is necessary if the configuration file has more than a list of variable. In this case, the tree is deeper than a rake and a node element if necessary to provide a new node within the tree.

In the Xorg example above, the options of Xorg::Screen need their own sub-branch in the tree:

Screen(Screen0)
  `--Option
     |--AllowGLXWithComposite=True
     `--DynamicTwinView=True

For this, a new dedicated class is necessary>Xorg::Screen::Option> (see its declaration above). This new class must be tied to the Screen class with a node element.

A node element has the following parameters:

type (set to node)
the name of the configuration class name (>config_class_name>)

So the Option node element is declared with:

Option => {
            type => 'node',
            config_class_name => 'Xorg::Screen::Option'
          },

Hash or list of nodes

Some configuration files can feature a set of rather complex configuration entities. For instance Xorg.pl can feature several Screen or Device definitions. These definitions are identified by the Identifier parameter:

Section "Device"
  Identifier     "Device0"
  Driver         "nvidia"
  BusID          "PCI:3:0:1"
EndSection

Section "Screen"
  Identifier     "Screen0"
  Device         "Device0"
  DefaultDepth    24
EndSection

The Xorg configuration tree features 2 elements (Screen and Device) that use the Identifier parameters as hash keys:

root
|--Device(Device0)
|  |--Driver=nvidia
|  `--BusId=PCI:3:0:1
`--Screen(Screen0)
   |--Device=Device0
   `--DefaultDepth=24

And the Xorg model must define these 2 parameters as hash. The cargo of this hash is of type node and refers to 2 different configuration classes, one for Device (Xorg::Device) and one for Screen (Xorg::Screen):

{
name => 'Xorg',
element => [
            Device => {
                       type => 'hash',
                       index_type => 'string'
                       cargo => {
                                  type => 'node',
                                  config_class_name => 'Xorg::Device'
                                },
                      },
            Screen => {
                       type => 'hash',
                       index_type => 'string'
                       cargo => {
                                 type => 'node',
                                 config_class_name => 'Xorg::Screen'
                                },
                      },
         ]
}

Configuration wizard

Both Perl/Tk and Curses interfaces feature a configuration wizard generated from a configuration model.

The wizard works by exploring the configuration tree and stopping on each important element and on each error (mostly missing mandatory parameter).

When designing a model, you have to ponder for each element:

The importance level of the parameter (important, normal or hidden). level is used to set how configuration data is presented to the user in wizard and browsing mode. Important elements are shown in the wizard. hidden elements are explained with the warp notion in Creating a model with advanced features.

Reading configuration files

Once the model is specified, Config::Model can generate a nice user interface, but there's still no way to load or write the configuration file.

For Config::Model to read the file, the model designer must declare in the model how to read and write the file (the read/write backend).

The read/write functionality is provided by a class inheriting Config::Model::Backend::Any class like Config::Model::Backend::IniFile

The name of the backend parameter must match the backend class name without Config::Model::Backend. As syntactic sugar, lower case backend name are transformed into upper case to match the backend class name.

E.g.

Yaml -> Config::Model::Backend::Yaml
plain_file -> Config::Model::Backend::PlainFile
ini_file -> Config::Model::Backend::IniFile

With the backend name, the following parameters must be defined:

config_dir: The configuration directory
file: Config file name (optional). defaults to <config_class_name>.[pl|ini|cds]

rw_config  => { backend    => 'ini_file' ,
                config_dir => '/etc/cfg_dir',
                file       => 'cfg_file.ini',
              },

See Config::Model::Backend::IniFile for details

Note that these parameters can also be set with the graphical configuration model editor (cme meta edit).

rw_config can also have custom parameters that are passed verbatim to Config::Model::Backend::Foo methods:

rw_config  => {
   backend    => 'my_backend',
   config_dir => '/etc/cfg_dir',
   my_param   => 'my_value',
}

This Config::Model::Backend::MyBackend class is expected to inherit Config::Model::Backend::Any and provide the following methods:

new
read
write

Their signatures are explained in Config::Model::BackendMgr doc on plugin backends

Feedback welcome

Feel free to send comments and suggestion about this page at

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

AUTHORS

Dominique Dumont <ddumont at cpan.org>

AUTHOR

Dominique Dumont

COPYRIGHT AND LICENSE

This is free software, licensed under:

The GNU Lesser General Public License, Version 2.1, February 1999

To install Config::Model, copy and paste the appropriate command in to your terminal.

cpanm

cpanm Config::Model

CPAN shell

perl -MCPAN -e shell
install Config::Model

For more information on module installation, please visit the detailed CPAN module installation guide.

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