Provided by: libconfig-model-perl_2.152-1_all bug

NAME

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

VERSION

       version 2.152

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 the"Display" 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'
                                         },
                               },
                  ]
        }

       The"Xorg::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

SEE ALSO

       •   More complex models: Config::Model::Manual::ModelCreationAdvanced

       •   Config::Model::Manual::ModelForUpgrade: Writing a model for configuration upgrades

       •   Configuration upgrades within Debian packages
           <http://wiki.debian.org/PackageConfigUpgrade>

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 software is Copyright (c) 2005-2022 by Dominique Dumont.

       This is free software, licensed under:

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

perl v5.34.0                                Config::Model::Manual::ModelCreationIntroduction(3pm)