Config::Model::Manual::ModelCreationIntroduction - Introduction to model
creation with Config::Model
version 2.152
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)
- 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
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).
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.
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.
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.
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'
}
]
}
];
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 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' ]
}
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.
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'
},
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'
},
},
]
}
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.
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
- •
- 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>
Feel free to send comments and suggestion about this page at
config-model-users at lists dot sourceforge dot net.
Dominique Dumont <ddumont at cpan.org>
Dominique Dumont
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