Provided by: libconfig-model-perl_2.082-1_all 
NAME
Config::Model::Manual::ModelCreationIntroduction - Introduction to model creation with
Config::Model
VERSION
version 2.082
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.
A tutorial is available in Creating a model from config file documentation.
Note that this document will show 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 will be 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 will use the configuration file
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
Some will 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 will 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).
In terms of models, the model will be 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 will lead 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 will be 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 will be:
{
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 configuration classes that will be required, you will have to 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 will also have to find several valid examples. These examples be
used as non-regression tests and verify that the documentation was understood.
Model declaration
Configuration class declaration
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'
}
]
}
];
Configuration class declaration (easier way)
Since writing a data structure is not fun (even with Perl), you are encouraged to use the
model editor provided by config-model-edit from Config::Model::Itself module. You will get
this type of GUI shown on the right with the command "config-model-edit -model Xorg"
Common attributes for all elements
This first set of attributes will help 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 will
be shown to the user no matter what. hidden elements will be explained with the warp
notion.
• "status": is "obsolete", "deprecated" or "standard" (default). Using a deprecated
element will issue a warning. Using an obsolete element will raise an exception.
See "Configuration_class" in Config::Model for details.
Simple leaf elements
Simple leaf elements will be used most often for configuration files. A leaf element will
represent 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 will have to 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 will use "any" if this parameter is left blank in "sshd_config" file.
Thus the model of this element will be :
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 repositories must be stored as a hash where the key will be debian or multimedia and
the associated value will 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 will be stored this way in the configuration tree:
root
|--distrib(debian)=http://ftp.fr.debian.org/debian
`--distrib(multimedia)=http://www.debian-multimedia.org
The model will need to declare that "distrib" 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 will feature 2 elements (Screen and Device) that will 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 will
of type "node" and will refer 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 will have to think about 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 will be shown in the wizard. hidden elements will be 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 the file (the read backend).
The read method can use one or more of the following mechanisms:
• Built-in, e.g Perl file, INI file...
• A plugin, i.e. a Perl "Config::Model::Backend::*" class like
"Config::Model::Backend::Augeas"
• A custom class where a read call-back must be provided
For more details, see Config::Model::BackendMgr.
The name of the backend parameter must be specified in all cases.
Using built-in read mechanism
"Config::Model" comes with 3 read/write built in mechanisms:
perl_file
A perl data structure (like the ones produced by Data::Dumper). See
Config::Model::DumpAsData for details.
ini_file
Windows INI file (note that only simple tree structure can use this backend)
cds_file
Config::Model own serialization format (a bit like YAML). See Config::Model::Dumper
for details.
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]"
read_config => [ { backend => 'cds_file' ,
config_dir => '/etc/cfg_dir',
file => 'cfg_file.cds', # optional
} ],
See "Built-in_backend" in Config::Model::BackendMgr.pm for details
Note that these parameters can also be set with the graphical configuration model editor.
Using a plugin read mechanism
A plugin backend class can also be specified with:
read_config => [ { backend => 'foo' ,
config_dir => '/etc/cfg_dir'
} ]
In this case, Config::Model will try to load "Config::Model::Backend::Foo". (The class
name is constructed with "ucfirst($backend_name)")
"read_config" can also have custom parameters that will passed verbatim to
"Config::Model::Backend::Foo" methods:
read_config => [ { backend => 'foo' ,
config_dir => '/etc/cfg_dir',
my_param => 'my_value',
} ]
This "Config::Model::Backend::Foo" class is expected to provide the following methods:
new
read
write
Their signatures are explained in Config::Model::BackendMgr doc on plugin backends
Using a custom class
In case the plugin mechanism is not possible, a class with an arbitrary name can be
specified:
read_config => [ { backend => 'custom' ,
class => 'MyRead',
config_dir => '/etc/foo', # optional
file => 'foo.conf', # optional
} ]
Even the read method can have an arbitrary name by specifying a "function" parameters.
For more details on available parameters on custom backends, see Config::Model::BackendMgr
doc on custom backends
Using several read mechanisms
Several read mechanism can be specified to enable:
• Migration from one syntax to another
• Usage of different libraries (e.g. Augeas <http://augeas.net> or pure Perl backend)
For instance, to try Augeas and fall back on a custom class in case of problem, specify:
read_config => [ {
save => 'backup',
file => 'sshd_config',
backend => 'augeas',
config_dir => '/etc/ssh'
},
{
function => 'sshd_read',
backend => 'custom',
class => 'Config::Model::OpenSsh',
config_dir => '/etc/ssh'
} ],
Both specifications are tried in order. If Augeas backend fails (e.g. Augeas is not
installed), the custom backend will be used.
An exception will be raised if both methods fails. This behavior is correct for "OpenSsh",
but it can be a problem if you want to use Config::Model to create a configuration file
from scratch. In this case you will also have to specify the "auto_create" parameter:
read_config => [ { backend => 'custom' ,
class => 'ProcessRead' ,
config_dir => '/etc/foo',
file => 'foo.conf',
auto_create => 1,
} ],
Writing configuration files
Read and write specifications were designed to be very similar. Most of the times, the
"read" and "write" specification will be identical. In this case, there's no need to enter
them: the data specified in the "read" specification will be used to write the
configuration file.
Here's an example:
write_config => [ { backend => 'custom',
class => 'NewFormat'
function => 'my_write',
}
],
Several "write" specification can be used. They are tried in order, until the first
succeeds.
For more information, see write specification doc
Syntax migration example
By combining multiple read specification with "'one"' write specification, a configuration
file can be migrated from old to new syntax. The following example will migrate a
configuration file from a custom syntax to a perl data file:
{
name => 'Example',
element => [ ... ] ,
read_config => [ { backend => 'perl_file',
config_dir => '/etc/my_cfg/'
} ,
{ backend => 'custom',
class => 'Bar'
},
],
write_config => [ { backend => 'perl_file',
config_dir => '/etc/my_cfg/'
}
],
}
How does this work ? Here's the sequence:
1. Configuration is stored in old file "/etc/my_cfg/bar.conf"
2. Config::Model tries to read the config with "perl_file" read backend and looks for
"/etc/my_cfg/example.pl". This file is not found so the read fails.
3. Config::Model tries the second backend which succeeds and load configuration data in
the configuration tree
4. Config::Model writes data back from configuration tree using "write_config" backend
which writes "/etc/my_cfg/example.pl"
5. At the next invocation, the first "read" backend will successfully read the perl
configuration file. The old file is left alone and can be removed later by the system
admin.
Thanks to this mechanism, this operation is idempotent so it can safely be scripted in
package scriplets.
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) 2005-2016 by Dominique Dumont.
This is free software, licensed under:
The GNU Lesser General Public License, Version 2.1, February 1999
perl v5.22.1 Config::Model::Manual::ModelCreationIntroduction(3pm)