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Dominique Dumont
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Config::Model::BackendMgr - Load configuration node on demand


version 2.117


 # Use BackendMgr to write data in Yaml file
 use Config::Model;

 # define configuration tree object
 my $model = Config::Model->new;
    name    => "Foo",
    element => [
        [qw/foo bar/] => {
            type       => 'leaf',
            value_type => 'string'

    name => "MyClass",

    # rw_config spec is used by Config::Model::BackendMgr
    rw_config => [
            backend     => 'yaml',
            config_dir  => '/tmp/',
            file        => 'my_class.yml',
            auto_create => 1,

    element => [
        [qw/foo bar/] => {
            type       => 'leaf',
            value_type => 'string'
        hash_of_nodes => {
            type       => 'hash',     # hash id
            index_type => 'string',
            cargo      => {
                type              => 'node',
                config_class_name => 'Foo'

 my $inst = $model->instance( root_class_name => 'MyClass' );

 my $root = $inst->config_root;

 # put data
 my $steps = 'foo=FOO hash_of_nodes:fr foo=bonjour -
   hash_of_nodes:en foo=hello ';
 $root->load( steps => $steps );


 # now look at file /tmp/my_class.yml


This class provides a way to specify how to load or store configuration data within the model.

With these specifications, all configuration information is read during creation of a node (which triggers the creation of a backend manager object) and written back when write_back method is called (either on the node or on this backend manager).

This load/store can be done with different backends:

  • Any of the Config::Model::Backend::* classes available on your system. For instance Config::Model::Backend::Yaml.

  • cds_file: Config dump string (cds) in a file. I.e. a string that describes the content of a configuration tree is loaded from or saved in a text file. This format is defined by this project. See "load string syntax" in Config::Model::Loader.

  • perl_file: Perl data structure (perl) in a file. See Config::Model::DumpAsData for details on the data structure. Now handled by Config::Model::Backend::PerlFile

When needed, write_back method can be called on the instance (See Config::Model::Instance) to store back all configuration information.

Backend specification

The backend specification is provided as an attribute of a Config::Model::Node specification. These attributes are optional: A node without rw_config attribute must rely on another node to read or save its data.

When needed (usually for the root node), the configuration class is declared with a rw_config parameter which specifies the read/write backend configuration.

Parameters available for all backends

The following parameters are accepted by all backends:


Specify configuration directory. This parameter is optional as the directory can be hardcoded in the backend class. config_dir beginning with '~' is munged so ~ is replaced by File::HomeDir->my_data. See File::HomeDir for details.


Specify configuration file name (without the path). This parameter is optional as the file name can be hardcoded in the backend class.

The configuration file name can be specified with &index keyword when a backend is associated to a node contained in a hash. For instance, with file set to &index.conf:

 service    # hash element
   foo      # hash index
     nodeA  # values of nodeA are stored in foo.conf
   bar      # hash index
     nodeB  # values of nodeB are  stored in bar.conf

Likewise, the keyword &element can be used to specify the file name. For instance, with file set to &element-&index.conf:

 service    # hash element
   foo      # hash index
     nodeA  # values of nodeA are stored in service.foo.conf
   bar      # hash index
     nodeB  # values of nodeB are  stored in service.bar.conf

Alternatively, file can be set to -, in which case, the configuration is read from STDIN.


file_mode parameter can be used to set the mode of the written file(s). file_mode value can be in any form supported by "chmod" in Path::Tiny. Example:

  file_mode => 0664,
  file_mode => '0664',
  file_mode => 'g+w'

Specify alternate location of a configuration directory depending on the OS (as returned by $^O, see "PLATFORMS" in perlport). For instance:

 config_dir => '/etc/ssh',
 os_config_dir => { darwin => '/etc' }

Optional. Specifies where to find a global configuration file that specifies default values. For instance, this is used by OpenSSH to specify a global configuration file (/etc/ssh/ssh_config) that is overridden by user's file:

        'default_layer' => {
            os_config_dir => { 'darwin' => '/etc' },
            config_dir    => '/etc/ssh',
            file          => 'ssh_config'

Only the 3 above parameters can be specified in default_layer.


By default, an exception is thrown if no read was successful. This behavior can be overridden by specifying auto_create => 1 in one of the backend specification. For instance:

    rw_config  => {
        backend => 'IniFile',
        config_dir => '/tmp',
        file  => 'foo.conf',
        auto_create => 1

Setting auto_create to 1 is necessary to create a configuration from scratch


Delete configuration files that contains no data. (default is to leave an empty file)

Config::Model::Backend::* backends

Specify the backend name and the parameters of the backend defined in their documentation.

For instance:

   rw_config => {
       backend     => 'yaml',
       config_dir  => '/tmp/',
       file        => 'my_class.yml',

See Config::Model::Backend::Yaml for more details for this backend.

Your own backend

You can also write a dedicated backend. See How to write your own backend for details.

Test setup

By default, configurations files are read from the directory specified by config_dir parameter specified in the model. You may override the root directory for test.


When both config_dir and file are specified, this class write-opens the configuration file (and thus clobber it) before calling the write call-back and pass the file handle with io_handle parameter. write should use this handle to write data in the target configuration file.

If this behavior causes problem (e.g. with augeas backend), the solution is either to set file to undef or an empty string in the rw_config specification.



Returns 1 if at least the backend supports read and write annotations (aka comments) in the configuration file.


Dominique Dumont, (ddumont at cpan dot org)


Config::Model, Config::Model::Instance, Config::Model::Node, Config::Model::Dumper


Dominique Dumont


This software is Copyright (c) 2005-2018 by Dominique Dumont.

This is free software, licensed under:

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