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Manual Reference Pages  -  CONFIG::MODEL::BACKENDMGR (3)

.ds Aq ’


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



version 2.082


 # Use BackendMgr to write data in perl data 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",

    # read_config spec is used by Config::Model::BackendMgr
    read_config => [
            backend     => perl_file,
            config_dir  => /tmp/,
            file        =>,
            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->node->instance( root_class_name => MyClass );

 my $root = $inst->config_root;

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


 # now look at file /tmp/


This class provides a way to specify how to load or store configuration data within the model (instead of writing dedicated perl code).

With these specifications, all the configuration information is read during creation of a node.

This load/store can be done with different backend:
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. See Config::Model::Dumper.
IniFile INI files (written with Config::Model::Backend::IniFile. See limitations in Limitations depending on storage.
perl_file Perl data structure (perl) in a file. See Config::Model::DumpAsData for details on the data structure.
custom Any format when the user provides a dedicated class and function to read and load the configuration tree.
After loading the data, the object registers itself to the instance. Then the user can call the write_back method on the instance (See Config::Model::Instance) to store all configuration information back.

    Built-in backend

cds_file, IniFile and perl_file backend must be specified with mandatory config_dir parameter. For instance:

   read_config  => { backend    => cds_file ,
                     config_dir => /etc/cfg_dir,
                     file       => cfg_file.cds, #optional

If file is not specified, a file name will be constructed with <config_class_name>.<suffix> where suffix is pl or ini or cds.

    Plugin backend classes

A plugin backend class can also be specified with:

  read_config  => [ { backend    => foo ,
                      config_dir => /etc/cfg_dir
                      file       => foo.conf, # optional

In this case, this class 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 must inherit Config::Model::Backend::Any and is expected to provide the following methods:
new Mandatory parameters:

 node => ref_to_config_model_node

new() must return the newly created object

read with parameters:

 %custom_parameters,      # model data
 root => $root_dir,       # mostly used for tests
 config_dir => $read_dir, # path below root
 file_path => $full_name, # full file name (root+path+file)
 io_handle => $io_file    # IO::File object
 check     => [ yes|no|skip]

Must return 1 if the read was successful, 0 otherwise.

Following the my_param example above, %custom_parameters will contain ( my_param , my_value ) , so read() will also be called with root, config_dir, file_path, io_handle <B>andB> my_param => my_value.

write with parameters:

 %$write,                     # model data
 auto_create => $auto_create, # from model
 backend     => $backend,     # backend name
 config_dir  => $write_dir,   # override from instance
 io_handle   => $fh,          # IO::File object
 write       => 1,            # always
 check       => [ yes|no|skip] ,
 root        => $root_dir,
 backup      => [ undef ||  || suffix ] # backup strategy required by user

Must return 1 if the write was successful, 0 otherwise

If io_handle is defined, the backup has already been done while opening the config file. If io_handle is not defined, there’s not enough information in the model to read the configuration file and create the backup. Your write() method will have to do the backup requested by user.

    Custom backend

Custom backend is provided to be backward compatible but should not be used for new project. Using a plugin backend as described above is preferred.

Custom backend must be specified with a class name that will features the methods used to write and read the configuration files:

  read_config  => [ { backend => custom ,
                      class => MyRead,
                      config_dir => /etc/foo, # optional
                      file => foo.conf,       # optional
                    } ]

custom backend parameters are:
class Specify the class that contain the read method
config_dir Specify configuration directory. This parameter is optional as the directory can be hardcoded in the custom class. config_dir beginning with ’~’ will be munged so ~ is replaced by File::HomeDir->my_data. See File::HomeDir for details.
os_config_dir 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 }

file optional. Configuration file. This parameter may not apply if the configuration is stored in several files. By default, the instance name is used as configuration file name.
default_layer 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.

function Function name that will be called back to read the file. See read callback for details. (default is read)
auto_create 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:

    read_config  => [ { backend => cds_file, config_dir => /etc/my_cfg/ } ,
                      { backend => custom, class => Bar ,
                        auto_create => 1

This feature is necessary to create a configuration from scratch

When set in write backend, missing directory and files will be created with current umask. Default is false.

Write specification is similar to read_specification. Except that the default value for function is write. Here’s an example:

   write_config  => [ { backend => cds_file, config_dir => /etc/my_cfg/ } ,
                      { backend => custom, class => Bar ,
                        function => my_write,

Limitations depending on storage

Some storage system will limit the structure of the model you can map to the file.

    Ini files limitation

Structure of the Config::Model must be very simple. Either:
o A single class with hash of leaves elements.
o 2 levels of classes. The top level has nodes elements. All other classes have only leaf elements.

Configuration class with auto read or auto write

    read specification

A configuration class will be declared with optional read_config parameter:

  read_config  => [ { backend => cds_file, config_dir => /etc/my_cfg/ } ,
                    { backend => custom, class => Bar },

The read backends will be tried in the specified order:
o First the cds file whose name depend on the parameters used in model creation and instance creation: <model_config_dir>/<instance_name>.cds The syntax of the cds file is described in Config::Model::Dumper.
o A callback to Bar::read. See "read callback for details.
When a read operation is successful, the remaining read methods will be skipped.

    write specification

A configuration class will be declared with optional write_config parameters (along with read_config parameter):

  write_config => [ { backend => cds_file, config_dir => /etc/my_cfg/,
                      auto_create => 1, },
                    { backend => custom, class => NewFormat } ],

By default, the specifications are tried in order, until the first succeeds.

When required by the user, all configuration information is written back using <B>allB> the write specifications. See write_back ( ... ) in Config::Model::Instance for details.

The write class declared with custom backend must provide a call-back. See write callback for details.

    read write directory

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.

    read callback

Read callback function will be called with these parameters:

  object     => $obj,         # Config::Model::Node object
  root       => ./my_test,  # fake root directory, used for tests
  config_dir => /etc/foo,    # absolute path
  file       => foo.conf,   # file name
  file_path  => ./my_test/etc/foo/foo.conf
  io_handle  => $io           # IO::File object with binmode :utf8
  check      => [yes|no|skip]

The IO::File object is undef if the file cannot be read.

The callback must return 0 on failure and 1 on successful read.

    write callback

Write callback function will be called with these parameters:

  object      => $obj,         # Config::Model::Node object
  root        => ./my_test,  # fake root directory, used for tests
  config_dir  => /etc/foo,    # absolute path
  file        => foo.conf,   # file name
  file_path  => ./my_test/etc/foo/foo.conf
  io_handle   => $io           # IO::File object opened in write mode
                               # with binmode :utf8
  auto_create => 1             # create dir as needed
  check      => [yes|no|skip]

The IO::File object is undef if the file cannot be written to.

The callback must return 0 on failure and 1 on successful write.

Configuration file will be deleted if callback returns 2.


When both config_dir and file are specified, this class will write-open 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:
o Set file to undef or an empty string in the write_config specification.
o Create a skip_open function in your backend class that returns 1


In the example below, only a cds file is written. But, both custom format and cds file are tried for read. This is also an example of a graceful migration from a customized format to a cds format.

  read_config  => [ { backend => cds_file, config_dir => /etc/my_cfg/ } ,
                    { backend => custom, class => Bar },
  write_config => [{ backend => cds_file, config_dir => /etc/my_cfg/ }],

You can choose also to read and write only customized files:

  read_config  => [{ backend => custom, class => Bar}],

Or to read and write only cds files :

  read_config  => [{ backend => cds_file}] ,

You can also specify more parameters that must be passed to your custom class:

  read_config  => [{ backend => custom, class => Bar,
                    config_dir => /etc/foo}],

To migrate from an old format to a new format:

  read_config  => [ { backend => custom,
                      class => OldFormat,
                      function => old_read
                    } ,
                    { backend => custom,
                      class => NewFormat,
                      function => new_read
  write_config => [ { backend => custom,
                      class => NewFormat

If write_config is missing, the data provided by read_config will be used. For instance:

  read_config  => [ { backend => custom,
                      class => Bar,
                      config_dir => /etc/foo
                  } ],

In this case, configuration data will be read by Bar::read in directory /etc/foo and will be written back there by Bar::write.


    write_back ( ... )

Try to run all subroutines registered by auto_write_init write the configuration information until one succeeds (returns true).

You can specify here a pseudo root directory or another config directory to write configuration data back with root and config_dir parameters. This will override the model specifications.

You can force to use a backend by specifying backend => xxx. For instance, backend => perl_file or backend => custom.

You can force to use all backend to write the files by specifying backend => all.

You can force a specific config file to write with config_file => foo/bar.conf

write_back will croak if no write call-back are known for this node.


Returns 1 if at least one of the backends support to 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) 2005-2016 by Dominique Dumont.

This is free software, licensed under:

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

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perl v5.20.3 CONFIG::MODEL::BACKENDMGR (3) 2016-04-03

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