libconfig-model-perl 2.097-2 / usr / share / perl5 / Config / Model / BackendMgr.pm

#
# This file is part of Config-Model
#
# 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
#
package Config::Model::BackendMgr;
$Config::Model::BackendMgr::VERSION = '2.097';
use Mouse;

use Carp;
use 5.10.1;

use Config::Model::Exception;
use Data::Dumper;
use File::Path;
use File::Copy;
use File::HomeDir;
use IO::File;
use Storable qw/dclone/;
use Scalar::Util qw/weaken/;
use Log::Log4perl qw(get_logger :levels);

my $logger = get_logger('BackendMgr');

# used only for tests
my $__test_home = '';
sub _set_test_home { $__test_home = shift; }

# one BackendMgr per file

has 'node' => (
    is       => 'ro',
    isa      => 'Config::Model::Node',
    weak_ref => 1,
    required => 1
);
has 'file_backup' => ( is => 'rw', isa => 'ArrayRef', default => sub { [] } );
has 'backend' => (
    is      => 'rw',
    isa     => 'HashRef[Config::Model::Backend::Any]',
    traits  => ['Hash'],
    default => sub { {} },
    handles => { set_backend => 'set', get_backend => 'get' } );

# Configuration directory where to read and write files. This value
# does not override the configuration directory specified in the model
# data passed to read and write functions.
has config_dir => ( is => 'ro', isa => 'Maybe[Str]', required => 0 );

has support_annotation => (
    is => 'ro',
    isa => 'Bool',
    default => 0,
);

sub get_tuned_config_dir {
    my ($self, %args) = @_;

    my $dir = $args{os_config_dir}{$^O} || $args{config_dir} || $self->config_dir || '';
    if ( $dir =~ /^~/ ) {
        my $home = $__test_home || File::HomeDir->my_home;
        $dir =~ s/^~/$home/;
    }

    $dir .= '/' if $dir and $dir !~ m(/$);

    return $dir;
}

sub get_cfg_dir_path {
    my $self = shift;
    my %args = @_;

    my $w = $args{write} || 0;
    my $dir = $self->get_tuned_config_dir(%args);

    $dir = $args{root} . $dir;

    if ( not -d $dir and $w and $args{auto_create} ) {
        $logger->info("creating directory $dir");
        mkpath( $dir, 0, 0755 );
    }

    unless ( -d $dir ) {
        $logger->info( "auto_" . ( $w ? 'write' : 'read' ) . " $args{backend} no directory $dir" );
        return ( 0, $dir );
    }

    $logger->debug( "dir: " . $dir // '<undef>' );

    return ( 1, $dir );
}

sub get_cfg_file_path {
    my $self = shift;
    my %args = @_;

    my $w = $args{write} || 0;

    # config file override
    my $cfo = $args{config_file};

    if ( defined $cfo and $cfo eq '-' and $w == 0 ) {
        $logger->trace("auto_read: $args{backend} override target file is STDIN");
        return ( 1, '-' );
    }

    if ( defined $cfo ) {
        my $override = $args{root} . $args{config_file};
        $logger->trace( "auto_"
                . ( $w ? 'write' : 'read' )
                . " $args{backend} override target file is $override" );
        return ( 1, $args{root} . $args{config_file} );
    }

    Config::Model::Exception::Model->throw(
        error  => "backend error: empty 'config_dir' parameter (and no config_file override)",
        object => $self->node
    ) unless $args{config_dir} or $self->config_dir;

    my ( $dir_ok, $dir ) = $self->get_cfg_dir_path(%args);

    if ( defined $args{file} ) {
        my $file = $args{file};
        $file =~ s/&index/$self->node->index_value/eg;
        my $res = $dir . $file;
        $logger->trace("get_cfg_file_path: returns $res");
        return ( $dir_ok, $res );
    }

    if ( not defined $args{suffix} ) {
        $logger->trace("get_cfg_file_path: returns undef (no suffix, no file argument)");
        return (0);
    }

    my $i    = $self->node->instance;
    my $name = $dir . $i->name;

    # append ":foo bar" if not root object
    my $loc = $self->node->location;    # not very good
    if ($loc) {
        if ( ( $w and not -d $name and $args{auto_create} ) ) {
            $logger->info( "get_cfg_file_path: auto_write create subdirectory ",
                "$name (location $loc)" );
            mkpath( $name, 0, 0755 );
        }
        $name .= '/' . $loc;
    }

    $name .= $args{suffix};

    $logger->trace( "get_cfg_file_path: auto_"
            . ( $w ? 'write' : 'read' )
            . " $args{backend} target file is $name" );

    return ( 1, $name );
}

sub open_read_file {
    my $self = shift;
    my %args = @_;

    my ( $file_ok, $file_path ) = $self->get_cfg_file_path(%args);

    if ( $file_ok and $file_path eq '-' ) {
        my $io = IO::Handle->new();
        if ( $io->fdopen( fileno(STDIN), "r" ) ) {
            return ( 1, '-', $io );
        }
        else {
            return ( 0, '-' );
        }
    }

    # not very clean
    return ( 0, $file_path )
        if $args{backend} =~ /_file$/
        and ( not $file_ok or not -r $file_path );

    my $fh = new IO::File;
    if ( $file_ok and -e $file_path ) {
        $logger->debug("open_read_file: open $file_path for read");
        $fh->open($file_path);
        $fh->binmode(":utf8");

        # store a backup in memory in case there's a problem
        $self->file_backup( [ $fh->getlines ] );
        $fh->seek( 0, 0 );    # go back to beginning of file
        return ( 1, $file_path, $fh );
    }
    else {
        return ( 0, $file_path );
    }
}

# called at configuration node creation
#
# New subroutine "load_backend_class" extracted - Thu Aug 12 18:32:37 2010.
#
sub load_backend_class {
    my $backend  = shift;
    my $function = shift;

    $logger->debug("load_backend_class: called with backend $backend, function $function");
    my %c;

    my $k = "Config::Model::Backend::" . ucfirst($backend);
    my $f = $k . '.pm';
    $f =~ s!::!/!g;
    $c{$k} = $f;

    # try another class
    $k =~ s/_(\w)/uc($1)/ge;
    $f =~ s/_(\w)/uc($1)/ge;
    $c{$k} = $f;

    foreach my $c ( keys %c ) {
        if ( $c->can($function) ) {

            # no need to load class
            $logger->debug("load_backend_class: $c is already loaded (can $function)");
            return $c;
        }
    }

    # look for file to load
    my $class_to_load;
    foreach my $c ( keys %c ) {
        $logger->debug("load_backend_class: looking to load class $c");
        foreach my $prefix (@INC) {
            my $realfilename = "$prefix/$c{$c}";
            $class_to_load = $c if -f $realfilename;
        }
    }

    return unless defined $class_to_load;
    my $file_to_load = $c{$class_to_load};

    $logger->debug("load_backend_class: loading class $class_to_load, $file_to_load");
    eval { require $file_to_load; };

    if ($@) {
        die "Could not parse $file_to_load: $@\n";
    }
    return $class_to_load;
}

sub read_config_data {
    my ( $self, %args ) = @_;

    {
        no warnings 'uninitialized';
        $logger->debug( "called with ", join( ' ', %args ) );
    }

    my $readlist_orig        = delete $args{read_config};
    my $check                = delete $args{check};
    my $r_dir                = delete $args{read_config_dir};
    my $config_file_override = delete $args{config_file};
    my $auto_create_override = delete $args{auto_create};

    croak "unexpected args " . join( ' ', keys %args ) . "\n" if %args;

    # r_dir is obsolete
    if ( defined $r_dir ) {
        die $self->node->config_class_name, " : read_config_dir is obsolete\n";
    }

    my $readlist = dclone $readlist_orig ;

    my $instance = $self->node->instance();

    # root override is passed by the instance
    my $root_dir = $instance->read_root_dir || '';

    croak "readlist must be array or hash ref\n"
        unless ref $readlist;

    my @list = ref $readlist eq 'ARRAY' ? @$readlist : ($readlist);
    my $pref_backend = $instance->backend || '';
    my $read_done    = 0;
    my $auto_create  = 0;
    my @tried;

    foreach my $read (@list) {
        warn $self->config_class_name, " deprecated 'syntax' parameter in backend\n"
            if defined $read->{syntax};
        my $backend = delete $read->{backend} || delete $read->{syntax} || 'custom';
        if ( $backend =~ /^(perl|ini|cds)$/ ) {
            warn $self->config_class_name,
                " deprecated  backend $backend. Should be '$ {backend}_file'\n";
            $backend .= "_file";
        }

        next if ( $pref_backend and $backend ne $pref_backend );

        if ( defined $read->{allow_empty} ) {
            warn "backend $backend: allow_empty is deprecated. Use auto_create";
            $auto_create ||= delete $read->{allow_empty};
        }

        $auto_create ||= delete $read->{auto_create} if defined $read->{auto_create};

        if ( $read->{default_layer} ) {
            $self->read_config_sub_layer( $read, $root_dir, $config_file_override, $check,
                $backend );
        }

        my ( $res, $file ) =
            $self->try_read_backend( $read, $root_dir, $config_file_override, $check, $backend );
        push @tried, $file;

        if ($res) {
            $read_done = 1;
            last;
        }
    }

    Config::Model::Exception::ConfigFile::Missing->throw(
        tried_files => \@tried,
        object      => $self->node,
        )
        unless $read_done
        or $auto_create_override
        or $auto_create;

}

sub read_config_sub_layer {
    my ( $self, $read, $root_dir, $config_file_override, $check, $backend ) = @_;

    my $layered_config = delete $read->{default_layer};
    my $layered_read   = dclone $read ;

    map { my $lc = delete $layered_config->{$_}; $layered_read->{$_} = $lc if $lc; }
        qw/file config_dir os_config_dir/;

    Config::Model::Exception::Model->throw(
        error => "backend error: unexpected default_layer parameters: "
            . join( ' ', keys %$layered_config ),
        object => $self->node,
    ) if %$layered_config;

    my $i                  = $self->node->instance;
    my $already_in_layered = $i->layered;

    # layered stuff here
    if ( not $already_in_layered ) {
        $i->layered_clear;
        $i->layered_start;
    }

    $self->try_read_backend( $layered_read, $root_dir, $config_file_override, $check, $backend );

    if ( not $already_in_layered ) {
        $i->layered_stop;
    }
}

# called at configuration node creation, NOT when writing
#
# New subroutine "try_read_backend" extracted - Sun Jul 14 11:52:58 2013.
#
sub try_read_backend {
    my $self                 = shift;
    my $read                 = shift;
    my $root_dir             = shift;
    my $config_file_override = shift;
    my $check                = shift;
    my $backend              = shift;

    my $read_dir = $self->get_tuned_config_dir(%$read);

    my @read_args = (
        %$read,
        root        => $root_dir,
        config_dir  => $read_dir,
        backend     => $backend,
        check       => $check,
        config_file => $config_file_override
    );

    my ( $file_ok, $res, $fh, $file_path );

    if ( $backend eq 'custom' ) {
        my $c = my $file = delete $read->{class};
        $file =~ s!::!/!g;
        my $f = delete $read->{function} || 'read';
        require $file . '.pm' unless $c->can($f);
        no strict 'refs';

        $logger->info("Read with custom backend $ {c}::$f in dir $read_dir");

        ( $file_ok, $file_path, $fh ) = $self->open_read_file(@read_args);
        eval {
            $res = &{ $c . '::' . $f }(
                @read_args,
                file_path => $file_path,
                io_handle => $fh,
                object    => $self->node
            );
        };
    }
    elsif ( $backend eq 'perl_file' ) {
        ( $file_ok, $file_path, $fh ) = $self->open_read_file( @read_args, suffix => '.pl' );
        return ( 0, $file_path ) unless $file_ok;
        eval { $res = $self->read_perl( @read_args, file_path => $file_path, io_handle => $fh ); };
    }
    elsif ( $backend eq 'cds_file' ) {
        ( $file_ok, $file_path, $fh ) = $self->open_read_file( @read_args, suffix => '.cds' );
        return ( 0, $file_path ) unless $file_ok;
        eval {
            $res = $self->read_cds_file(
                @read_args,
                file_path => $file_path,
                io_handle => $fh,
            );
        };
    }
    else {
        # try to load a specific Backend class
        my $f = delete $read->{function} || 'read';
        my $c = load_backend_class( $backend, $f );
        return ( 0, 'unknown' ) unless defined $c;

        no strict 'refs';
        my $backend_obj = $c->new( node => $self->node, name => $backend );
        $self->set_backend( $backend => $backend_obj );
        my $suffix;
        $suffix = $backend_obj->suffix if $backend_obj->can('suffix');
        ( $file_ok, $file_path, $fh ) = $self->open_read_file( @read_args, suffix => $suffix );
        if ($logger->is_info) {
            my $fp = defined $file_path ? " on $file_path":'' ;
            $logger->info( "Read with $backend " . $c . "::$f".$fp);
        }

        eval {
            $res = $backend_obj->$f(
                @read_args,
                file_path => $file_path,
                io_handle => $fh,
                object    => $self->node,
            );
        };

        # only backend based on C::M::Backend::Any can support annotations
        if ($backend_obj->can('annotation')) {
            $self->{support_annotation} ||= $backend_obj->annotation ;
        }

    }

    # catch eval errors done in the if-then-else block before
    my $e = $@;
    if ( ref($e) and $e->isa('Config::Model::Exception::Syntax') ) {

        $e->parsed_file( $file_path) unless $e->parsed_file;
        $e->rethrow;
    }
    elsif ( ref $e ) {
        $e->rethrow ;
    }
    elsif ( $e ) {
        die "Backend error: $e";
    }

    return ( $res, $file_path );
}

sub auto_write_init {
    my ( $self, %args ) = @_;
    my $wrlist_orig = delete $args{write_config};
    my $w_dir       = delete $args{write_config_dir};

    weaken($self);    # avoid leak: $self is stored in write_back closure

    croak "auto_write_init: unexpected args " . join( ' ', keys %args ) . "\n"
        if %args;

    # w_dir is obsolete
    if ( defined $w_dir ) {
        die $self->config_class_name, " : write_config_dir is obsolete\n";
    }

    my $wrlist = dclone $wrlist_orig ;

    my $instance = $self->node->instance();

    # root override is passed by the instance
    my $root_dir = $instance->write_root_dir || '';

    my @array = ref $wrlist eq 'ARRAY' ? @$wrlist : ($wrlist);

    # ensure that one auto_create specified applies to all wr backends
    my $auto_create = 0;
    foreach my $write (@array) {
        $auto_create ||= delete $write->{auto_create}
            if defined $write->{auto_create};
    }

    # provide a proper write back function
    foreach my $write (@array) {
        warn $self->config_class_name, " deprecated 'syntax' parameter in auto_write\n"
            if defined $write->{syntax};
        my $backend = delete $write->{backend} || delete $write->{syntax} || 'custom';
        if ( $backend =~ /^(perl|ini|cds)$/ ) {
            warn $self->config_class_name,
                " deprecated backend $backend. Should be '$ {backend}_file'\n";
            $backend .= "_file";
        }

        my $write_dir = $self->get_tuned_config_dir(%$write);

        $logger->debug( "auto_write_init creating write cb ($backend) for ", $self->node->name );

        my @wr_args = (
            %$write,    # model data
            auto_create => $auto_create,
            backend     => $backend,
            config_dir  => $write_dir,     # override from instance
            write       => 1,              # for get_cfg_file_path
            root        => $root_dir,      # override from instance
        );

        my $wb;
        if ( $backend eq 'custom' ) {
            my $c = my $file = $write->{class};
            $file =~ s!::!/!g;
            my $f = $write->{function} || 'write';
            require $file . '.pm' unless $c->can($f);

            $wb = sub {
                no strict 'refs';
                $logger->debug( "write cb ($backend) called for ", $self->node->name );
                my ( $file_ok, $file_path, $fh ) =
                    $self->open_file_to_write( $backend, @wr_args, @_ )
                    unless ( $c->can('skip_open') and $c->skip_open );
                my $res;
                $res = eval {

                    # override needed for "save as" button
                    &{ $c . '::' . $f }(
                        @wr_args,
                        io_handle => $fh,
                        file_path => $file_path,
                        conf_dir  => $write_dir,    # legacy FIXME
                        object    => $self->node,
                        @_                          # override from user
                    );
                };
                $logger->warn( "write backend $c" . '::' . "$f failed: $@" ) if $@;
                $self->close_file_to_write( $@, $fh, $file_path );
                return defined $res ? $res : $@ ? 0 : 1;
            };
            $self->{auto_write}{custom} = 1;
        }
        elsif ( $backend eq 'perl_file' ) {
            $wb = sub {
                $logger->debug( "write cb ($backend) called for ", $self->node->name );
                my ( $file_ok, $file_path, $fh ) =
                    $self->open_file_to_write( $backend, suffix => '.pl', @wr_args, @_ );
                my $res;
                $res = eval {
                    $self->write_perl( @wr_args, io_handle => $fh, file_path => $file_path, @_ );
                };
                $self->close_file_to_write( $@, $fh, $file_path );
                $logger->warn("write backend $backend failed: $@") if $@;
                return defined $res ? $res : $@ ? 0 : 1;
            };
            $self->{auto_write}{perl_file} = 1;
        }
        elsif ( $backend eq 'cds_file' ) {
            $wb = sub {
                $logger->debug( "write cb ($backend) called for ", $self->node->name );
                my ( $file_ok, $file_path, $fh ) =
                    $self->open_file_to_write( $backend, suffix => '.cds', @wr_args, @_ );
                my $res;
                $res = eval {
                    $self->write_cds_file(
                        @wr_args,
                        io_handle => $fh,
                        file_path => $file_path,
                        @_
                    );
                };
                $logger->warn("write backend $backend failed: $@") if $@;
                $self->close_file_to_write( $@, $fh, $file_path );
                return defined $res ? $res : $@ ? 0 : 1;
            };
            $self->{auto_write}{cds_file} = 1;
        }
        else {
            my $f = $write->{function} || 'write';
            my $c = load_backend_class( $backend, $f );

            $wb = sub {
                no strict 'refs';
                $logger->debug( "write cb ($backend) called for ", $self->node->name );
                my $backend_obj = $self->get_backend($backend)
                    || $c->new( node => $self->node, name => $backend );
                my $suffix = $backend_obj->suffix if $backend_obj->can('suffix');
                my ( $file_ok, $file_path, $fh ) =
                    $self->open_file_to_write( $backend, suffix => $suffix, @wr_args, @_ )
                    unless ( $c->can('skip_open') and $c->skip_open );

                # override needed for "save as" button
                my %backend_args = (
                        @wr_args,
                        io_handle => $fh,
                        file_path => $file_path,
                        object    => $self->node,
                        @_    # override from user
                    );
                my $res = eval { $backend_obj->$f( %backend_args ); };
                $logger->warn( "write backend $backend $c" . '::' . "$f failed: $@" ) if $@;
                $self->close_file_to_write( $@, $fh, $file_path );

                $self->auto_delete($file_path, \%backend_args) if $write->{auto_delete};

                return defined $res ? $res : $@ ? 0 : 1;
            };
        }

        # FIXME: enhance write back mechanism so that different backend *and* different nodes
        # work as expected
        $logger->debug( "registering write $backend in node " . $self->node->name );
        push @{ $self->{write_back} }, [ $backend, $wb ];
        $instance->register_write_back( $self->node->location );
    }
}

sub auto_delete {
    my ($self, $file_path, $args) = @_;

    my $perl_data = $self->node->dump_as_data( full_dump => $args->{full_dump} // 0);

    my $size = ref($perl_data) eq 'HASH'  ? scalar keys %$perl_data
             : ref($perl_data) eq 'ARRAY' ? scalar @$perl_data
             :                              $perl_data ;
    if (not $size) {
        $logger->info( "Removing $file_path (no data to store)" );
        unlink($file_path);
    }
}

sub write_back {
    my $self = shift;
    my %args = @_;

    my $force_backend = delete $args{backend} || '';

    croak "write_back: no subs registered in node", $self->node->location, ". cannot save data\n"
        unless @{ $self->{write_back} };

    my @backends = @{ $self->{write_back} };
    $logger->debug(
        "write_back called on node '",
        $self->node->name, "' for ", scalar @backends,
        " backends"
    );

    my $dir = $args{config_dir};
    mkpath( $dir, 0, 0755 ) if $dir and not -d $dir;

    foreach my $wb_info (@backends) {
        my ( $backend, $wb ) = @$wb_info;
        if (   not $force_backend
            or $force_backend eq $backend
            or $force_backend eq 'all' ) {

            # exit when write is successfull
            my $res = $wb->(%args);
            $logger->info( "write_back called with $backend backend, result is ",
                defined $res ? $res : '<undef>' );
            last if ( $res and not $force_backend );
        }
    }
    $logger->debug( "write_back on node '", $self->node->name, "' done" );
}

sub open_file_to_write {
    my ( $self, $backend, %args ) = @_;

    my $backup    = delete $args{backup};
    my $do_backup = defined $backup;
    $backup ||= 'old';    # use old only if defined
    $backup = '.' . $backup unless $backup =~ /^\./;

    my ( $file_ok, $file_path ) = $self->get_cfg_file_path(%args);

    if ( $file_ok and $file_path eq '-' ) {
        my $io = IO::Handle->new();
        if ( $io->fdopen( fileno(STDOUT), "w" ) ) {
            return ( 1, '-', $io );
        }
        else {
            return ( 0, '-' );
        }
    }
    elsif ($file_ok) {
        if ( $do_backup and -r $file_path ) {
            copy( $file_path, $file_path . $backup ) or die "Backup copy failed: $!";
        }
        $logger->debug("$backend backend opened file $file_path to write");
        my $fh = new IO::File;
        $fh->open("> $file_path") || die "Cannot open $file_path:$!";
        $fh->binmode(':utf8');
        return ( $file_ok, $file_path, $fh );
    }
    else {
        return ( 0, $file_path );
    }
}

sub close_file_to_write {
    my ( $self, $error, $fh, $file_path ) = @_;

    return unless defined $file_path;

    if ($error) {

        # restore backup and display error
        my $data = $self->file_backup;
        $logger->debug(
            "Error during write, restoring backup in $file_path with " . scalar @$data . " lines" );
        $fh->seek( 0, 0 );    # go back to beginning of file
        $fh->print(@$data);
        $fh->close;
        $error->rethrow if ref($error) and $error->can('rethrow');
        die $error;
    }

    $fh->close;

    # check file size and remove empty files
    unlink($file_path) if -z $file_path and not -l $file_path;
}

sub is_auto_write_for_type {
    my $self = shift;
    my $type = shift;
    return $self->{auto_write}{$type} || 0;
}

sub read_cds_file {
    my $self = shift;
    my %args = @_;

    my $file_path = $args{file_path};
    $logger->info("Read cds data from $file_path");

    $self->node->load( step => [ $args{io_handle}->getlines ] );
    return 1;
}

sub write_cds_file {
    my $self      = shift;
    my %args      = @_;
    my $file_path = $args{file_path};
    $logger->info("Write cds data to $file_path");

    my $dump = $self->node->dump_tree( skip_auto_write => 'cds_file', check => $args{check} );
    $args{io_handle}->print($dump);
    return 1;
}

sub read_perl {
    my $self = shift;
    my %args = @_;

    my $file_path = $args{file_path};
    $file_path = "./$file_path" unless $file_path =~ m!^\.?/!;
    $logger->info("Read Perl data from $file_path");

    my $pdata = do $file_path || die "Cannot open $file_path:$!";
    $self->node->load_data($pdata);
    return 1;
}

sub write_perl {
    my $self      = shift;
    my %args      = @_;
    my $file_path = $args{file_path};
    $logger->info("Write perl data to $file_path");

    my $p_data = $self->node->dump_as_data( skip_auto_write => 'perl_file', check => $args{check} );
    my $dumper = Data::Dumper->new( [$p_data] );
    $dumper->Terse(1);

    $args{io_handle}->print( $dumper->Dump, ";\n" );
    return 1;
}

__PACKAGE__->meta->make_immutable;

1;

# ABSTRACT: Load configuration node on demand

__END__

=pod

=encoding UTF-8

=head1 NAME

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

=head1 VERSION

version 2.097

=head1 SYNOPSIS

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

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

 $model->create_config_class(
    name => "MyClass",

    # read_config spec is used by Config::Model::BackendMgr
    read_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 );

 $inst->write_back;

 # now look at file /tmp/my_class.yml

=head1 DESCRIPTION

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 L<write_back|/"write_back ( ... )">
method is called (either on the node or on this backend manager).

=begin comment

This feature is also useful if you want to read configuration class
declarations at run time. (For instance in a C</etc> directory like
C</etc/some_config.d>). In this case, each configuration class must
specify how to read and write configuration information.

Idea: sub-files name could be <instance>%<location>.cds

=end comment

This load/store can be done with different backends:

=over

=item *

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

=item *

C<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
L<Config::Model::Loader/"load string syntax">.

=item *

C<perl_file>: Perl data structure (perl) in a file. See L<Config::Model::DumpAsData>
for details on the data structure.

=item * 

C<custom>: specifies a dedicated class and function to read and load the
configuration tree. This is provided for backward compatibility and
should not be used for new projects.

=back

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

=head1 Backend specification

The backend specification is provided as an attribute of a
L<Config::Model::Node> specification. These attributes are optional:
A node without C<read_config> attribute must rely on another node for
its data to be read and saved.

When needed (usually for the root node), the configuration class is
declared with a C<read_config> parameter. This parameter is a list
of possible backend. Usually, only one read backend is needed.

=head2 Parameters available for all backends

The following parameters are accepted by all backends:

=over 4

=item config_dir

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

=item file

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

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

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

=item os_config_dir

Specify alternate location of a configuration directory depending on the OS
(as returned by C<$^O>, see L<perlport/PLATFORMS>).
For instance:

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

=item 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 (C</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 C<default_layer>.

=item auto_create

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

    read_config  => [ {
        backend => 'IniFile',
        config_dir => '/tmp',
        file  => 'foo.conf',
        auto_create => 1
    } ],

Setting C<auto_create> to 1 is necessary to create a configuration
from scratch

When C<auto_create> is set in write backend, missing directory and
files are created with current umask. Default is false.

=item auto_delete

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

=back

=head2 Config::Model::Backend::* backends

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

For instance:

   read_config => [{
       backend     => 'yaml',
       config_dir  => '/tmp/',
       file        => 'my_class.yml',
   }],

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

=head2 Your own backend

You can also write a dedicated backend. See
L<How to write your own backend|Config::Model::Backend::Any/"How to write your own backend">
for details.

=head2 Built-in backend

C<cds_file> and C<perl_file> backend must be specified with
mandatory C<config_dir> parameter. For instance:

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

When C<file> is not specified, a file name is constructed with
C<< <config_class_name>.<suffix> >> where suffix is C<pl> or C<cds>.

=head2 Custom backend

Custom backend is provided to be backward compatible but should not be used
for new project.
L<Writing your own backend|Config::Model::Backend::Any/"How to write your own backend">
is preferred.

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

  read_config  => [ {
      backend    => 'custom' ,
      class      => 'MyRead',
      function   => 'read_it",  # optional, defaults to 'read'
      config_dir => '/etc/foo', # optional
      file       => 'foo.conf', # optional
  } ]

C<custom> backend parameters are:

=over

=item class

Specify the class that contains the read methods

=item function

Function name that is called back to read the file.
See L</"read callback"> for details. (default is C<read>)

=item 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.

=back

Most of the times, there's no need to create a write specification:
the read specification is enough for this module to write back the
configuration file. 

The write method must be specified if the writer class is not the same as the
reader class or if the writer method is not C<write>:

  write_config  => [ {
      backend    => 'custom' ,
      class      => 'MyWrite',
      function   => 'write_it", # optional, defaults to 'read'
      config_dir => '/etc/foo', # optional
      file       => 'foo.conf', # optional
  } ]

Read callback function is 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 L<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 function is 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 L<IO::File> object is undef if the file cannot be written to.

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

=head1 Using backend to change configuration file syntax

C<read_config> tries all the specified backends. This feature 
can be used to migrate from one syntax to another.

In this example, backend manager first tries to read an INI file
and then to read a YAML file:

  read_config  => [ 
    { backend => 'IniFile', ... },
    { backend => 'yaml',    ... },
  ],

When a read operation is successful, the remaining read methods are
skipped.

Likewise, the C<write_config> specification accepts several backends.
By default, the specifications are tried in order, until the first succeeds.

In the example above, the migration from INI to YAML can be achieved
by specifying only the YAML backend:

  write_config => [
    { backend => 'yaml',    ... },
  ],

=head1 Test setup

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

=head1 CAVEATS

When both C<config_dir> and C<file> are specified, this class
write-opens the configuration file (and thus clobber it) before calling
the C<write> call-back and pass the file handle with C<io_handle>
parameter. C<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 C<file> to undef or an empty string in the
C<write_config> specification.

=head1 Methods

=head2 write_back ( ... )

Try to run all subroutines registered by L<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 C<root> and
C<config_dir> parameters. This overrides the model specifications.

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

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

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

C<write_back> croaks if no write call-back are known for this node.

=head2 support_annotation

Returns 1 if at least one of the backends support to read and write annotations
(aka comments) in the configuration file.

=head1 AUTHOR

Dominique Dumont, (ddumont at cpan dot org)

=head1 SEE ALSO

L<Config::Model>, L<Config::Model::Instance>,
L<Config::Model::Node>, L<Config::Model::Dumper>

=head1 AUTHOR

Dominique Dumont

=head1 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

=cut