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NAME

       MLDBM - store multi-level Perl hash structure in single level tied hash

SYNOPSIS

           use MLDBM;                          # this gets the default, SDBM
           #use MLDBM qw(DB_File FreezeThaw);  # use FreezeThaw for serializing
           #use MLDBM qw(DB_File Storable);    # use Storable for serializing

           $dbm = tie %o, 'MLDBM' [..other DBM args..] or die $!;

DESCRIPTION

       This module can serve as a transparent interface to any TIEHASH package
       that is required to store arbitrary perl data, including nested
       references.  Thus, this module can be used for storing references and
       other arbitrary data within DBM databases.

       It works by serializing the references in the hash into a single
       string. In the underlying TIEHASH package (usually a DBM database), it
       is this string that gets stored.  When the value is fetched again, the
       string is deserialized to reconstruct the data structure into memory.

       For historical and practical reasons, it requires the Data::Dumper
       package, available at any CPAN site. Data::Dumper gives you really
       nice-looking dumps of your data structures, in case you wish to look at
       them on the screen, and it was the only serializing engine before
       version 2.00.  However, as of version 2.00, you can use any of
       Data::Dumper, FreezeThaw or Storable to perform the underlying
       serialization, as hinted at by the SYNOPSIS overview above.  Using
       Storable is usually much faster than the other methods.

       See the BUGS section for important limitations.

   Changing the Defaults
       MLDBM relies on an underlying TIEHASH implementation (usually a DBM
       package), and an underlying serialization package.  The respective
       defaults are SDBM_File and Data::Dumper.  Both of these defaults can be
       changed.  Changing the SDBM_File default is strongly recommended.  See
       WARNINGS below.

       Three serialization wrappers are currently supported: Data::Dumper,
       Storable, and FreezeThaw.  Additional serializers can be supported by
       writing a wrapper that implements the interface required by
       MLDBM::Serializer.  See the supported wrappers and the
       MLDBM::Serializer source for details.

       In the following, $OBJ stands for the tied object, as in:

               $obj = tie %o, ....
               $obj = tied %o;

       $MLDBM::UseDB  or   $OBJ->UseDB([TIEDOBJECT])
           The global $MLDBM::UseDB can be set to default to something other
           than "SDBM_File", in case you have a more efficient DBM, or if you
           want to use this with some other TIEHASH implementation.
           Alternatively, you can specify the name of the package at "use"
           time, as the first "parameter".  Nested module names can be
           specified as "Foo::Bar".

           The corresponding method call returns the underlying TIEHASH object
           when called without arguments.  It can be called with any object
           that implements Perl's TIEHASH interface, to set that value.

       $MLDBM::Serializer  or   $OBJ->Serializer([SZROBJECT])
           The global $MLDBM::Serializer can be set to the name of the
           serializing package to be used. Currently can be set to one of
           "Data::Dumper", "Storable", or "FreezeThaw". Defaults to
           "Data::Dumper".  Alternatively, you can specify the name of the
           serializer package at "use" time, as the second "parameter".

           The corresponding method call returns the underlying MLDBM
           serializer object when called without arguments.  It can be called
           with an object that implements the MLDBM serializer interface, to
           set that value.

   Controlling Serializer Properties
       These methods are meant to supply an interface to the properties of the
       underlying serializer used.  Do not call or set them without
       understanding the consequences in full.  The defaults are usually
       sensible.

       Not all of these necessarily apply to all the supplied serializers, so
       we specify when to apply them.  Failure to respect this will usually
       lead to an exception.

       $MLDBM::DumpMeth    or  $OBJ->DumpMeth([METHNAME])
           If the serializer provides alternative serialization methods, this
           can be used to set them.

           With Data::Dumper (which offers a pure Perl and an XS verion of its
           serializing routine), this is set to "Dumpxs" by default if that is
           supported in your installation.  Otherwise, defaults to the slower
           "Dump" method.

           With Storable, a value of "portable" requests that serialization be
           architecture neutral, i.e. the deserialization can later occur on
           another platform. Of course, this only makes sense if your database
           files are themselves architecture neutral.  By default, native
           format is used for greater serializing speed in Storable.  Both
           Data::Dumper and FreezeThaw are always architecture neutral.

           FreezeThaw does not honor this attribute.

       $MLDBM::Key  or  $OBJ->Key([KEYSTRING])
           If the serializer only deals with part of the data (perhaps because
           the TIEHASH object can natively store some types of data), it may
           need a unique key string to recognize the data it handles.  This
           can be used to set that string.  Best left alone.

           Defaults to the magic string used to recognize MLDBM data. It is a
           six character wide, unique string. This is best left alone, unless
           you know what you are doing.

           Storable and FreezeThaw do not honor this attribute.

       $MLDBM::RemoveTaint  or  $OBJ->RemoveTaint([BOOL])
           If the serializer can optionally untaint any retrieved data subject
           to taint checks in Perl, this can be used to request that feature.
           Data that comes from external sources (like disk-files) must always
           be viewed with caution, so use this only when you are sure that
           that is not an issue.

           Data::Dumper uses "eval()" to deserialize and is therefore subject
           to taint checks.  Can be set to a true value to make the
           Data::Dumper serializer untaint the data retrieved. It is not
           enabled by default.  Use with care.

           Storable and FreezeThaw do not honor this attribute.

EXAMPLES

       Here is a simple example.  Note that does not depend upon the
       underlying serializing package--most real life examples should not,
       usually.

           use MLDBM;                          # this gets SDBM and Data::Dumper
           #use MLDBM qw(SDBM_File Storable);  # SDBM and Storable
           use Fcntl;                          # to get 'em constants

           $dbm = tie %o, 'MLDBM', 'testmldbm', O_CREAT|O_RDWR, 0640 or die $!;

           $c = [\ 'c'];
           $b = {};
           $a = [1, $b, $c];
           $b->{a} = $a;
           $b->{b} = $a->[1];
           $b->{c} = $a->[2];
           @o{qw(a b c)} = ($a, $b, $c);

           #
           # to see what was stored
           #
           use Data::Dumper;
           print Data::Dumper->Dump([@o{qw(a b c)}], [qw(a b c)]);

           #
           # to modify data in a substructure
           #
           $tmp = $o{a};
           $tmp->[0] = 'foo';
           $o{a} = $tmp;

           #
           # can access the underlying DBM methods transparently
           #
           #print $dbm->fd, "
";              # DB_File method

       Here is another small example using Storable, in a portable format:

           use MLDBM qw(DB_File Storable);     # DB_File and Storable

           tie %o, 'MLDBM', 'testmldbm', O_CREAT|O_RDWR, 0640 or die $!;

           (tied %o)->DumpMeth('portable');    # Ask for portable binary
           $o{'ENV'} = \%ENV;                  # Stores the whole environment

BUGS

       1.  Adding or altering substructures to a hash value is not entirely
           transparent in current perl.  If you want to store a reference or
           modify an existing reference value in the DBM, it must first be
           retrieved and stored in a temporary variable for further
           modifications.  In particular, something like this will NOT work
           properly:

                   $mldb{key}{subkey}[3] = 'stuff';        # won't work

           Instead, that must be written as:

                   $tmp = $mldb{key};                      # retrieve value
                   $tmp->{subkey}[3] = 'stuff';
                   $mldb{key} = $tmp;                      # store value

           This limitation exists because the perl TIEHASH interface currently
           has no support for multidimensional ties.

       2.  The Data::Dumper serializer uses eval().  A lot.  Try the Storable
           serializer, which is generally the most efficient.

WARNINGS

       1.  Many DBM implementations have arbitrary limits on the size of
           records that can be stored.  For example, SDBM and many ODBM or
           NDBM implementations have a default limit of 1024 bytes for the
           size of a record.  MLDBM can easily exceed these limits when
           storing large data structures, leading to mysterious failures.
           Although SDBM_File is used by MLDBM by default, it is not a good
           choice if you're storing large data structures.  Berkeley DB and
           GDBM both do not have these limits, so I recommend using either of
           those instead.

       2.  MLDBM does well with data structures that are not too deep and not
           too wide.  You also need to be careful about how many "FETCH"es
           your code actually ends up doing.  Meaning, you should get the most
           mileage out of a "FETCH" by holding on to the highest level value
           for as long as you need it.  Remember that every toplevel access of
           the tied hash, for example $mldb{foo}, translates to a MLDBM
           "FETCH()" call.

           Too often, people end up writing something like this:

                   tie %h, 'MLDBM', ...;
                   for my $k (keys %{$h{something}}) {
                       print $h{something}{$k}[0]{foo}{bar};  # FETCH _every_ time!
                   }

           when it should be written this for efficiency:

                   tie %h, 'MLDBM', ...;
                   my $root = $h{something};                  # FETCH _once_
                   for my $k (keys %$root) {
                       print $k->[0]{foo}{bar};
                   }

AUTHORS

       Gurusamy Sarathy <gsar@umich.edu>.

       Support for multiple serializing packages by Raphael Manfredi
       <Raphael_Manfredi@grenoble.hp.com>.

       Test suite fixes for perl 5.8.0 done by Josh Chamas.

       Copyright (c) 1995-98 Gurusamy Sarathy.  All rights reserved.

       Copyright (c) 1998 Raphael Manfredi.

       Copyright (c) 2002 Josh Chamas, Chamas Enterprises Inc.

       Copyright (c) 2010-2013 Alexandr Ciornii (alexchorny@gmail.com).

       This program is free software; you can redistribute it and/or modify it
       under the same terms as Perl itself.

VERSION

       Version 2.05

SEE ALSO

       perl(1), perltie(1), perlfunc(1), Data::Dumper, FreezeThaw, Storable,
       DBM::Deep, MLDBM::Serializer::JSON.



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