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MLDBM(3)                             User Contributed Perl Documentation                            MLDBM(3)



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, "\n";              # 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 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 2.04   08 Mar 2010

SEE ALSO
       perl(1), perltie(1), perlfunc(1), Data::Dumper(3), FreezeThaw(3), Storable(3), DBM::Deep.



perl v5.12.5                                     2013-08-25                                         MLDBM(3)