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


    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 $!;


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;

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.


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.


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


  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.


  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 FETCHes 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};


Gurusamy Sarathy <>.

Support for multiple serializing packages by Raphael Manfredi <>.

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.

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


Version 2.01 07 July 2002


perl(1), perltie(1), perlfunc(1), Data::Dumper(3), FreezeThaw(3), Storable(3).