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MLDBM(3pm) User Contributed Perl Documentation MLDBM(3pm)
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 AT umich.edu>.
Support for multiple serializing packages by Raphael Manfredi
<Raphael_Manfredi AT grenoble.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 AT 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.
perl v5.32.0 2021-01-01 MLDBM(3pm)
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