Sync(3)
NAME
MLDBM - store multi-level 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 refer-
ences. 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 ver-
sion 2.00. However, as of version 2.00, you can use any of
Data::Dumper, FreezeThaw or Storable to perform the underlying serial-
ization, 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::Serial-
izer 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. Alterna-
tively, 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 serial-
izing 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 serial-
izer 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 under-
standing 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 for-
mat 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 underly-
ing 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 modifica-
tions. 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 stor-
ing 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.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
VERSION
Version 2.01 07 July 2002
SEE ALSO
perl(1), perltie(1), perlfunc(1), Data::Dumper(3), FreezeThaw(3),
Storable(3).
perl v5.8.8 2002-07-07 MLDBM(3)
See also MLDBM::Sync(3)
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