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) 2010-2013 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.