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14.7. Persistent Data

14.7.1. Problem

You want your variables to retain their values between calls to your program.

14.7.2. Solution

Use a MLDBM to store the values between calls to your program:

use MLDBM "DB_File";
my ($VARIABLE1,$VARIABLE2);
my $Persistent_Store = "/projects/foo/data";
BEGIN {
my %data;
tie(%data, "MLDBM", $Persistent_Store)
or die "Can't tie to $Persistent_Store : $!";
$VARIABLE1 = $data{VARIABLE1};
$VARIABLE2 = $data{VARIABLE2};
# ...
untie %data;
}
END {
my %data;
tie (%data, "MLDBM", $Persistent_Store)
or die "Can't tie to $Persistent_Store : $!";
$data{VARIABLE1} = $VARIABLE1;
$data{VARIABLE2} = $VARIABLE2;
# ...
untie %data;
}

14.7.3. Discussion

An important limitation of MLDBM is that you can't add to or alter the structure in the reference without assignment to a temporary variable. We do this in the sample program in Example 14-4, assigning to $array_ref before we push. You can't simply do this:

push(@{$db{$user}}, $duration);

For a start, MLDBM doesn't allow it. Also, $db{$user} might not be in the database (the array reference isn't automatically created as it would be if %db weren't tied to a DBM file). This is why we test exists $db{$user} when we give $array_ref its initial value. We're creating the empty array for the case where it doesn't already exist.

Example 14-4. mldbm-demo

  #!/usr/bin/perl -w
# mldbm_demo - show how to use MLDBM with DB_File
use MLDBM "DB_File";
$db = "/tmp/mldbm-array";
tie %db, "MLDBM", $db
or die "Can't open $db : $!";
while(<DATA>) {
chomp;
($user, $duration) = split(/\s+/, $_);
$array_ref = exists $db{$user} ? $db{$user} : [  ];
push(@$array_ref, $duration);
$db{$user} = $array_ref;
}
foreach $user (sort keys %db) {
print "$user: ";
$total = 0;
foreach $duration (@{ $db{$user} }) {
print "$duration ";
$total += $duration;
}
print "($total)\n";
}
_ _END_ _
gnat        15.3
tchrist     2.5
jules       22.1
tchrist     15.9
gnat        8.7

Newer versions of MLDBM allow you to select not just the database module (we recommend DB_File), but also the serialization module (we recommend Storable). Early versions limited you to Data::Dumper for serializing, which is slower than Storable. Here's how you use DB_File with Storable:

use MLDBM qw(DB_File Storable);

14.7.4. See Also

The documentation for the standard Data::Dumper and Storable modules; the documentation for the FreezeThaw and MLDBM modules from CPAN; Recipe 11.13; Recipe 14.6