Persistent Data

Problem

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

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;
}

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 14.6, assigning to $array_ref before we push. You simply can't 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 14.6: 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). Previous 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);

See Also

The documentation for the Data::Dumper, MLDBM, and Storable modules from CPAN; ;