Recipe 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