Hi, I have just started learning OOPerl a couple months ago and I'm still a little bit confused as to how some stuff works. Let's say I set up a calendar class that is tied to a file on disk using MLDBM::Sync. I need to use MLDBM::Sync, because I need concurrent access to this file by multiple users (through a web interface). Below is how the beginning of my calendar class looks like including the constructor: 

package Calendar;

# class variables
my $sync_dbm_obj;

# constructor
sub new {
    my $class = shift;
    my %args = @_;
    
    my $calendar = {};
    
    $sync_dbm_obj = tie(%{$calendar}, 'MLDBM::Sync', "$args{DIR}\\$arg
+s{year}.db", O_CREAT|O_RDWR, 0640) 
                        or die "can't open tie to $args{year}.db: $!";
    
    # if just created, initialize hash with default values
    if( !$calendar->{months} ) {

        $sync_dbm_obj->Lock;

        $calendar->{year} = $args{year};
        $calendar->{months} = {};
        my $aref = $calendar->{months};
        foreach my $month (1..12) {
            $aref->{$month} = _init_days($calendar->{year}, $month);
        }
        $calendar->{months} = $aref;

        $sync_dbm_obj->UnLock;
    }
    
    return bless $calendar, $class;
}

[download]

Now, if I need to create two calendar objects I would do something like: 

my $calendar1 = Calendar->new(year => "2002", DIR => "c:\a_dir");
my $calendar2 = Calendar->new(year => "2003", DIR => "c:\a_dir");

[download]

Once I create the second object, the $sync_dbm_obj class variable points to the second file (2003.db in this example), so if I call any methods for $calendar1 object, file 2002.db is not going to be locked. And if I were to destroy $calendar1 by either letting it go out of scope or calling undef $calendar1, then $sync_dbm_obj becomes undefined and methods for $calendar2 object stop functioning, because they're trying to lock 2003.db using $sync_dbm_obj, but it is now undefined.

Can you think of how to fix this or suggest a nice workaround? One workaround would, of course, be to use just one calendar object at a time, but there's a part in my program where I need to have two calendar objects active at the same time. And I do need fine-grained persistant and syncronized access to files offered by MLDBM and MLDBM::Sync to prevent concurrency issues. Any input is welcome!

In reply to Need a workaround: Class variables with MLDBM::Sync by relax99

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