NAME

mod_perl and dbm files

Description

Small databases can be implemented pretty efficiently using dbm files, but there are still some precautions that must be taken to use properly under mod_perl.

DB_File::Lock2

Here is code/DB_File-Lock2.pm which does the locking by using an external lockfile.

This allows you to gain the lock before the file is tied. Note that it's not yet on CPAN and so is linked from here in its entirety. Note also that this code still needs some testing, so be careful if you use it on a production machine.

You use it like this:

  use DB_File::Lock2 ();

A simple tie, READ lock and untie

  use DB_File::Lock2 ();
  my $dbfile = "/tmp/test";
  tie my %mydb, 'DB_File::Lock2', $dbfile, 'read';
  print $mydb{foo} if exists $mydb{foo};
  untie %mydb;

You can even skip the untie() call. When $mydb goes out of scope everything will be done automatically. However it is better to use the explicit call, to make sure the critical sections between lock and unlock are as short as possible. This is especially important when requesting an exclusive (write) lock.

The following example shows how it might be convenient to skip the explicit untie(). In this example, we don't need to save the intermediate result, we just return and the cleanup is done automatically.

  use DB_File::Lock2 ();
  my $dbfile = "/tmp/test";
  print user_exists("stas") ? "Yes" : "No";
  sub user_exists{
    my $username = shift || '';
  
    warn("No username passed\n"), return 0 unless $username;
  
    tie my %mydb, 'DB_File::Lock2', $dbfile, 'read';
  
    # if we match the username return 1, else 0
    return $mydb{$username} ? 1 : 0;
  
  } # end of sub user_exists

Now let's write all the upper case characters and their respective ASCII values to a dbm file. Then read the file and print the contents of the DB, unsorted.

  use DB_File::Lock2 ();
  my $dbfile = "/tmp/test";
  
    # write 
  tie my %mydb, 'DB_File::Lock2', $dbfile,'write';
  for (0..26) {
    $mydb{chr 65+$_} = $_;
  }
  untie %mydb;
  
    # now, read them and printout (unsorted) 
    # notice that 'read' is a default lock mode
  tie %mydb, 'DB_File::Lock2', $dbfile;
  while (my($k,$v) = each %mydb) {
    print "$k => $v\n";
  }
  untie %mydb;

If your CGI script is interrupted, the DESTROY block will take care of unlocking the dbm file and flush any changes. So your DB will be safe against possible corruption because of unclean program termination.

Maintainers

Maintainer is the person(s) you should contact with updates, corrections and patches.

• Stas Bekman <stas (at) stason.org>

Authors

• Stas Bekman <stas (at) stason.org>

Only the major authors are listed above. For contributors see the Changes file.