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David Harris


DB_File::Lock - Locking with flock wrapper for DB_File


 use DB_File::Lock;

 $locking = "read";
 $locking = "write";
 $locking = {
     mode            => "read",
     nonblocking     => 0,
     lockfile_name   => "/path/to/shared.lock",
     lockfile_mode   => 0600,

 [$X =] tie %hash,  'DB_File::Lock', [$filename, $flags, $mode, $DB_HASH], $locking;
 [$X =] tie %hash,  'DB_File::Lock', $filename, $flags, $mode, $DB_BTREE, $locking;
 [$X =] tie @array, 'DB_File::Lock', $filename, $flags, $mode, $DB_RECNO, $locking;

 ...use the same way as DB_File for the rest of the interface...


This module provides a wrapper for the DB_File module, adding locking.

When you need locking, simply use this module in place of DB_File and add an extra argument onto the tie command specifying if the file should be locked for reading or writing.

The alternative is to write code like:

  open(LOCK, "<$db_filename.lock") or die;
  flock(LOCK, LOCK_SH) or die;
  tie(%db_hash, 'DB_File', $db_filename,  O_RDONLY, 0600, $DB_HASH) or die;
  ... then read the database ...

This module lets you write

  tie(%db_hash, 'DB_File', $db_filename,  O_RDONLY, 0600, $DB_HASH, 'read') or die;
  ... then read the database ...

This is better for two reasons:

(1) Less cumbersome to write.

(2) A fatal exception in the code working on the database which does not lead to process termination will probably not close the lockfile and therefore cause a dropped lock.


The filename used for the lockfile defaults to "$filename.lock" (the filename of the DB_File with ".lock" appended). Using a lockfile separate from the database file is recommended because it prevents weird interactions with the underlying database file library

The additional locking argument added to the tie call, can be:

(1) "read" -- aquires a shared lock for reading

(2) "write" -- aquires an exclusive lock for writing

(3) A hash with the following keys (all optional except for the "mode"):


the locking mode, "read" or "write".


specifies the name of the lockfile to use. Default is "$filename.lock". This is useful for locking multiple resources with the same lockfiles.


determines if the flock call on the lockfile should block waiting for a lock, or if it should return failure if a lock can not be immediately attained. If "nonblocking" is set and a lock can not be attained, the tie command will fail. Currently, I'm not sure how to differentiate this between a failure form the DB_File layer.


determines the mode for the sysopen call in opening the lockfile. The default mode will be formulated to allow anyone that can read or write the DB_File permission to read and write the lockfile. (This is because some systems may require that one have write access to a file to lock it for reading, I understand.) The umask will be prevented from applying to this mode.

Note: One may import the same values from DB_File::Lock as one may import from DB_File.


There are three locking wrappers for DB_File in CPAN right now. Each one implements locking differently and has different goals in mind. It is therefore worth knowing the difference, so that you can pick the right one for your application.

Here are the three locking wrappers:

Tie::DB_Lock -- DB_File wrapper which creates copies of the database file for read access, so that you have kind of a multiversioning concurrent read system. However, updates are still serial. Use for databases where reads may be lengthy and consistency problems may occur.

Tie::DB_LockFile -- DB_File wrapper that has the ability to lock and unlock the database while it is being used. Avoids the tie-before-flock problem by simply re-tie-ing the database when you get or drop a lock. Because of the flexibility in dropping and re-acquiring the lock in the middle of a session, this can be massaged into a system that will work with long updates and/or reads if the application follows the hints in the POD documentation.

DB_File::Lock (this module) -- extremely lightweight DB_File wrapper that simply flocks a lockfile before tie-ing the database and drops the lock after the untie. Allows one to use the same lockfile for multiple databases to avoid deadlock problems, if desired. Use for databases where updates are reads are quick and simple flock locking semantics are enough.

(This text duplicated in the POD documentation, by the way.)


David Harris <dharris@drh.net>

Helpful insight from Stas Bekman <sbekman@iil.intel.com>