NAME

       DB_File::Lock - Locking with flock wrapper for DB_File

SYNOPSIS

        use DB_File::Lock;
        use Fcntl qw(:flock O_RDWR O_CREAT);

        $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;

        # or place the DB_File arguments inside a list reference:
        [$X =] tie %hash,  'DB_File::Lock', [$filename, $flags, $mode, $DB_HASH], $locking;

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

DESCRIPTION

       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 ...
         untie(%db_hash);
         close(LOCK);

       This module lets you write

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

       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.

USAGE DETAILS

       Tie to the database file by adding an additional locking argument to the list of arguments
       to be passed through to DB_File, such as:

         tie(%db_hash, 'DB_File::Lock', $db_filename,  O_RDONLY, 0600, $DB_HASH, 'read');

       or enclose the arguments for DB_File in a list reference:

         tie(%db_hash, 'DB_File::Lock', [$db_filename,  O_RDONLY, 0600, $DB_HASH], 'read');

       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" -- acquires a shared lock for reading

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

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

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

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

       nonblocking
           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.

       lockfile_mode
           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.

GOOD LOCKING ETIQUETTE

       To avoid locking problems, realize that it is critical that you release the lock as soon
       as possible. See the lock as a "hot potato", something that you must work with and get rid
       of as quickly as possible. See the sections of code where you have a lock as "critical"
       sections. Make sure that you call "untie" as soon as possible.

       It is often better to write:

         # open database file with lock
         # work with database
         # lots of processing not related to database
         # work with database
         # close database and release lock

       as:

         # open database file with lock
         # work with database
         # close database and release lock

         # lots of processing not related to database

         # open database file with lock
         # work with database
         # close database and release lock

       Also realize that when acquiring two locks at the same time, a deadlock situation can be
       caused.

       You can enter a deadlock situation if two processes simultaneously try to acquire locks on
       two separate databases. Each has locked only one of the databases, and cannot continue
       without locking the second. Yet this will never be freed because it is locked by the other
       process. If your processes all ask for their DB files in the same order, this situation
       cannot occur.

OTHER LOCKING MODULES

       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.)

AUTHOR

       David Harris <dharris@drh.net>

       Helpful insight from Stas Bekman <stas@stason.org>

SEE ALSO

       DB_File(3).