Locking Strategies
The issues of safe access to databases that plagued flat-file databases still apply to Berkeley databases. Therefore, it is a good idea to implement a locking strategy that allows safe multi-user access to the databases, if this is required by your applications.
The way in which
flock()
is used regarding DBM files is slightly different than that of
locking standard Perl filehandles, as there is no direct reference to
the underlying filehandle when we create a DBM file within a Perl
script.
Fortunately, the DB_File
module defines a method
that can be used to locate the underlying file descriptor
for a DBM file, allowing us to use
flock()
on it. This can be achieved by invoking
the fd()
method on the object reference returned from the database
initialization by tie()
. For example:
### Create the new database ... $db = tie %database, 'DB_File', "megaliths.dat" or die "Can't initialize database: $!\n"; ### Acquire the file descriptor for the DBM file my $fd = $db->fd(); ### Do a careful open() of that descriptor to get a Perl filehandle open DATAFILE, "+<&=$fd" or die "Can't safely open file: $!\n"; ### And lock it before we start loading data ... print "Acquiring an exclusive lock..."; flock( DATAFILE, LOCK_EX ) or die "Unable to acquire exclusive lock: $!. Aborting"; print "Acquired lock. Ready to update database!\n\n";
This code looks a bit gruesome, especially with the additional call
to open()
. It is written in such a way that the original file descriptor being ...
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