One might like to do flock manually or use a CPAN module. Meta::cpan is equipped with many flock modules. The Mutex module was made to work realiably with parallelization, supports threads and processes, including nested sessions.

The path option to Mutex constructs a Mutex::Flock object. One may specify the script path or a temporary path. Typically, the system flock requires read-write access to work. Thus, the Mutex module does a check and emits an error if the path isn't writeable by the calling process-userid.

use Mutex;

my $mutex = Mutex->new( path => $0 );  # choose $0 or tmp path
my $mutex = Mutex->new( path => "/tmp/db.lock" );
 
# The script terminates after some time if a previous
# instance is still running with a held lock.

exit unless $mutex->timedwait( 5 );    # seconds

# Has exclusive lock at this point. Due to using flock,
# the lock is released no matter how the process dies,
# even if omitting $mutex->unlock.

# Do DB insert here.

$mutex->unlock;

# Do other things, optionally.

...

[download]

The example lacks logic for storing the item to insert into a state file, in the event unable to obtain an exclusive lock. Running again with exclusive lock inserts pending items.

my $state_mutex = Mutex->new( path => "/tmp/state.lock" );

$state_mutex->enter( sub {
    # Unable to obtain exclusive lock, then append pending inserts
    # to a state file. Otherwise, read pending items, insert into DB,
    # then unlink or empty the state file.
} );

[download]

Regards, Mario

In reply to Re^2: perl script and queue by marioroy
in thread perl script and queue by k_manimuthu

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