Perl : File Locking using flock

As a application server admin, it do have to write scripts for various automation operations in weblogic. One such task to do deploy application in weblogic. As we know that Weblogic domain does not allow multiple operations to be performed at same time. We have to take a lock first and then perform action, if a lock is already been taken by a different operation a message is sent back saying “lock already taken“.

In my case ,we have our own scripts which will do a deployments in weblogic for us . we have used WLST for doing the actual deployment. Now my task is make sure that only 1 deployment is being performed on a domain and every second one should wait until the first one is completed. This is much like a locking stuff.

Perl provide us with many modules that help many of the administrative tasks easy. One such module is perl::FLOCK. This module allows one to take a lock on a file and hold it. Lets write a sample script and see how it works

Here is a Sample Script that i wrote for my testing purpose.

#!/usr/bin/perl

#  --------

#  PROGRAM:  lock.pl  (Sample Script for Flock Test)

#  --------

$LOCK_EXCLUSIVE                   = 2;

$UNLOCK                      = 8;

$LOCK_SHARED                        = 1;

$LOCK_NONBLOCKING = 4;

# open the file, lock the file, sleep, then write, then unlock the file, then close the file.

open (FILE, ">> test.dat") || die "problem opening test.dat\n"; 

flock FILE, $LOCK_EXCLUSIVE;

sleep 10;

print FILE "this line printed by lock.pl\n";

flock FILE, $UNLOCK;

close(FILE);

In the above case , we are taking a lock on the test.dat file and sleeping for a 10 second , then writing content to it and then un locking.

There are 4 locks available

shared lock : Shared lock can be shared with other process too. If a process takes a lock for reading a file which has a Shared lock , another process can take the lock on the same file for reading .This lock is normally applied when you just want to read the file

exclusive lock : This is the lock iam planning to use in the code, this lock is used when you want to make changes to the file. Only one exclusive lock can be on a file, so that only one process at a time can make changes. 

non-blocking : non-blocking lock request so that the process does not have to wait if an incompatible lock is held by another process; instead the process can take some other action.

unlock: unlock the lock. normally call to this is not use full. coz once a process releases the lock , the unlock is called automatically.

Now this sample also checks whether the file is already been locked. This is a sample logic

#!/usr/bin/perl

$LOCK_EXCLUSIVE         = 2;

$UNLOCK                     = 8;

$LOCK_SHARED            = 1;

$LOCK_NONBLOCKING    = 4;

# -----------------------

# What's about to happen:

# -----------------------

# open the file, try to lock the file, then write, then unlock the

# file, then close the file.

open (FILE, ">> test.dat") || die "problem opening test.dat\n";

if ( is_file_locked($FILE) ){ print "locked\n";} else { print "not locked\n";}

flock FILE, $LOCK_EXCLUSIVE;

print FILE "this line printed by try.pl\n";

flock FILE, $UNLOCK;

close(FILE);

sub is_file_locked

{

  my $theFile;

  my $theRC;

  ($theFile) = @_;

  $theRC = open(my $HANDLE, ">>", $theFile);

  $theRC = flock($HANDLE, LOCK_EX|LOCK_NB);

  close($HANDLE);

  return !$theRC;

}

The above script will also check whether the lock is being taken or not

Now for my actual task , I have to use the same lock technology and make sure my second process checks the lock continuously and when it is available we need to perform some action.

Here is the sample code for that

#!/usr/bin/perl

#  --------

#  PROGRAM:  try.pl  (Sample Flock Script)

#  --------

$LOCK_EXCLUSIVE         = 2;

$UNLOCK                 = 8;

$LOCK_SHARED            = 1;

$LOCK_NONBLOCKING       = 4;

#Get the Domain Name for the Cluster Passed

my @values = split('-', $ARGV[0]);

my $cluster= $values[0];

my $lock_file="/logs/jas/jagadish/$cluster.lock";

for (;;)

{

   # See if lock Status has changed.

   my $lock_status=check_lock_exists();

   if ($lock_status == 0)

   {

       print "-----Lock  exist,sleeping for 10------\n";

      # lock Status Changed.

      # Sleep for 10 to Check the Lock status again

       sleep(10);

   }

   else

   {

      get_lock();

      exit 0;

   }

}

#Sub Routine to Check Whether the Lock File Exists

sub check_lock_exists() {

  

   my $status;

   if (-e $lock_file) {

        $status = 0;

    } else {

        $status  = 1;

    }

  

   return $status;

 }

sub get_lock()    {

     open (FILE, ">>$lock_file") || die "problem opening lock File\n";

     flock FILE, $LOCK_EXCLUSIVE;

     print "----Obtained Lock----\n";

     sleep 20;

     flock FILE, $UNLOCK;

     close(FILE);

     unlink($lock_file) or die "can't remove lockfile: $lockfile ($!)";

}

This script works in such a way ,

1. For the First time deployment , the script will checks for the Domain and create a lock file in the specified location with the domain name , like MyDomain.lock. For every domain , the lock file is created with domainName.lock so that we can check if multiple deployments are going to same domain.

2. The second  deployment will check the domain and if a domain lock file exists in the location ( the deployment is happening or lock was taken ) , it will continuously wait until the lock is released ( The lock file is deleted at the end and the second deployment will create the same lock file for the deployment ).Here are the testing

First Window

[djas999@vx181d jagadish]$ perl getlock.pl MyDomain

----Obtained Lock----

Second Window

[djas999@vx181d jagadish]$ perl getlock.pl MyDomain

-----Lock  exist,sleeping for 10------

-----Lock  exist,sleeping for 10------

----Obtained Lock----

Thrid Window

[djas999@vx181d jagadish]$ perl getlock.pl MyDomain

-----Lock  exist,sleeping for 10------

-----Lock  exist,sleeping for 10------

-----Lock  exist,sleeping for 10------

-----Lock  exist,sleeping for 10------

----Obtained Lock----

In the above case ( all ran at same time ) , the domain i have chosen is MyDomain and the  script will continuously check for the lock until it is released or not available.