URLS

   http://rubyforge.org/projects/codeforpeople/
   http://codeforpeople.com/lib/ruby/lockfile/
   http://raa.ruby-lang.org/project/lockfile/

SYNOPSIS

   lib/lockfile.rb : a ruby library for creating NFS safe lockfiles

   bin/rlock : ruby command line tool which uses this library to create 
lockfiles
               and to run arbitrary commands while holding them.

               for example

                 rlock lockfile cp -r huge/ huge.bak/

               run 'rlock -h' for more info

INSTALL

   sudo ruby install.rb


BASIC ALGORITHIM

   * create a globally uniq filename in the same filesystem as the desired
     lockfile - this can be nfs mounted

   * link(2) this file to the desired lockfile, ignore all errors

   * stat the uniq filename and desired lockfile to determine is they are the
     same, use only stat.rdev and stat.ino - ignore stat.nlink as NFS can cause
     this report incorrect values

   * iff same, you have lock.  either return or run optional code block with
     optional refresher thread keeping lockfile fresh during execution of code
     block, ensuring that the lockfile is removed..

   * iff not same try again a few times in rapid succession (poll), then, iff
     this fails, sleep using incremental backoff time.  optionally remove
     lockfile if it is older than a certain time, timeout if more than a certain
     amount of time has passed attempting to lock file.


BASIC USAGE

   1)
     lockfile = Lockfile.new 'file.lock'
     begin
       lockfile.lock
       p 42
     ensure
       lockfile.unlock
     end


   2)
     require 'pstore'           # which is NOT nfs safe on it's own

     opts = {                    # the keys can be symbols or strings

       :retries        => nil,   # we will try forever to aquire the lock

       :sleep_inc      => 2,     # we will sleep 2 seconds longer than the
                                 # previous sleep after each retry, cycling from
                                 # min_sleep upto max_sleep downto min_sleep 
upto
                                 # max_sleep, etc., etc.

       :min_sleep      => 2,     # we will never sleep less than 2 seconds

       :max_sleep      => 32,    # we will never sleep longer than 32 seconds

       :max_age        => 1024,  # we will blow away any files found to be older
                                 # than this (lockfile.thief? #=> true)

       :suspend        => 64,    # iff we steal the lock from someone else - 
wait
                                 # this long to give them a chance to realize it

       :refresh        => 8,     # we will spawn a bg thread that touches file
                                 # every 8 sec.  this thread also causes a
                                 # StolenLockError to be thrown if the lock
                                 # disappears from under us - note that the
                                 # 'detection' rate is limited to the refresh
                                 # interval - this is a race condition

       :timeout        => nil,   # we will wait forever

       :poll_retries   => 16,    # the initial attempt to grab a lock is done in 
a
                                 # polling fashion, this number controls how 
many
                                 # times this is done - the total polling 
attempts
                                 # are considered ONE actual attempt (see 
retries
                                 # above)

       :poll_max_sleep => 0.08,  # when polling a very brief sleep is issued
                                 # between attempts, this is the upper limit of
                                 # that sleep timeout

       :dont_clean     => false, # normally a finalizer is defined to clean up
                                 # after lockfiles, settin this to true prevents 
this

       :dont_sweep     => false, # normally locking causes a sweep to be made. 
a
                                 # sweep removes any old tmp files created by
                                 # processes of this host only which are no
                                 # longer alive

       :debug => true,           # trace execution step on stdout
     }

     pstore = PStore.new 'file.db'
     lockfile = Lockfile.new 'file.db.lock', opts
     lockfile.lock do
       pstore.transaction do
         pstore[:last_update_time] = Time.now
       end
     end

   3) same as 1 above - Lockfile.new takes a block and ensures unlock is called

     Lockfile.new('file.lock') do
       p 42
     end

   4) watch locking algorithim in action (debugging only)

       Lockfile.debug = true
       Lockfile.new('file.lock') do
         p 42
       end

     you can also set debugging via the ENV var LOCKFILE_DEBUG, eg.

     ~ > LOCKFILE_DEBUG=true rlock lockfile


SAMPLES

   * see samples/a.rb
   * see samples/nfsstore.rb
   * see samples/lock.sh
   * see bin/rlock

AUTHOR

   Ara T. Howard

EMAIL

   Ara.T.Howard / noaa.gov

BUGS

   bugno > 1 && bugno < 42

HISTORY

   1.4.0:
     - gem'd up
     - added Lockfile::create method which atomically creates a file and opens
       it:

         Lockfile::create("atomic_even_on_nfs", "r+") do |f|
           f.puts 42
         end

       arguments are the same as those for File::open.  note that there is no 
way
       to accomplish this otherwise since File::O_EXCL fails silently on nfs,
       flock does not work on nfs, and posixlock (see raa) can only lock a file
       after it is open so the open itself is still a race.

   1.3.0:
     - added sweep functionality.  hosts can clean up any tmp files left around
       by other processes that may have been killed using -9 to prevent proper
       clean up.  it is only possible to clean up after processes on the same
       host as there is no other way to determine if the process that created 
the
       file is alive.  in summary - hosts clean up after other processes on that
       same host if needed.
     - added attempt/try_again methods
   1.2.0:
     - fixed bug where stale locks not removed when retries == 0
   1.1.0
     - unfortunately i've forgotten
   1.0.1:
     - fixed bugette in sleep cycle where repeated locks with same lockfile 
would
       not reset the cycle at the start of each lock
   1.0.0:
     - allow rertries to be nil, meaning to try forever
     - default timeout is now nil - never timeout
     - default refresh is now 8
     - fixed bug where refresher thread was not actually touching lockfile! 
(ouch)
     - added cycle method to timeouts
         1-2-3-2-1-2-3-1...
       pattern is constructed using min_sleep, sleep_inc, max_sleep
   0.3.0:
     - removed use of yaml in favour of hand parsing the lockfile contents, the
       file as so small it just wasn't worth and i have had one core dump when 
yaml
       failed to parse a (corrupt) file
   0.2.0:
     - added an initial polling style attempt to grab lock before entering 
normal
       sleep/retry loop.  this has really helped performance when lock is under
       heavy contention: i see almost no sleeping done by any of in the 
interested
       processes
   0.1.0:
     - added ability of Lockfile.new to accept a block
   0.0.0:
     - initial version


enjoy.

-a
-- 
===============================================================================
| ara [dot] t [dot] howard [at] gmail [dot] com
| all happiness comes from the desire for others to be happy.  all misery
| comes from the desire for oneself to be happy.
| -- bodhicaryavatara
===============================================================================