ProcessLock

A simple class to acquire and check process-id file based locks on a unix filesystem. Can also be used to see if a process is already running or designate a master process when running concurrent applications.

Add this line to your application's Gemfile:

gem 'process_lock'

And then execute:

$ bundle

Or install it yourself as:

$ gem install process_lock

Create an instance of ProcessLock with a filename as the lock. You may have more than one lock (with different names) per process.

Methods:

• acquire - Acquires a lock if it can. Returns true (or value of block if block is passed) if a lock was acquired, otherwise false.
• acquire! - Same as acquire except it throws an exception if a lock could not be obtained.
• release - Releases the lock if we are the owner. Returns true if the lock was released.
• release! - Same as release except it throws an exception if a lock was not released.
• filename - the filename passed when the instance was created
• read - the process id in the lock file, otherwise 0 (zero)

Note:

• locks don't stack - if we have already acquired the lock subsequent calls will reacquire the lock. releasing an already released lock will fail.
• If Rails.root is defined then lock files without path separators (/) will be put in tmp/pids. If no extension is specified then .pid will be appended.

To acquire a lock, do some work and then release it:

pl = ProcessLock.new('tmp/name_of_lock.lock')

acquired = pl.acquire do
  puts "Do some work!"
end
puts "Unable to obtain a lock" unless acquired

# OR

while ! pl.acquire
  puts "Trying to acquire a lock"
  sleep(1)
end
puts "Do some work!"
pl.release

To allow many worker processes to self organise and identify a leader process. (Simon and my implementation have diverged).

IRB 1>>
pl = ProcessLock.new('example.tmp')
pl.acquire
=> true
pl.read
=> "2435"

IRB 2>>
pl = ProcessLock.new('example.tmp')
pl.acquire
=> false
pl.read
=> "2435"

example.tmp will contain the pid of the leader process

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request (Tests appreciated)

• Copyright (c) 2008 Simon Engledew, released under the MIT license: https://github.com/simon-engledew/ruby-process-lock .
• Subsequent work by Ian Heggie: enhanced library and packaged into a gem, added tests and acquire method, fixed some bugs.
• See git log for other contributers