Locking with Doozer

doozer_lock.rb

require 'rubygems'
require 'fraggle/block'

# # Locks with doozerd
#
# Doozerd is a little coordination service. I think of it like Zookeeper
# Lite (TM). In today's episode, we'll naively manage exclusive advisory locks.
#
# Disclaimer: If part of this implementation is total lolscale, 
# please to let me know what papers/sources I should read to avoid lolscaling.
#
# To run this example (assuming you have doozerd installed and the fraggle-block
# gem):
#
# $ doozerd # In shell 0
# $ ruby locksmith2.rb # In shell 1
# $ ruby locksmith2.rb # In shell 2
# $ doozer watch '/example' # In shell 3
#
# You should observe the two clients trading the lock. In the shell 3, you can
# watch the clients acquire and release the lock as doozerd sees it.

# Gracefully exit if we're in the middle of doing work.
$run = true
Signal.trap('TERM') { puts "Quittin' time, namean!"; $run = false }
Signal.trap('INT') { puts "Quittin' time, namean!"; $run = false }

# A locker is where you put your valuable state while you're doing 
# something else.
module Locker

  # We'll share a connection between all locks via a module instance variable.
  @connection = nil

  # Grab that connection.
  def self.connection
    @connection
  end

  # Set that connection.
  def self.connection=(c)
    @connection = c
  end

end

# A shared, advisory lock. Kids, only use locks if all else fails.
class Locker::Lock

  # This is the whole API. Specify a lock (`filename`) to wrap around a `block`
  # of work. Doesn't handle timing out the lock, just retries until the heat 
  # death of the universe.
  def self.with_lock(filename, &block)
    lock = new
    rev = lock.acquire(filename)
    block.call
    lock.release(filename, rev)
  end

  # Grabs the lock. It first grabs the lock file in doozer to see if anyone
  # already has the lock. If it's already acquired, it sleeps for a second and
  # tries again. Otherwise, it tries to set the lock. If the revision on the
  # lock file is greater after setting the lock than when it was first 
  # observed, the lock was successfully acquired. Otherwise it tries again.
  #
  # This is using the client process's PID as the lock value. In a 
  # distributed system, you'd want to use something more unique, probably MAC
  # address and PID/thread ID.
  def acquire(filename)
    rev = nil
    holder = connection.get(filename)
    while rev.nil?
      if holder.value == ''
        resp = connection.set(filename, Process.pid.to_s, holder.rev)
        if resp.err_code.nil? && resp.rev > holder.rev
          rev = resp.rev
        else
        end
      else
        sleep 1
        holder = connection.get(filename)
      end
    end
    rev
  end

  # Surrender the lock. Blindly assumes we really have the lock and deletes
  # the file, passing along the revision so doozerd can verify we really own
  # the lock.
  def release(filename, rev)
    resp = connection.del(filename, rev)
  end

  # Just a helper, nothing interesting here.
  def connection
    Locker.connection
  end

end

if __FILE__ == $PROGRAM_NAME
  # Connect to Doozerd with the non-EM, blocking driver.
  Locker.connection = Fraggle::Block.connect

  # Run until a signal is received.
  while $run do
    # Use the doozerd file '/example' as our coordination point.
    Locker::Lock.with_lock('/example') do
      # This block is executed whenever we hold the lock. Presumably we'd do
      # something fancier than writing to STDOUT in a real system.
      5.times { puts "Doing work"; sleep 1 }
    end
    # Sleeping here gives other clients a chance to acquire the lock. Not sure
    # if doozerd has primitives that make it possible for other clients to
    # watch the lock file while avoiding the herd effect.
    sleep 1
  end

  # Once we're done running, disconnect from doozerd.
  Locker.connection.disconnect
end