module MonitorMixin

In concurrent programming, a monitor is an object or module intended to be used safely by more than one thread. The defining characteristic of a monitor is that its methods are executed with mutual exclusion. That is, at each point in time, at most one thread may be executing any of its methods. This mutual exclusion greatly simplifies reasoning about the implementation of monitors compared to reasoning about parallel code that updates a data structure.

You can read more about the general principles on the Wikipedia page for Monitors.

Examples¶ ↑

Simple object.extend¶ ↑

require 'monitor.rb'

buf = []
buf.extend(MonitorMixin)
empty_cond = buf.new_cond

# consumer
Thread.start do
  loop do
    buf.synchronize do
      empty_cond.wait_while { buf.empty? }
      print buf.shift
    end
  end
end

# producer
while line = ARGF.gets
  buf.synchronize do
    buf.push(line)
    empty_cond.signal
  end
end

The consumer thread waits for the producer thread to push a line to buf while buf.empty?. The producer thread (main thread) reads a line from ARGF and pushes it into buf then calls empty_cond.signal to notify the consumer thread of new data.

Simple Class include¶ ↑

require 'monitor'

class SynchronizedArray < Array

  include MonitorMixin

  def initialize(*args)
    super(*args)
  end

  alias :old_shift :shift
  alias :old_unshift :unshift

  def shift(n=1)
    self.synchronize do
      self.old_shift(n)
    end
  end

  def unshift(item)
    self.synchronize do
      self.old_unshift(item)
    end
  end

  # other methods ...
end

SynchronizedArray implements an Array with synchronized access to items. This Class is implemented as subclass of Array which includes the MonitorMixin module.

# File ext/monitor/lib/monitor.rb, line 152
def self.extend_object(obj)
  super(obj)
  obj.__send__(:mon_initialize)
end

# File ext/monitor/lib/monitor.rb, line 222
def initialize(...)
  super
  mon_initialize
end

# File ext/monitor/lib/monitor.rb, line 169
def mon_enter
  @mon_data.enter
end

# File ext/monitor/lib/monitor.rb, line 176
def mon_exit
  mon_check_owner
  @mon_data.exit
end

# File ext/monitor/lib/monitor.rb, line 184
def mon_locked?
  @mon_data.mon_locked?
end

# File ext/monitor/lib/monitor.rb, line 191
def mon_owned?
  @mon_data.mon_owned?
end

# File ext/monitor/lib/monitor.rb, line 200
def mon_synchronize(&b)
  @mon_data.synchronize(&b)
end

# File ext/monitor/lib/monitor.rb, line 160
def mon_try_enter
  @mon_data.try_enter
end

# File ext/monitor/lib/monitor.rb, line 209
def new_cond
  unless defined?(@mon_data)
    mon_initialize
    @mon_initialized_by_new_cond = true
  end
  return ConditionVariable.new(@mon_data)
end