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.

Namespace
Methods
E
M
N
S
T
Class Public methods
extend_object(obj)

Source: show 

# File lib/monitor.rb, line 159
def self.extend_object(obj)
  super(obj)
  obj.__send__(:mon_initialize)
end
new(*args)

Use extend MonitorMixin or include MonitorMixin instead of this constructor. Have look at the examples above to understand how to use this module.

Source: show 

# File lib/monitor.rb, line 231
def initialize(*args)
  super
  mon_initialize
end
Instance Public methods
mon_enter()

Enters exclusive section.

Source: show 

# File lib/monitor.rb, line 183
def mon_enter
  if @mon_owner != Thread.current
    @mon_mutex.lock
    @mon_owner = Thread.current
  end
  @mon_count += 1
end
mon_exit()

Leaves exclusive section.

Source: show 

# File lib/monitor.rb, line 194
def mon_exit
  mon_check_owner
  @mon_count -=1
  if @mon_count == 0
    @mon_owner = nil
    @mon_mutex.unlock
  end
end
mon_synchronize()

Enters exclusive section and executes the block. Leaves the exclusive section automatically when the block exits. See example under MonitorMixin.

Also aliased as: synchronize

Source: show 

# File lib/monitor.rb, line 208
def mon_synchronize
  mon_enter
  begin
    yield
  ensure
    mon_exit
  end
end
mon_try_enter()

Attempts to enter exclusive section. Returns false if lock fails.

Also aliased as: try_mon_enter

Source: show 

# File lib/monitor.rb, line 167
def mon_try_enter
  if @mon_owner != Thread.current
    unless @mon_mutex.try_lock
      return false
    end
    @mon_owner = Thread.current
  end
  @mon_count += 1
  return true
end
new_cond()

Creates a new MonitorMixin::ConditionVariable associated with the receiver.

Source: show 

# File lib/monitor.rb, line 222
def new_cond
  return ConditionVariable.new(self)
end
synchronize()
Alias for: mon_synchronize
try_mon_enter()

For backward compatibility

Alias for: mon_try_enter