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.

Public Class Methods

extend_object(obj) click to toggle source
Calls superclass method
# File monitor/lib/monitor.rb, line 152
def self.extend_object(obj)
  super(obj)
  obj.__send__(:mon_initialize)
end
new(...) click to toggle source

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

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

Public Instance Methods

mon_enter() click to toggle source

Enters exclusive section.

# File monitor/lib/monitor.rb, line 169
def mon_enter
  @mon_data.enter
end
mon_exit() click to toggle source

Leaves exclusive section.

# File monitor/lib/monitor.rb, line 176
def mon_exit
  mon_check_owner
  @mon_data.exit
end
mon_locked?() click to toggle source

Returns true if this monitor is locked by any thread

# File monitor/lib/monitor.rb, line 184
def mon_locked?
  @mon_data.mon_locked?
end
mon_owned?() click to toggle source

Returns true if this monitor is locked by current thread.

# File monitor/lib/monitor.rb, line 191
def mon_owned?
  @mon_data.mon_owned?
end
mon_synchronize(&b) click to toggle source

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

# File monitor/lib/monitor.rb, line 200
def mon_synchronize(&b)
  @mon_data.synchronize(&b)
end
Also aliased as: synchronize
mon_try_enter() click to toggle source

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

# File monitor/lib/monitor.rb, line 160
def mon_try_enter
  @mon_data.try_enter
end
Also aliased as: try_mon_enter
new_cond() click to toggle source

Creates a new MonitorMixin::ConditionVariable associated with the Monitor object.

# File 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
synchronize(&b)
Alias for: mon_synchronize
try_mon_enter()

For backward compatibility

Alias for: mon_try_enter

Private Instance Methods

mon_check_owner() click to toggle source
# File monitor/lib/monitor.rb, line 241
def mon_check_owner
  @mon_data.mon_check_owner
end
mon_initialize() click to toggle source

Initializes the MonitorMixin after being included in a class or when an object has been extended with the MonitorMixin

# File monitor/lib/monitor.rb, line 229
def mon_initialize
  if defined?(@mon_data)
    if defined?(@mon_initialized_by_new_cond)
      return # already initialized.
    elsif @mon_data_owner_object_id == self.object_id
      raise ThreadError, "already initialized"
    end
  end
  @mon_data = ::Monitor.new
  @mon_data_owner_object_id = self.object_id
end