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 tmp/rubies/ruby-master/ext/monitor/lib/monitor.rb, line 152
def self.extend_object(obj)
super(obj)
obj.__send__(:mon_initialize)
end
# File tmp/rubies/ruby-master/ext/monitor/lib/monitor.rb, line 222
def initialize(...)
super
mon_initialize
end
Use extend MonitorMixin
or include MonitorMixin
instead of this constructor. Have look at the examples above to understand how to use this module.
# File tmp/rubies/ruby-master/ext/monitor/lib/monitor.rb, line 241
def mon_check_owner
@mon_data.mon_check_owner
end
# File tmp/rubies/ruby-master/ext/monitor/lib/monitor.rb, line 169
def mon_enter
@mon_data.enter
end
Enters exclusive section.
# File tmp/rubies/ruby-master/ext/monitor/lib/monitor.rb, line 176
def mon_exit
mon_check_owner
@mon_data.exit
end
Leaves exclusive section.
# File tmp/rubies/ruby-master/ext/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
Initializes the MonitorMixin
after being included in a class or when an object has been extended with the MonitorMixin
# File tmp/rubies/ruby-master/ext/monitor/lib/monitor.rb, line 184
def mon_locked?
@mon_data.mon_locked?
end
Returns true if this monitor is locked by any thread
# File tmp/rubies/ruby-master/ext/monitor/lib/monitor.rb, line 191
def mon_owned?
@mon_data.mon_owned?
end
Returns true if this monitor is locked by current thread.
# File tmp/rubies/ruby-master/ext/monitor/lib/monitor.rb, line 200
def mon_synchronize(&b)
@mon_data.synchronize(&b)
end
Enters exclusive section and executes the block. Leaves the exclusive section automatically when the block exits. See example under MonitorMixin
.
# File tmp/rubies/ruby-master/ext/monitor/lib/monitor.rb, line 160
def mon_try_enter
@mon_data.try_enter
end
Attempts to enter exclusive section. Returns false
if lock fails.
# File tmp/rubies/ruby-master/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
Creates a new MonitorMixin::ConditionVariable
associated with the Monitor
object.