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-2.6.10/lib/monitor.rb, line 165
def self.extend_object(obj)
super(obj)
obj.__send__(:mon_initialize)
end
# File tmp/rubies/ruby-2.6.10/lib/monitor.rb, line 255
def initialize(*args)
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-2.6.10/lib/monitor.rb, line 272
def mon_check_owner
if @mon_owner != Thread.current
raise ThreadError, "current thread not owner"
end
end
# File tmp/rubies/ruby-2.6.10/lib/monitor.rb, line 190
def mon_enter
if @mon_owner != Thread.current
@mon_mutex.lock
@mon_owner = Thread.current
@mon_count = 0
end
@mon_count += 1
end
Enters exclusive section.
# File tmp/rubies/ruby-2.6.10/lib/monitor.rb, line 278
def mon_enter_for_cond(count)
@mon_owner = Thread.current
@mon_count = count
end
# File tmp/rubies/ruby-2.6.10/lib/monitor.rb, line 202
def mon_exit
mon_check_owner
@mon_count -=1
if @mon_count == 0
@mon_owner = nil
@mon_mutex.unlock
end
end
Leaves exclusive section.
# File tmp/rubies/ruby-2.6.10/lib/monitor.rb, line 283
def mon_exit_for_cond
count = @mon_count
@mon_owner = nil
@mon_count = 0
return count
end
# File tmp/rubies/ruby-2.6.10/lib/monitor.rb, line 262
def mon_initialize
if defined?(@mon_mutex) && @mon_mutex_owner_object_id == object_id
raise ThreadError, "already initialized"
end
@mon_mutex = Thread::Mutex.new
@mon_mutex_owner_object_id = object_id
@mon_owner = nil
@mon_count = 0
end
Initializes the MonitorMixin after being included in a class or when an object has been extended with the MonitorMixin
# File tmp/rubies/ruby-2.6.10/lib/monitor.rb, line 214
def mon_locked?
@mon_mutex.locked?
end
Returns true if this monitor is locked by any thread
# File tmp/rubies/ruby-2.6.10/lib/monitor.rb, line 221
def mon_owned?
@mon_mutex.locked? && @mon_owner == Thread.current
end
Returns true if this monitor is locked by current thread.
# File tmp/rubies/ruby-2.6.10/lib/monitor.rb, line 230
def mon_synchronize
# Prevent interrupt on handling interrupts; for example timeout errors
# it may break locking state.
Thread.handle_interrupt(EXCEPTION_NEVER){ mon_enter }
begin
yield
ensure
Thread.handle_interrupt(EXCEPTION_NEVER){ mon_exit }
end
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-2.6.10/lib/monitor.rb, line 173
def mon_try_enter
if @mon_owner != Thread.current
unless @mon_mutex.try_lock
return false
end
@mon_owner = Thread.current
@mon_count = 0
end
@mon_count += 1
return true
end
Attempts to enter exclusive section. Returns false if lock fails.
# File tmp/rubies/ruby-2.6.10/lib/monitor.rb, line 246
def new_cond
return ConditionVariable.new(self)
end
Creates a new MonitorMixin::ConditionVariable associated with the receiver.