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
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.
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 lib/monitor.rb, line 159 def self.extend_object(obj) super(obj) obj.__send__(: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 lib/monitor.rb, line 233 def initialize(*args) super mon_initialize end
Enters exclusive section.
# File lib/monitor.rb, line 184 def mon_enter if @mon_owner != Thread.current @mon_mutex.lock @mon_owner = Thread.current @mon_count = 0 end @mon_count += 1 end
Leaves exclusive section.
# File lib/monitor.rb, line 196 def mon_exit mon_check_owner @mon_count -=1 if @mon_count == 0 @mon_owner = nil @mon_mutex.unlock end end
Enters exclusive section and executes the block. Leaves the exclusive section automatically when the block exits. See example under MonitorMixin
.
# File lib/monitor.rb, line 210 def mon_synchronize mon_enter begin yield ensure mon_exit end end
Attempts to enter exclusive section. Returns false
if lock fails.
# 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 @mon_count = 0 end @mon_count += 1 return true end
Creates a new MonitorMixin::ConditionVariable associated with the receiver.
# File lib/monitor.rb, line 224 def new_cond return ConditionVariable.new(self) end
# File lib/monitor.rb, line 246 def mon_check_owner if @mon_owner != Thread.current raise ThreadError, "current thread not owner" end end
# File lib/monitor.rb, line 252 def mon_enter_for_cond(count) @mon_owner = Thread.current @mon_count = count end
# File lib/monitor.rb, line 257 def mon_exit_for_cond count = @mon_count @mon_owner = nil @mon_count = 0 return count end
Initializes the MonitorMixin after being included in a class or when an object has been extended with the MonitorMixin
# File lib/monitor.rb, line 240 def mon_initialize @mon_owner = nil @mon_count = 0 @mon_mutex = Thread::Mutex.new end
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