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.