Ruby Language Thread Accessing shared resources
Example
Use a mutex to synchronise access to a variable which is accessed from multiple threads:
counter = 0
counter_mutex = Mutex.new
# Start three parallel threads and increment counter
3.times.map do |index|
Thread.new do
counter_mutex.synchronize { counter += 1 }
end
end.each(&:join) # Wait for all threads to finish before killing the process
Otherwise, the value of counter currently visible to one thread could be changed by another thread.
Example without Mutex (see e.g. Thread 0, where Before and After differ by more than 1):
2.2.0 :224 > counter = 0; 3.times.map { |i| Thread.new { puts "[Thread #{i}] Before: #{counter}"; counter += 1; puts "[Thread #{i}] After: #{counter}"; } }.each(&:join)
[Thread 2] Before: 0
[Thread 0] Before: 0
[Thread 0] After: 2
[Thread 1] Before: 0
[Thread 1] After: 3
[Thread 2] After: 1
Example with Mutex:
2.2.0 :226 > mutex = Mutex.new; counter = 0; 3.times.map { |i| Thread.new { mutex.synchronize { puts "[Thread #{i}] Before: #{counter}"; counter += 1; puts "[Thread #{i}] After: #{counter}"; } } }.each(&:join)
[Thread 2] Before: 0
[Thread 2] After: 1
[Thread 1] Before: 1
[Thread 1] After: 2
[Thread 0] Before: 2
[Thread 0] After: 3
