Thread

Basic Thread Semantics

A new thread separate from the main thread’s execution, can be created using Thread.new.

thr = Thread.new {
  sleep 1 # 1 second sleep of sub thread
  puts "Whats the big deal"
}

This will automatically start the execution of the new thread.

To freeze execution of the main Thread, until the new thread stops, use join:

thr.join #=> ... "Whats the big deal"

Note that the Thread may have already finished when you call join, in which case execution will continue normally. If a sub-thread is never joined, and the main thread completes, the sub-thread will not execute any remaining code.

Accessing shared resources

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

How to kill a thread

You call use Thread.kill or Thread.terminate:

thr = Thread.new { ... }
Thread.kill(thr)

Terminating a Thread

A thread terminates if it reaches the end of its code block. The best way to terminate a thread early is to convince it to reach the end of its code block. This way, the thread can run cleanup code before dying.

This thread runs a loop while the instance variable continue is true. Set this variable to false, and the thread will die a natural death:

require 'thread'

class CounterThread < Thread
  def initialize
    @count = 0
    @continue = true

    super do
      @count += 1 while @continue
      puts "I counted up to #{@count} before I was cruelly stopped."
    end
  end

  def stop
    @continue = false
  end
end

counter = CounterThread.new
sleep 2
counter.stop