cache.rb

@semaphore = Mutex.new

def cache
  @cache ||= {}
end

def get_y
  sleep 1
  Object.new
end

def read(x)
  cache[x] ||= begin
    y = get_y
    y
  end
end

def read2(x)
  cache[x] ||= begin
    y = get_y
    cache[x] = y
  end
end

def read3(x)
  @semaphore.synchronize do
    read(x)
  end
end

10.times.map do
  Thread.new do
    puts read(123).object_id
  end
end.each(&:join)
# 70364185000120
# 70364184999880
# 70364184999740
# 70364184999560
# 70364184999440
# 70364184999340
# 70364184999120
# 70364184998920
# 70364184998820
# 70364184998680

10.times.map do
  Thread.new do
    puts read(124).object_id
  end
end.each(&:join)
# 70325148659660
# 70325148659060
# 70325148658860
# 70325148658380
# 70325148657920
# 70325148657380
# 70325148656320
# 70325148655840
# 70325148663580
# 70325148662820

10.times.map do
  Thread.new do
    puts read3(125).object_id
  end
end.each(&:join)
# 70276222056500
# 70276222056500
# 70276222056500
# 70276222056500
# 70276222056500
# 70276222056500
# 70276222056500
# 70276222056500
# 70276222056500
# 70276222056500