Why A.perform and A.new.perform aren't the same thing, from http://blog.codeclimate.com/blog/2012/11/14/why-ruby-class-methods-resist-refactoring/ - A.perform is not threadsafe (unless it calls A.new.perform, heh), B.perform is

sample_output.txt

All A outputs:
AA
A
AAAAAAAAAA
AAA
AAAA
AAAAA
AAAAAA
AAAAAAA
AAAAAAAA
AAAAAAAAA
All B outputs:
B
B
B
B
B
B
B
B
B
B

thread_test.rb

class A  # class method approach
  def self.perform
    @counter ||= ''
    @counter = @counter + 'A'
  end
end

class B  # instance method approach
  def perform
    @counter ||= ''
    @counter = @counter + 'B'
  end
end

threads = 10.times.map do
  Thread.new do
    Thread.current[:a] = A.perform
    Thread.current[:b] = B.new.perform
  end
end
threads.each(&:join)

# let's inspect outputs

puts "All A outputs:"
puts threads.map{ |t| t[:a] }

puts "All B outputs:"
puts threads.map{ |t| t[:b] }

Yup, in the first case @counter refers to an instance variable of A (remember classes are objects too), whereas in the second example, @counter refers to an instance variable of an instance of B.