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] }

Just found my way here from the Code Climate blog post, and wanted to point out that the difference in behaviour has nothing to do with thread-safety: in A you're updating a single, shared class variable, whereas in B you're creating a new instance every time which means using an empty "counter" each time.

You see exactly the same behaviour without the threading (other than that the results are guaranteed to be in-order):

a, b = [], []

10.times do
  a << A.perform
  b << B.new.perform
end

a #=> ["A", "AA", "AAA", "AAAA", "AAAAA", "AAAAAA", "AAAAAAA", "AAAAAAAA", "AAAAAAAAA", "AAAAAAAAAA"]
b #=> ["B", "B", "B", "B", "B", "B", "B", "B", "B", "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.