bank.rb

#  Example of concurrency issue:

class Account
  attr_accessor :amount

  def initialize
    @amount = 0
  end

  def get_balance
    @amount
  end

  def update_balance val
    return false if @amount.nil?

    @amount = @amount + val
  end

  def close_account
    @amount = nil
  end
end

account = Account.new()

puts account.get_balance
5.times do |i|
  Thread.new do
    account.update_balance 10
  end
end

puts account.get_balance
# output 30