understand_pool_exhaustion.rb

# To had if you want to understand why even with same max thread and connection pool size 
# you still see "could not obtain a connection from the pool within 5.000 seconds"
class ActiveRecord::ConnectionAdapters::ConnectionPool::Queue
  def wait_poll(timeout)
    super(timeout)
  rescue ActiveRecord::ConnectionTimeoutError => e
    puts "listing #{Thread.list.count} threads:"
    Thread.list.each_with_index do |t,i|
      puts "---- thread #{i}: #{t.inspect}"
      puts t.backtrace.take(5)
    end
    raise e
  end
end