isolation_level=

# File activesupport/lib/active_support/isolated_execution_state.rb, line 15
      def isolation_level=(level)
        unless %(thread fiber).include?(level)
          raise ArgumentError, "isolation_level must be `:thread` or `:fiber`, got: `#{level.inspect}`"
        end

        if level != isolation_level
          clear
          singleton_class.alias_method(:current, "current_#{level}")
          singleton_class.send(:private, :current)
          @isolation_level = level
        end
      end