isolation_level=

# File activesupport/lib/active_support/isolated_execution_state.rb, line 15
      def isolation_level=(level)
        return if level == @isolation_level

        unless %(thread fiber).include?(level)
          raise ArgumentError, "isolation_level must be `:thread` or `:fiber`, got: `#{level.inspect}`"
        end

        clear if @isolation_level

        @scope =
          case level
          when :thread; Thread
          when :fiber; Fiber
          end

        @isolation_level = level
      end