protected function buildStacks()
    {
        $stacks   = [];
        $stackNo  = 0;
        $currSize = 0;

        foreach ($this->requests as $request) {
            if ($currSize === $this->stackSize) {
                $currSize = 0;
                $stackNo++;
            }

            $stacks[$stackNo][] = $request;
            $currSize++;
        }

        return $stacks;
    }