Ansible 2.9.0 新增的 throttle 關鍵字

Ansible 2.9的CHANGELOG裏是這麼描述的：

- Added new `throttle` keyword, which can be used at the task, block, or play level to limit the number of workers (up to the specified forks or serial setting) allowed.

定義在lib/ansible/playbook/base.py裏作爲Base的成員，也就是說Play,Task,Block都可以使用這個關鍵字。
它的用途主要體現在Strategy裏，跟StrategyBase.ALLOW_BASE_THROTTLING共同作用與WorkerProcess的創建。在StrategyBase中，ALLOW_BASE_THROTTLING默認爲True，也就是允許節流。註釋是這麼寫的（158-161行）：

    # by default, strategies should support throttling but we allow individual
    # strategies to disable this and either forego supporting it or managing
    # the throttling internally (as `free` does)
    ALLOW_BASE_THROTTLING = True

在StrategyBase._queue_task裏原本的代碼上加了一塊：

                # Determine the "rewind point" of the worker list. This means we start
                # iterating over the list of workers until the end of the list is found.
                # Normally, that is simply the length of the workers list (as determined
                # by the forks or serial setting), however a task/block/play may "throttle"
                # that limit down.
                rewind_point = len(self._workers)
                if throttle > 0 and self.ALLOW_BASE_THROTTLING:
                    if task.run_once:
                        display.debug("Ignoring 'throttle' as 'run_once' is also set for '%s'" % task.get_name())
                    else:
                        if throttle <= rewind_point:
                            display.debug("task: %s, throttle: %d" % (task.get_name(), throttle))
                            rewind_point = throttle
                if self._cur_worker >= rewind_point:
                    self._cur_worker = 0

簡單解釋就是throttle的值是在作用在forks基礎之上的，因爲forks是個選項，寫在配置文件裏的默認值是5。
Ansible啓動的時候還是按照forks的大小去創建workers列表，但這個列表裏具體有多少個槽位能用，是取決於throttle的。由於throttle可以在play，block，task中使用，然而至少就我們目前的應用場景而言，這個關鍵字比較雞肋，我還沒想到它能有啥用。