Ansible 2.9.0 新增的 throttle 关键字

Ansible 2.9的CHANGELOG里是这么描述的：

- 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.

定义在lib/ansible/playbook/base.py里作为Base的成员，也就是说Play,Task,Block都可以使用这个关键字。
它的用途主要体现在Strategy里，跟StrategyBase.ALLOW_BASE_THROTTLING共同作用与WorkerProcess的创建。在StrategyBase中，ALLOW_BASE_THROTTLING默认为True，也就是允许节流。注释是这么写的（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里原本的代码上加了一块：

                # 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

简单解释就是throttle的值是在作用在forks基础之上的，因为forks是个选项，写在配置文件里的默认值是5。
Ansible启动的时候还是按照forks的大小去创建workers列表，但这个列表里具体有多少个槽位能用，是取决于throttle的。由于throttle可以在play，block，task中使用，然而至少就我们目前的应用场景而言，这个关键字比较鸡肋，我还没想到它能有啥用。