窗口的状态
Q：想在每个key的窗口执行完成后，清除那个key的state，因为用不到了，那个key后面不再出现。
A：
when you use Flink's Windowing API, the state of an event time window is
cleared once the watermark passes the end time of the window (that's when
the window fires) + the allowed lateness. So, as long as you don't
configure additional allowed lateness (default=0), Flink will already
behave as described by default.
一旦水印超过了窗口，那个窗口的state会自动清除，如果不设定延时关闭的话，默认是容忍延时0.
窗口的同步时间很大，checkpoint的
A：即使异步模式，窗口的timer是同步保存的，目前还在修复？1.6版本之前没修复，还没去看这个解决了没。
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