??????????Async I/O??exactly-once

??io?? ??at least once??The asynchronous I/O operator offers full exactly-once fault tolerance guarantees. It stores the records for in-flight asynchronous requests in checkpoints and restores/re-triggers the

????Async I/O??exactly-once

??failover??at least-once?? ck1:??a b cck2d??e??f??ck1??checkpoint??a ??b??c??.