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For such kind of operators, runtime would keep waiting till the finalize future is done. If the operator is running in the mail thread, the waiting is achieve by keeping checking whether the future is done and yield the mailbox thread, otherwise the runtime would block via future.get() (e.g., the operator is chained with a legacy source).

Failover and Recovery

Logically we could not restart the finished tasks on failover. We could only restart those tasks who have operators not fully finished. Specially, if all the source operators have been finished, the remaining operator would directly start the process of finishing. 

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