In a failure scenario, when a container fails, it is redeployed along with all the operators in it. The operators downstream to these operators are also redeployed within their containers. The operators are restored from their checkpoint and connect to the appropriate point in the stream according to the processing mode. In at least once mode, for example, the data is replayed from the same checkpoint
Restoring an operator state from checkpoint could turn out to be a costly operation depending on the size of the state. In some use cases, based on the operator logic, when there is an upstream failure, the operator state without being restored to the checkpoint i.e., remaining as is, will still produce the same results with the data replayed from the last fully processed window. This is true with some operators in batch use cases. The operator state can remain the same as it was before the upstream failure by reusing the same operator instance from before and only the streams and window reset to the window after the last fully processed window to guarantee the at least once processing of tuples. If the container where the operator itself is running goes down, it would need to be restored from the checkpoint of course. I would like to propose adding the ability for a user to explicitly identify operators to be of this type and the corresponding functionality in the engine to handle their recovery in the way described above by not restoring their state from checkpoint, reusing the instance and restoring the stream to the window after the last fully processed window for the operator. When operators are not identified to be of this type, the default behavior is what it is today and nothing changes. I have done some prototyping on the engine side to ensure that this is possible with our current code base without requiring a massive overhaul, especially the restoration of the operator instance within the Node in the streaming container, the re-establishment of the subscriber stream to a window in the buffer server where the publisher (upstream) hasn't yet reached as it would be restarting from checkpoint and have been able to get it all working successfully. Thanks
