[jira] [Created] (BEAM-6758) Potential Bug, BeamFnDataClient future finalization

["Alex Amato (JIRA)" <ji...@apache.org>]

Alex Amato created BEAM-6758:
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             Summary: Potential Bug, BeamFnDataClient future finalization
                 Key: BEAM-6758
                 URL: https://issues.apache.org/jira/browse/BEAM-6758
             Project: Beam
          Issue Type: New Feature
          Components: java-fn-execution
            Reporter: Alex Amato


Just wanted to write down this thought before I forget, maybe a second person can think through this and close it out if I am wrong.

I think that there is a bug here, which could leave to data loss when processing the last few elements in a bundle

 

I recently learned that a java CompletableFuture cannot be completedExecptionally (AKA failed) if the future has already completed. So in QueueingBeamFnDataClient if the future is already marked done, we cannot fail it. I noticed this behaviour in a unit test for a bundle receiving data for multiple inputs (where I could not fail one of them execptionally, since it was already marked done). 

 

The potential bug I see would occur if the future is already completed before we fail the element (I think that we rely on this in the allDone method of the QueuingBeamFnDataClient). Imagine processing the last few elements in a bundle, the InBoundDataClient is marked completed because there are no more elements on the GRPC channel coming in, but we fail when processing it.

 

I could be wrong, if somehow the inbound data client futures are guaranteed to not complete until we finish processing the elements themselves. But I don't think this is the case, I think there is some code (GRPCBeamFnDataClient) which will complete the future once it has received all the elements on the channel.

Also we might have other code which mitigates this problem entirely, because the ProcessBundleHandler.processBundle will also throw an exception in this case, which should be enough to fail the bundle and hopefully prevent data loss.

One potential solution is to have two future in the inboundDataClient:
- waitUntilAllElementsReceivedOnGrpc

- waitUntilAllElementsFinishedProcessing (which can be marked in the QueueingBeamFnDataClient).

 

 



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