bezmax commented on issue #6161: Cloning subscriptions URL: https://github.com/apache/pulsar/issues/6161#issuecomment-580068138 Yes, as I mentioned in my last paragraph, Reader is the only alternative that would work. I would like to avoid using it if possible, as we would need to reimplement full functionality of subscriptions but on the client side, as we need an out-of-order acknowledgements as well. Maybe explaining the full flow of the system (Apache Flink based) would be useful: Message is read from pulsar and sent asynchronously down the processing pipeline. Given that some messages go down the pipeline faster than others - ordering might be lost. At a point of time, a synchronization signal is sent for all processors to "checkpoint" their state changes, similar to a "commit" of a transaction. At this moment our reader needs to ack the messages that have reached the end of the processing pipeline (which again, might miss a few late ones). Every "commit" that the system has made is in theory restorable. That is, for every other component we can say "restore your state from 3 commits before now". Except Pulsar subscriber. In pulsar subscriber the only "commit" I can go back is the set of messages that have been acked by sending a `redeliverUnacked` command. That's where this feature request comes in, I am not very good in Pulsar architecture, but it sounds like it should be much easier to do on the backend with existing infrastructure for subscriptions than reimplement it all on top of Reader on the client. As we would essentially need to keep a local watermark and a set of unacked messages around that watermark, which is not ideal. Thanks, hope that clarifies the feature request.
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