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This new mode (name is enable.parallel.rebalance) will by default not be used by KafkaConsumer (e.g. will be by default equal to false). If the user chooses, he or she can activate this mode by setting the config's value to true. With the enabling of this mode, there will be an increased burden on a computer's processor cores because an extra consumer thread will be instantiated  and run in parallel with the old consumer. In this manner, we could retrieve records at twice that of the current design. Note that when using this mode, offsets would no longer be given to the user in correct sequence (or order), but rather in a more haphazard manner. The mentality of this mode could be described as "give me all the offsets as fast as you can, I don't care about order." We will be able to guarantee at-least-once semantics, however the sequence which they come in will be thrown off. To illustrate, offsets could be sent to the user in this order:  1,2,3,6,4,7,5,8

A method called childConsumerIsAlive() will be introduced which returns a boolean value indicating if the secondary thread created has started or not. If true, then the thread is alive. This allows the user to check if a second thread is running.

Design

Here is the current design of a consumer and what happens when it recovers from a crash:

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