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The Streams protocol also assigns warm-up tasks. However, the classic protocol does not have any notion of a warm-up task. If the group coordinator assigns a warm-up task to a Streams client on the classic protocol, that warm-up task is translated to a standby task in the assignment for the Streams client on the classic protocol. The group coordinator chooses one of the Streams clients on the classic protocol to trigger a probing rebalance.

A more detailed description of this process can be found in KIP-848 in Section Supporting Online Consumer Group Upgrade.  

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