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We will add a new error code for consumer to wait for pending transaction clearance. In order to be able to return corresponding exceptions for old/new clients, we shall also bump the OffsetFetch protocol version.

PENDING_TRANSACTION(85, "There are pending transactions for the offset topic that need to be cleared", PendingTransactionException::new),

In the meantime, this offset fetch back-off should be only applied to EOS use cases, not general offset fetch use case such as admin client access. we shall also define isolation level within offset fetch request so that we only trigger back-off logic when the request is on isolation level read_committed.

PENDING_TRANSACTION(85, "There are pending transactions for the offset topic that need to be cleared", PendingTransactionException::new),OffsetFetchRequest => Partitions GroupId IsolationLevel
  Partitions          => List<TopicPartition>
  GroupId             => String
  IsolationLevel      => int8 // NEW

Fence Zombie

A zombie process may invoke InitProducerId after falling out of the consumer group. In order to distinguish zombie requests, we need to leverage group coordinator to fence out of sync client.

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