Steven Aerts created KAFKA-7635:
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             Summary: FetcherThread stops processing after "Error processing 
data for partition"
                 Key: KAFKA-7635
                 URL: https://issues.apache.org/jira/browse/KAFKA-7635
             Project: Kafka
          Issue Type: Bug
          Components: replication
    Affects Versions: 2.0.0
            Reporter: Steven Aerts
         Attachments: stacktraces.txt

After disabling unclean leader leader again after recovery of a situation where 
we enabled unclean leader due to a split brain in zookeeper, we saw that some 
of our stopped replicating their partitions.

Digging into the logs, we saw that the replica thread was stopped because one 
partition had a failure which threw a [{{Error processing data for partition}} 
exception|https://github.com/apache/kafka/blob/2.0.0/core/src/main/scala/kafka/server/AbstractFetcherThread.scala#L207].
  But the broker kept running and serving the partitions from which it was 
leader.

We saw three different types of exceptions triggering this (example stacktraces 
attached):
* {{kafka.common.UnexpectedAppendOffsetException}}
* {{Trying to roll a new log segment for topic partition partition-b-97 with 
start offset 1388 while it already exists.}}
* {{Kafka scheduler is not running.}}

We think there are two acceptable ways for the kafka broker to handle this:
* Mark those partitions as a partition with error and handle them accordingly.  
As is done [when a {{CorruptRecordException}} or 
{{KafkaStorageException}}|https://github.com/apache/kafka/blob/2.0.0/core/src/main/scala/kafka/server/AbstractFetcherThread.scala#L196]
 is thrown.
* Exit the broker as is done [when log truncation is not 
allowed|https://github.com/apache/kafka/blob/2.0.0/core/src/main/scala/kafka/server/ReplicaFetcherThread.scala#L189].
 
Maybe even a combination of both.  Our probably naive idea is that for the 
first two types the first strategy would be the best, but for the last type, it 
is probably better to re-throw a {{FatalExitError}} and exit the broker.





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