Narayan Periwal created KAFKA-6681:
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             Summary: Two instances of kafka consumer reading the same 
partition within a consumer group
                 Key: KAFKA-6681
                 URL: https://issues.apache.org/jira/browse/KAFKA-6681
             Project: Kafka
          Issue Type: Bug
          Components: consumer
    Affects Versions: 0.10.2.1
            Reporter: Narayan Periwal


We have seen this issue with the Kafka consumer, the new library that got 
introduced in 0.9

With this new client, the group management is done by kafka coordinator, which 
is one of the kafka broker.

We are using Kafka broker 0.10.2.1 and consumer client version is also 0.10.2.1 

The issue that we have faced is that, after rebalancing, some of the partitions 
gets consumed by 2 instances within a consumer group, leading to duplication of 
the entire partition data. They continue to read until the next rebalancing, or 
the restart of those clients. 

It looks like that a particular consumer goes on fetching the data from a 
partition, but the broker is not able to identify this "stale" consumer 
instance. 

During this time, we also see the underreplicated partition metrics spiking. 

We have hit this twice in production. Please look at it the earliest. 



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