[jira] [Created] (KAFKA-5337) Partition assignment strategy that distributes lag evenly across consumers in each group

["Grant Neale (JIRA)" <ji...@apache.org>]

Grant Neale created KAFKA-5337:
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             Summary: Partition assignment strategy that distributes lag evenly across consumers in each group
                 Key: KAFKA-5337
                 URL: https://issues.apache.org/jira/browse/KAFKA-5337
             Project: Kafka
          Issue Type: New Feature
          Components: consumer
    Affects Versions: 0.10.2.1
            Reporter: Grant Neale
            Priority: Minor


Existing partition assignment strategies (Range Assignor and RoubdRobinAssignor) do not account for the current consumer group lag on each partition.  This can result in sub-optimal assignments when the distribution of lags for a given topic and consumer group is skewed.

The LagBasedAssignor operates on a per-topic basis, and attempts to assign partitions such that lag is distributed as evenly across a consumer group.

**Algorithm:**

For each topic, first obtain the lag on all partitions. Lag on a given partition is the difference between the end offset and the last offset committed by the consumer group. If no offsets have been committed for a partition we determine the lag based on the code auto.offset.reset property. If auto.offset.reset=latest, we assume a lag of 0. If auto.offset.reset=earliest (or any other value) we assume lag equal to the total number of message currently available in that partition.

Next, create a map storing the current total lag of all partitions assigned to each member of the consumer group. Partitions are assigned in decreasing order of lag, with each partition assigned to the consumer with least total number of assigned partitions, breaking ties by assigning to the consumer with the least total currently assigned lag.

Assigning partitions evenly across consumers (by partition count) ensures that the assignment is reasonably balanced (by partition count) when all partitions have a current lag of 0 or if the distribution of lags is heavily skewed. It also gives the consumer group the best possible chance of remaining balanced if the assignment is retained for a long period (assuming throughput is consistent across members of the consumer group).



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