Dragos Dena created KAFKA-1183:
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Summary: DefaultEventHandler causes unbalanced distribution of
messages across partitions
Key: KAFKA-1183
URL: https://issues.apache.org/jira/browse/KAFKA-1183
Project: Kafka
Issue Type: Bug
Components: producer
Affects Versions: 0.8.0
Reporter: Dragos Dena
Assignee: Jun Rao
Fix For: 0.8.1, 0.9.0
KAFKA-959 introduced an optimisation in {{DefaultEventHandler}} that was
supposed to have the effect of sending all messages from the same batch to a
single partition if no key is specified.
The problem is that the {{sendPartitionPerTopicCache}} cache, which holds the
current selected partition for each topic, isn't actually invalided at the
start or end of each batch.
The observed result is that, after the first request chooses a random
partition, all subsequent messages from that producer land in the same
partition. If you have a large number of producers, then it should be fine, but
if your producer count is comparable to the partition count, then it will get
unbalanced.
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