Kafka: Key cardinality decides whether partitions stay balanced

Few distinct keys means most partitions sit idle while one does all the work.

Code

// 3 distinct keys, 12 partitions
send("orders", "IN", ...);
send("orders", "US", ...);
send("orders", "UK", ...);

Output

At most 3 of 12 partitions ever receive data.
Parallelism is 3, whatever the partition count says.
Use a key with high cardinality - order id, not country.

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Published 2026-08-11