Partition = ordering unit
Order guaranteed WITHIN a partition. Across partitions, no ordering. Choose key so same-key messages hash to same partition.
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Partition = ordering unit
Order guaranteed WITHIN a partition. Across partitions, no ordering. Choose key so same-key messages hash to same partition.
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Partition count = max parallelism
N consumers in a group ≤ N partitions. Add partitions to enable more consumers. Reducing is hard.
Producer key routing
hash(key) mod partition_count. Same key → same partition. No key → round-robin.
Consumer groups
All consumers in group share partitions. Different groups get full copy of stream. Great for fan-out.
Rebalancing pain
Consumer joins/leaves → partitions reassign. Cooperative rebalancing (Kafka 2.4+) minimizes disruption.