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.