Partitioned Consumption

1. The Pattern

Split the stream into partitions by key: every event for a given key lands in the same partition, each partition is consumed by exactly one consumer at a time, and order holds strictly within a partition. The contract: order per key, parallelism across keys.

This lesson formalizes a deal the course has been making informally for eight modules. The queue lesson's Q3 (the refund processed before its charge) prescribed "partition by customer ID" without the fine print