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 read by exactly one consumer at a time. Order holds strictly within a partition. The contract: order per key, parallelism across keys.

This lesson formalizes a contract used throughout the previous eight modules; in the queue lesson's Q3, a refund was processed before its charge. Partitioning by customer ID corrected the order, but that lesson omitted the implementation details