Event-Driven Pitfalls

1. Ordering Is a Per-Partition Promise

Message ordering is the guarantee most often assumed and least often read carefully.

Kafka orders records within a partition. It promises nothing across partitions, and nothing at all across topics.

Since a keyed record is assigned to a partition by hashing its key, ordering is really a promise about keys. Records sharing a key share a partition and arrive in order. Records with different keys may arrive in any relative order, and usually do under load.

That turns key selection into the ordering decision