Kafka ordering guarantees

Order is guaranteed only within a single partition. Cross-partition order is undefined. Producers must use the same key for related events; consumers must not parallelize processing of the same key across threads if order matters.

Trade-off: global ordering requires one partition — limiting throughput; per-key ordering scales with key distribution.

On interviews: guarantee order of login then logout for one user; why retries can reorder without idempotent sequence handling.

Common pitfalls: multiple keys for same entity; consumer thread pool breaking partition order; reprocessing from old offset interleaving with new events.

Checklist:

• Key all causally related events consistently.
• One in-flight handler per partition or per key.
• Design for out-of-order across services anyway.
• Test reorder scenarios under retries.