Ordering, Exactly-Once and Durability

Where Kafka Ordering Holds

Kafka guarantees order only within a partition. Records for one key share a partition, so per-key order is solid; cross-partition global order does not exist. Design consumers to rely on per-partition order.

Durability Chain

1. Producer sends with acks=all.
2. Leader writes to its log and waits for ISR.
3. Followers replicate before acknowledgment.
4. Consumer sees committed records only.

Retention vs Compaction

Topic retention (default 7 days) deletes old records by age. Log compaction instead keeps the latest value per key, ideal for key-value-like topics in event sourcing.

Exactly-Once Semantics

EOS combines an idempotent producer with transactions and read_committed consumers: the producer writes atomically across partitions, and consumers only see committed records, eliminating duplicates even on retries.

Key Points

• Order is per-partition, keyed and committed.
• acks=all plus ISR delivers durable writes.
• Compaction substitutes for retention on state topics.
• EOS needs transactions plus idempotence.