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Integration / Apache Kafka Interview questions

What are the types of message delivery semantics in Kafka?

Delivery semantics describe the guarantee a consumer gets about whether it might see a message zero, one, or more than once, and Kafka can be configured to support any of the three, depending on how offsets are committed relative to processing.

SemanticGuarantee
At-most-onceA message may be lost, but is never processed twice (offset committed before processing).
At-least-onceA message is never lost, but may be processed more than once (offset committed after processing).
Exactly-onceA message is processed once and only once, using Kafka's idempotent producer and transactional APIs.

At-least-once is the most common default in real applications, since it's the simplest to implement correctly and downstream processing can often be made idempotent to tolerate the occasional duplicate; exactly-once requires more setup (transactions, idempotent writes) but removes the need for that downstream deduplication logic entirely.

Which delivery semantic guarantees a message is never lost but might be processed more than once?
Which semantic is achieved using Kafka's idempotent producer and transactional APIs?

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