Which of the following is a stateful Kafka Streams operator?

• A. join
• B. map
• C. filter
• D. foreach

Answer: (A)

In Kafka Streams, distinguishing between stateful and stateless operators is key. A stateful operator keeps data from previous records so it can relate new records to what was seen before. Joining streams is the classic example: to produce a joined result you must remember records from one side while processing the other, typically using a state store and, often, a time window. This reliance on stored state makes joining stateful.

The other options operate without needing to retain history. map transforms each input record independently, producing a new value without consulting past records. filter makes a pass/fail decision for each record on its own. foreach performs a side effect for every record, again without maintaining state between records.

So, joining is the stateful choice because it requires maintaining and coordinating state to produce joined outputs.