How to ensure sequential processing of transactions for one user in Kafka?

To guarantee the sequential processing of transactions from a single user in Kafka, message keys are usually used, which determine the partition to which the message will go. If all messages from one user are sent with the same key (for example, userId), Kafka guarantees that they will go to the same partition and be processed in order of arrival.

It is also important that the consumer processes messages from a partition sequentially, not parallelizing processing within a single partition.

Example in Java using KafkaProducer:

ProducerRecord<String, String> record = new ProducerRecord<>("topic", userId, transactionData);
producer.send(record);

Here, userId is the key that ensures all messages from the user go to the same partition.

If transactional processing is needed, Kafka transactions can be used, but for sequence, a key and sequential processing of partitions are sufficient.