How to ensure that messages for a single user always go to the same Kafka partition?

To ensure that messages related to the same user always go to the same Kafka partition, you need to use a key when sending messages that uniquely identifies the user. Kafka uses the key to determine the partition via a hash function.

Example in Java using KafkaProducer:

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

Here, userId is the key by which Kafka will determine the partition. Thus, all messages with the same key will go to the same partition, ensuring order and data locality for that user.

It is important that the number of partitions does not change dynamically, otherwise the key distribution may change.