Did you use Kafka transactional features? How is the key related to the partition?

In Kafka, transactions are used to ensure atomicity when writing messages to multiple topics or partitions, and to guarantee that consumers see either all messages of a transaction or none. This is important for maintaining data consistency in complex processing flows.

The key in Kafka is used to determine the partition to which a message will be sent. When a message is sent, the key is hashed, and based on the hash result, a specific partition is selected. Thus, messages with the same key will always go to the same partition, ensuring the order of their processing.

Example in Python using kafka-python for transactional sending:

from kafka import KafkaProducer

producer = KafkaProducer(bootstrap_servers='localhost:9092', transactional_id='my_txn_id')
producer.init_transactions()

try:
    producer.begin_transaction()
    producer.send('topic1', key=b'key1', value=b'message1')
    producer.send('topic2', key=b'key2', value=b'message2')
    producer.commit_transaction()
except Exception:
    producer.abort_transaction()