Can you explain what a key in the Kafka system is and what role it plays?

In Apache Kafka, the key is a value used to determine the partition where the message will be stored. When sending a message with a key, Kafka applies a hash function to the key and uses the result to select a specific partition of the topic.

Role of the key:

• Ensures that all messages with the same key go to the same partition, which is important for ordered processing.
• Allows implementing partitioning and load balancing logic.

If the key is not set, messages are distributed randomly or by round-robin, which can lead to loss of order among related messages.