Does a Kafka Consumer receive a list of offsets first, before receiving the bytes/data?

Question

I'm quite new to Apache Kafka and I'm currently reading Learning Apache Kafka, 2ed, (2015). Chapter 3, paragraph Kafka Design fundamentals says the following:

Consumers always consume messages from a particular partition sequentially and also acknowledge the message offset. This acknowledgement implies that the consumer has consumed all prior messages. Consumers issue an asynchronous pull request containing the offset of the message to be consumed to the broker and get the buffer of bytes.

I'm a bit thrown off by the word 'acknowledge'. Do I understand it correctly that Kafka sends the offset first and then the consumer uses the list of offsets to pull request the data it has not consumed yet?

Thanks in advance,

Nick

Answer 1

On startup, KafkaConsumer issues a offset lookup request to the brokers for the specific consumer group that was configured on this consumer. If valid offsets are returned those are used. Otherwise, the consumer uses an initial offset according to auto.offset.reset parameter.

Afterwards, offsets are maintained mainly in-memory within the consumer. Each poll() sends the current offset to the broker and on broker reply consumer updates the in-memory offsets.

Additionally, in-memory offset are committed/acked to the broker from time to time. This can happen automatically within poll() if auto commit is enabled, or commit() must be called explicitly to send offsets to the broker for reliably storing them.