Difference between heartbeat.interval.ms and session.timeout.ms in Kafka consumer config

The heartbeat.interval.ms specifies the frequency of sending heart beat signal by the consumer. So if this is 3000 ms (default), then every 3 seconds the consumer will send the heartbeat signal to the broker.

The session.timeout.ms specifies the amount of time within which the broker needs to get at least one heart beat signal from the consumer. Otherwise it will mark the consumer as dead. The default value 10000 ms (10 seconds) makes provision for missing three heart beat signals before a broker will mark the consumer as dead.

In a network setup under heavy load, it is normal to miss few heartbeat signals. So it is recommended to wait for missing 3 heart beat signals before marking the consumer as dead. That is the reason for the 1/3 recommendation.

The code makes a hard limit that you cannot set heartbeat.interval.ms no less than request.timeout.ms, otherwise Kafka complains "Heartbeat must be set lower than the session timeout".

If you really have these two configs be the same value, a possible situation is network client will never heartbeat anymore because the session timeout nearly always happens before doing heartbeat.

As for the 1/3, I prefer to think it sort of being a heuristic value.