How to check the actual number of incremental fetch session cache slots used in Kafka cluster?

Question

I am reading this question Kafka: Continuously getting FETCH_SESSION_ID_NOT_FOUND, and I am trying to apply the solution suggested by Hrishikesh Mishra, as we also face the

Answer 1

The metric named kafka.server:type=FetchSessionCache,name=NumIncrementalFetchSessions is the correct way to monitor the number of FetchSessions.

The size is configurable via max.incremental.fetch.session.cache.slots. Note that this setting is applied per-broker, so each broker can cache up to max.incremental.fetch.session.cache.slots sessions.

The other metric you saw, kafka.server:type=FetchSessionCache,name=NumIncrementalFetchPartitionsCached, is the total number of partitions used across all FetchSession. Many FetchSessions will used several partitions so it's expected to see a larger number of them.

As you said, the low number of FetchSessions you saw was likely due to the restart.